AGM OrndaemtaIeinlnsctloaufldAaelllslium AGM OrnAallmiuemntal RHS Grower GuideRHS Grower Guide 1 Helping gardeners choose the best plants The Royal Horticultural Society trials a wide range of ornamental and edible garden plants to test their health, performance and weather endurance. The best performers may receive a prestigious RHS Award of Garden Merit (AGM). Plant Trials provide the opportunity for multiple selections of the same plant to be grown side by side for the purpose of comparison. The selections are assessed by an expert RHS forum panel and the trial results are used to inform their decision to award plants the RHS AGM status. The plants are usually grown from seed and vegetative material, submitted by nurseries, individuals and breeders. Visitors to RHS Garden Wisley are encouraged to view ongoing plant trials on the Trials Field. Off-site trials are also held at gardens and nurseries across the UK. First published in 2018 by the Royal Horticultural Society: RHS Media, Churchgate, New Road, Peterborough PE1 1TT Registered office: Royal Horticultural Society, 80 Vincent Square, London SW1P 2PE Registered charity number 222879 / SC038262 rhs.org.uk © Royal Horticultural Society Front cover image: Allium ‘Gladiator’ All images: © RHS, unless otherwise indicated 2 RHS Grower Guide The Centenary Border at the Sir Harold Hillier Gardens, Hampshire. Welcome Ornamental alliums are almost continually in fashion and their deserved popularity shows no sign of abating. Loved by home gardeners, plant breeders and the horticultural industry, the Allium genus offers some of the most show-stopping garden bulbs available. There are different types of allium, broadly categorised as border alliums – those that grow on tall stems with ball-like flowerheads – and those that are suitable for growing in a rock or alpine garden. The latter category are generally smaller in stature, often with graceful, nodding, pendant bells. Border alliums start to appear in late spring after the first flush of other bulbs and blooms. With their striking, globular flowerheads that sit atop stout stems, these alliums create an impressive display in any garden and provide a spectacular prelude to summer’s luxuriant and colourful borders. From simple drumsticks to vast cricket balls that look like fireworks caught mid-display, each tightly-packed flowerhead is made up of myriad star- or cup-shaped blooms. Planted in clusters, drifts or dotted through borders, ornamental alliums give a distinct architectural presence and rhythm to the garden. Their colourful pom-pom heads create the horticultural equivalent of exclamation marks in the border. Rock garden alliums provide attractive accent colours to displays and are perfect for growing in containers, or in niches where their more dainty flowers can be appreciated. Together, their colour range includes mauve, deep purple, lilac, blue, white and, less commonly, yellow. Lady Christine Skelmersdale, Border Allium Plant Trial Forum Chair RHS Grower Guide 3 Contents 3 5 The Allium trials 6 RHS hardiness ratings 8 The Award of Garden Merit 26 AGM-winning border Allium A–Z 28 Short-lived Allium 32 Smaller AGM Allium 34 AGMs rescinded 36 How to grow Allium 38 Allium at a glance 40 Where to use Allium 42 Planting partners 43 Pests and diseases 45 Ornamental Allium Trial 2013–16 Suppliers to the Plant Trial 4 RHS Grower Guide The Allium trials There have been two ornamental Allium trials at RHS Garden Wisley – the earlier one concentrated on rock-garden types while the most recent trial focused on drumstick-type alliums. Each trial was judged by assessment by a panel of experts who made up a forum. Allium trial 1993–95 Growing consumer interest in ornamental alliums was the catalyst for this two-year-long trial. It assessed, compared and demonstrated a range of small ornamental Allium species suitable for rock gardens, many of which are rhizomatous with slender foliage. Planting took place in September 1993 with assessments following in the flowering season of the subsequent two years. In total, 62 Allium species and cultivars, up to 50cm (20in) tall and suitable for growing in rock gardens or in pots and containers, were trialled. Each was judged for its habit, impact, colour, foliage, height of flowerheads, length of flowering and hardiness. Allium trial 2013–16 For this most recent trial of border alliums (planted October 2013), more than 100 entries were trialled, all of which were grown from bulbs. The trial included selections from UK nurseries as well as Dutch bulb growers. This trial was held over three years after concern was raised about the perennial nature of the bulbs available. In the UK, alliums are grown in entirely different climatic conditions to those in which they originate and are grown. Specifically, drumstick alliums are native to the Middle East and central Asia and experience extremes of seasonal climates through the growing period. They begin to grow as the snow melts and come into flower as temperatures rise. Seed is distributed before a hot, dry and dormant period. In Holland, from where most bulbs are sourced, bulbs are typically lifted once flowering is over. Neither scenario tallies with the UK climate or gardening culture, and as such it was thought possible that flower quality of some selections may deteriorate over the trial term. The objectives of the trial were to: ♦ judge for garden-worthiness (AGM) ♦ demonstrate the range and use as garden plants ♦ compare established and new cultivars ♦ produce a permanent record through herbarium specimens, photographs and written descriptions for the RHS Herbarium at Wisley. RHS Grower Guide 5 Judging the trial The Allium Trials Assessment Forum assessed the entries in 2014 on 13 and 21 May, 4 and 11 June and 10 July. In 2015 and 2016, each member of the panel assessed the Trial Field on three different occasions through the growing season and recorded their findings. Out of a possible 50 points, marks were awarded for: ♦ o verall appearance (flower size, height, stability and colour) ♦ f lower quality (size, density of colour and length of flowering time) ♦ stem (uniformity of height, straightness and rigidity) ♦ seedhead and leaves ♦ resistance to pests and diseases The date of the first bud and first flowering were recorded, as well as flower diameter, stem length and date the flowers faded. RHS Horticultural Database RHS Plant Trials provide an important record for RHS research into cultivated plants and their names, and data is stored on the RHS Horticultural Database. RHS Herbarium Trials are an important source of material for the Herbarium, which is based at Wisley. Its primary focus is to acquire and maintain reference material of plants in cultivation. Colour references In all plant descriptions that follow, the letter and number codes after the colour names refer to the RHS Colour Chart. The chart is available to buy at rhsshop.co.uk. 6 RHS Grower Guide RHS hardiness ratings To help gardeners determine which plants may be hardy in their locality, the RHS hardiness rating scheme is listed below. Temperatures given are minimum winter requirements. Rating Temperature Category Definition H1a ranges ºC (ºF) Warmer than Heated Needs to be grown as H1b 15 (>59) glasshouse – a house plant or under tropical glass year round. 10 to 15 Heated Can be grown outdoors (50 to 59) glasshouse – in summer in sunny/ subtropical sheltered locations but perform best as H1c 5 to 10 Heated a house plant or in (41 to 50) glasshouse glasshouse conditions. – warm Can be grown outdoors temperate in summer through most of the UK while H2 1 to 5 Tender – cool daytime temperatures (34 to 41) or frost-free are high enough to glasshouse promote growth. Tolerant of low H3 -5 to 1 Half-hardy temperatures but (23 to 34) – unheated will not survive being glasshouse/ frozen. Except in mild winter some inner-city or coastal areas, requires H4 -10 to -5 Hardy – glasshouse conditions (14 to 23) average in winter, but can be winter grown outdoors after last frost. H5 -15 to -10 Hardy – cold Hardy in coastal/mild (5 to 14) winter areas except in hard winters and at risk from H6 -20 to -15 Hardy – very early frosts. May be (-4 to 5) cold winter hardy elsewhere with wall shelter or good H7 colder than -20 Very hardy microclimate. Can (<-4) survive with artificial winter protection. Hardy through most of the UK, apart from inland valleys, at altitude and central/northerly locations. Possible foliage damage and stem dieback in winter. Potted plants vulnerable. Hardy through most of the UK even in severe winters. May not withstand open or exposed sites or central/northerly locations. Evergreens can suffer foliage damage, potted plants are at increased risk. Hardy throughout the UK and northern Europe. Many plants grown in containers will be damaged unless given protection. Hardy in severe European continental climates including the UK’s exposed uplands. RHS Grower Guide 7 The Award of What is the RHS Award of Garden Merit? The RHS Award of Garden Merit (AGM) is intended to help gardeners select plants that have good all-round garden value, and is only awarded to plants that are: ♦ excellent for ordinary use in appropriate conditions ♦ a vailable ♦ of good constitution ♦ e ssentially stable in form and colour ♦ reasonably resistant to pests and diseases Plants of all kinds may be considered for the AGM, including fruit and vegetables. Currently, more than 7,500 plants have an AGM, but new awards are made each year. Awards are made after assessment by an RHS forum of experts, including nursery owners, specialist growers and horticulturists, all appointed by the RHS. Each assessment is ratified by the relevant RHS plant committee. RHS awards can also be given by ‘round-table assessment’. An award is given when a forum of horticultural experts meets to debate the characteristics and garden performance of plants that the RHS is unable to trial in one of the RHS Gardens or other location. 8 RHS Grower Guide The AGM logo on a plant’s label shows it has the RHS seal of approval. Garden Merit To ensure consistent standards are maintained, the AGM is subject to regular review to make sure that every plant still merits its place on the list. Since 2013, reviews have been conducted by each RHS plant committee on a rolling basis. Buying plants with the AGM It is easy to find and buy AGM plants: ♦ s earch using the RHS ‘Find a Plant’ feature on rhs.org.uk/plants or using our AGM Plant Search ♦ d ownload a list at rhs.org.uk/agmlists ♦ check the latest edition of the RHS Plant Finder book, available to buy at rhsshop.co.uk ♦ look out for the AGM logo the next time you are in a garden centre or nursery The following border alliums have all received the RHS Award of Garden Merit, either following trial or by round-table assessment. RHS Grower Guide 9 Border Allium Allium ‘Ambassador’ AGM 2016 Flowering time: June to July. Habit: to 130cm (51in) tall. Foliage: green strap-like leaves wither before the flowers open. Stems: strong stems. Flowers: compact giant globes to 18cm (7in) diameter made up of rose-violet flowers. Attractive seedheads continue through summer. Hardiness rating: H5. Note: flowers and dried seedheads are useful for floral arrangements. THE FORUM SAID: This cultivar has a glorious vibrant colour. A giant that grows from an enormous bulb. 10 RHS Grower Guide Allium ‘Beau Regard’ AGM 1995, reconfirmed 2016 Flowering time: May and June. Habit: to 90cm (35in) tall. Foliage: leaves spreading, 50cm (20in) long and 10cm (4in) wide, glossy green. Stems: strong, bare stems. Flowers: globular flowerhead to 20cm (8in) diameter. Each is compact and crowded with star-shaped flowers, 3cm (1¼in) diameter with petals of light purple (77C) with a strong purple (77B) midrib. Hardiness rating: H7. THE FORUM SAID: Good colour. Makes a great cut flower. Stems may need supporting if planting in exposed sites. Exceptional uniformity. RHS Grower Guide 11 Allium cristophii CROCUS AGM 1993, reconfirmed 2016 Flowering time: June. Habit: upright, to 60cm (24in). Foliage: strap-shaped, slightly glaucous, smooth leaves form a basal rosette and wither by flowering time. Each leaf has a line of silvery hairs at the margins. Stems: leafless stem. Flowers: Star-shaped, rosy violet flowers (77B) are borne in globose heads to 20cm (8in) wide. Each flower is 4cm (1¾in) across and has a metallic cast. Attractive seedheads. Hardiness rating: H5. THE FORUM SAID: The trial plants were not considered to be representative of the species, though those growing in the Glasshouse Borders at RHS Garden Wisley at the same time as the trial were considered to be excellent examples of their type. 12 RHS Grower Guide Allium ‘Gladiator’ AGM 1995, reconfirmed 2016 Flowering time: May. Habit: 110cm (43in) tall. Foliage: light yellow-green (143B), erect strap-shaped leaves to 45cm (18in) long and 10cm (4in) wide growing around the base of the stem. Stems: strong, upright stems. Flowers: each compact, dome-shaped flowerhead to 10cm (4in) high and 12cm (5in) wide is crowded with hundreds of star-shaped flowers. Each is 2cm (¾in) diameter, 1cm (⅜in) long, light purple (77C), with a midrib and anthers that are deep reddish purple (77A) with silvery tips. Hardiness rating: H6. Note: This cultivar is a cross between A. aflatunense and A. macleanii. THE FORUM SAID: Attractive skeletal seedheads prolong the display into autumn. Sweetly-scented flower. One of the tallest cultivars. RHS Grower Guide 13 Allium ‘Globemaster’ AGM 1995, reconfirmed 2016 Flowering time: May and June. Habit: 1m (40in) high. Foliage: neat foliage, not too long. Stems: thick, bright green stem. Flowers: compact, globe-shaped flowerhead, 18cm (7in) diameter. Each is crowded with star-shaped flowers, 2.5cm (1in) in diameter and 1.5cm (¾in) long with petals 1.5cm (¾in) long and 3mm (⅛in) wide. The flowers are a strong purple (77B) colour with a deep reddish purple (77A) midrib. Hardiness rating: H6. THE FORUM SAID: Excellent performer, long-flowering. As expected, the flowerheads become smaller as the bulbs bulk up. Perfect for a sunny border. 14 RHS Grower Guide Allium ‘Guna’ AGM 2016, subject to availability Flowering time: June. Habit: particularly tall cultivar to 1.2m (4ft). Foliage: relatively narrow leaves. Stems: stiff stems. Flowers: mid-purple flowerheads that are a good size and have a ‘fluffy’ appearance. Hardiness rating: H5. THE FORUM SAID: It has an attractive flowerhead with good proportions. Rich, appealing colour. Increased well over the trial and maintained its flowerhead size. RHS Grower Guide 15 Allium hollandicum ‘Purple Sensation’ AGM 1993, reconfirmed 2016 Flowering time: May–June. Habit: to 90cm (35in) tall. Foliage: short, basal, strap-shaped, grey-green leaves. Stems: upright stems. Flowers: small, vivid, rosy purple, star-shaped flowers appear in crowded, spherical heads followed by attractive seedheads. Hardiness rating: H6. THE FORUM SAID: Striking silhouette – perfect to combine with grasses in a gravel garden. It is a seed-raised cultivar but has excellent performance and is reliably perennial. 16 RHS Grower Guide Allium jesdianum ‘Akbulak’ AGM 2016 Flowering time: May. Habit: sturdy habit to 90cm (35in) tall. Foliage: unusually narrow leaves. Stems: ribbed stems. Flowers: globular, starry, dark purple, loose flowerheads, each about the size of a tennis ball (10–12cm/4–5in). It has excellent dried seedheads. Hardiness rating: H5. THE FORUM SAID: Attractive plants – their ribbed stems and white exserted stamens contrast with starry, purple flowers. Holds its colour well. Exceptional and prolific over the three years of the trial and maintained its flowerhead size. RHS Grower Guide 17 Allium jesdianum ‘Early Emperor’ AGM 2016 Flowering time: May. Habit: upright, clump-forming to 75cm (30in) tall. Foliage: glossy, slightly aromatic, strap-shaped, dark green leaves. Stems: erect and sturdy, strongly-ribbed stems. Flowers: dense, rounded flowerheads, each 10–15cm (4–6in) diameter, made up of many violet flowers, which have long white stamens. Hardiness rating: H5. Note: plant in sheltered locations. THE FORUM SAID: This cultivar is one of the earliest to flower. Has a nice habit, good colour in the deep purple, starry flowers, which contrast nicely with the white stamens. 18 RHS Grower Guide Allium ‘Powder Puff’ AGM 2016 Flowering time: May to June. Habit: low-growing and compact cultivar to 40cm (16in) tall. Foliage: blade-like leaves. Stems: short in comparison to other selections with similar large flowerheads. Flowers: bright purple flowerheads, each the size of a tennis ball. Hardiness rating: H5. Note: a hybrid of A. nevskianum and A. sarawschanicum. THE FORUM SAID: Good performer, bulked up well. Shorter than most. One of the best in the trial. Attractive seedheads. Great for patios or to grow at the front of the border. RHS Grower Guide 19 Allium ‘Purple Rain’ AGM 2016 Flowering time: May and June. Habit: outstanding habit – increases fast and with long-lasting flowers. Foliage: green leaves that curl at the base before withering. Stems: upright and sturdy stems to 80–90cm (31–35in). Flowers: globe-shaped flowerhead to 15cm (6in), made up of open and airy, slightly star-shaped florets. Produces masses of flowers with an intense colour. Hardiness rating: H5. Note: A cross between A. hollandicum ‘Purple Sensation’ and A. cristophii. THE FORUM SAID: Great for the back of the border and to follow on from A. hollandicum ‘Purple Sensation’. Lovely colour, increasing significantly. Superb – best in trial. The seedheads are excellent. 20 RHS Grower Guide Allium rosenorum ‘Michael H. Hoog’ AGM 2016 Flowering time: May. Habit: increased well over the three years of the trial. Foliage: narrow leaves that fade as the flowers open. Stems: firm stems to 120cm (4ft). Flowers: large, evenly-toned, violet flowers. Produces good seedheads. Hardiness rating: H5. THE FORUM SAID: Graceful with sturdy stems. Persistent. RHS Grower Guide 21 Allium ‘Round ‘n’ Purple’ AGM 2016 Flowering time: June. Habit: compact and strong grower with a tendency to produce double stems to 90–120cm (35in–4ft) tall. Increases well. Foliage: broad, pointed leaves that were in good condition at flowering time. Stems: robust stems. Flowers: silvery-lilac flowers forming a massive 15cm (6in) globe. Even colour, but faded as the plant aged. Hardiness rating: H5. THE FORUM SAID: Performed well, bulked up well. Good seedheads. These long-lasting blooms are excellent as cut flowers. Naturalises easily – looks spectacular when planted in sweeping drifts. 22 RHS Grower Guide Allium sphaerocephalon AGM 2016 Flowering time: July and August. Habit: drumstick-like appearance to 65cm (26in) tall. Foliage: mid-green, small, narrow and stem-clasping. Stems: slender stems. Flowers: egg-shaped, dense heads of lime-green flowers. The flowers gradually turn crimson-maroon to reddish brown. Each is 1.5cm (¾in) wide. The flowers fade as they age. Hardiness rating: H6. Note: has a strong onion odour. THE FORUM SAID: Best when planted in a drift in a sunny border. This species is later flowering than most. RHS Grower Guide 23 Allium stipitatum ‘Mount Everest’ AGM 2016 Flowering time: May. Habit: statuesque – tall and upright to about 110cm (43in). Foliage: strap-shaped leaves. Stems: firm, ribbed stems. Flowers: pure white flowerheads to 15cm (6in) diameter. Each globe is made up of 50 or more white, star-shaped flowers with a green eye. The flowerheads mature to produce attractive seedheads. Hardiness rating: H5. THE FORUM SAID: A good performer. Best of the whites. It has stiff seedheads – a good green colour. 24 RHS Grower Guide Allium stipitatum ‘Violet Beauty’ AGM 2016 Flowering time: May and June. Habit: upright to approximately 70cm (27in) tall, clump-forming and not as large as many. Foliage: glossy, strap-shaped, mid-green leaves. Stems: strong, upright stems. Flowers: large, light violet drumstick heads to 10cm (4in) diameter, composed of tight clusters of star- shaped flowers. Each flowerhead has a flat base. Excellent cut flowers with a soft fragrance. Hardiness rating: H5. THE FORUM SAID: Pretty colour, which is retained well. Good height. Increases in volume, though subsequent bulbs produce smaller heads. Good for cut flower displays; can be pulled rather than cut. RHS Grower Guide 25 Allium suworowii AGM 2016 Flowering time: May to June. Habit: statuesque to 1m (40in) high. Foliage: between five and seven broad, bright green leaves form a rosette around the base of the stem. Stems: strong stems. Flowers: large and full, globe-shaped flowerhead made up of star-like florets, each 1cm (⅜in) diameter and with protruding stamens. Fabulous colour, almost luminous. Hardiness rating: H5. Note: a sterile, wild selection. THE FORUM SAID: Good persistence – maintained colour and flower size over the three years of the trial but didn’t increase much. Stands well. Strong colour. 26 RHS Grower Guide Allium ‘Universe’ AGM 2016 Flowering time: May to June. Habit: to 50cm (20in tall). Upright and a well- balanced plant. Increased well. Foliage: wide leaves. Stems: green (146B), sturdy and relatively short for the size of the flowerhead. Flowers: dark violet-purple (N87C) globes made up of around 40 loosely-packed florets, each 4cm (1¾in) diameter with purple (N79B) anthers and a green (137A) eye. One of the largest allium flowerheads at 23cm (9in) diameter and very full. Maintained flowerhead size over the three years of the trial. Hardiness rating: H5. THE FORUM SAID: This is an outstanding cultivar, with one of the largest flowerheads in the trial. RHS Grower Guide 27 Short-lived border selections The border alliums listed so far all performed well in the first year of the trial. Some already held the Award of Garden Merit on the basis of their exceptional flowering quality. However, because they did not perform as well in the second and third years of the trial, their AGMs were rescinded. This is because the parameters set for the 2013–16 trial were that the selections had to flower as well in year three as they did in year one to be worthy of the AGM. All Allium listed in this panel are exceptional flowerers in year one, and for that they are deemed worthy of a space in any garden. However, if they are left in the ground once they have flowered, they may not flower as spectacularly in subsequent years. If you choose to follow the Dutch method of growing large, bulbous alliums instead of the British method, then the flowering quality of year one will be replicated. The Dutch method refers to lifting the bulbs once the flowerheads have faded, then drying and storing them in a cool, dry place until the autumn, when they are replanted. Flowerheads can be dried for decorative use. Allium ‘Atlas’ Similar to A. ‘Universe’ but slightly shorter with paler, violet-purple stars. From a cross between A. hollandicum ‘Purple Sensation’ and A. cristophii. It has excellent performance in its first year and a decent seed head. Height: 90cm (35in). Heads: 20cm (8in). Allium ‘Mercurius’ Stunning in first year but low perenniality. The flowers are globular and blue-purple. This cultivar shows some A. cristophii parentage. Foliage remains attractive at peak flowering. Height: 75cm (30in). Heads: 10cm (4in). Allium ‘Bolero’/‘Miami’ Plant known by both names. Good colour, attractive heads, stems can become rather lax later in the season. Shows some A. cristophii parentage. Height: 90cm (35in). Heads: 10cm (4in). 28 RHS Grower Guide Allium ‘Summer Drummer’ Exceptionally tall, rather dramatic and quick to bulk up. Flowering is late and irregular. The stem-clasping, glaucous leaves are attractive early in the year but die back and can detract from the flowers. Grows best with grasses or at the back of a sunny border. Height: 150cm (59in). Allium schubertii This is the most dramatic of all alliums with its huge firework-like head. The flowerhead has 50 or more star-shaped, pinky purple florets on stems of differing lengths, producing a starburst effect. It requires a much warmer position than most but will perform well if planted against a sunny wall. The bright green, strap-shaped leaves appear in spring, and die back before the flowers appear. Will not bulk up. Dries exceptionally well. Colour: H4. Head: up to 30cm (12in). Short-lived allium relatives Nectaroscordum meliophyllum Similar appearance to N. tripedale but with slightly taller stems. Leaves narrow-keeled, 40cm (16in) long. Slow but spectacular in flower. Produces umbels of nodding green-tinged, cream flowers with purple markings followed by erect seed capsules. Colour: H5. Height 150cm (59in). Nectaroscordum tripedale Linear, keeled basal leaves up to 30cm (12in) long. Stout stems produce dense umbels to 8cm (3in) across. Slow but spectacular in flower. Heads have 20–30 bell-shaped flowers, white in colour, sometimes with purple spots. Colour: H5. Height: 1m (3ft). Heads: 14cm (5½in). RHS Grower Guide 29 GAP PHOTOS / TORIE CHUGG Smaller AGM Allium The following rhizomatous and small bulbous ornamental Allium also hold AGMs. These are shorter selections suitable for growing in a rock or alpine garden and were not retrialled as part of the 2013–16 trial because of their categorisation as rock garden alliums. GAP PHOTOS / TOMMY TONSBERG Allium beesianum AGM 2012 Nodding clusters of hanging, bell-shaped, cornflower-blue flowers with white margins and blue stamens. The species is slow to establish. Clump-forming habit to 25cm (10in) tall. Hardiness rating: H5. 30 RHS Grower Guide Allium WIKI / H. ZELL caesium LE CLOS D’ARMOISE NURSERY, BRITTANYPERENNIALS.UK AGM 2002 Forms dense umbels to 2cm (¾in) wide, composed of tiny, highly variable, green- veined, violet-blue flowers. Naturally inhabits deserts and dry fields. Hardiness rating: H5. Allium carinatum subsp. pulchellum AGM 1993 Loose umbels of nodding, bell-shaped flowers to 6cm (2½in). Rich red-purple (84B). White stamens marked with green. Hardiness rating: H5. Allium carinatum subsp. pulchellum f. album AGM 2002 Loose, elongated umbels, each 5cm (2in) wide. Forms handsome drifts. Hardiness rating: H5. Allium cernuum ‘Hidcote’ AGM 1993 Nodding umbels of small, bell-shaped, magenta-rose flowers, followed by attractive seed heads. To 45cm (18in) tall. Hardiness rating: H6. RHS Grower Guide 31 GAP PHOTOS / CHARLES HAWES Allium cyaneum AGM 1993 Small flowerheads each made up of 10–12 hanging bells that are intense violet- blue, often with a white margin. This dainty ornamental dislikes dry, shady locations. To 30cm (12in) tall. Hardiness rating: H5. Allium flavum AGM 1993 Small, bell-shaped, golden- yellow flowers are borne in loose umbels. The anthers protrude from the flowers. This clump-forming species naturalises easily. To 40cm (16in) tall. Hardiness rating: H5. Allium insumbicum AGM 1993 Large, nodding, rosy pink, bell-shaped flowers, five per cluster, each 15mm (¾in) long are followed by similarly nodding seedheads. To 30cm (12in) tall. Hardiness rating: H5. Allium moly ‘Jeannine’ AGM 1993 Each stem bears an umbel the size of a golf ball made up of bright yellow, star- shaped flowers. Two flower spikes per bulb. Perfect for growing in a border beneath trees and taller shrubs. Hardiness rating: H6. 32 RHS Grower Guide GAP PHOTOS / MARCUS HARPUR Allium oreophilum AVON BULBS ‘Zwanenburg’ AGM 1993 Large spherical heads of bell-shaped, deep carmine- pink flowers each 1cm (⅜in) long. Naturalises well. Creates attractive drifts. To 30cm (12in) tall. Hardiness rating: H6. Allium senescens AGM 2014 Round heads of pink-lilac flowers. The foliage stays on the plant until well after the flowers have finished. Has a stronger onion scent than most others. A late flowerer that needs little attention once established. Hardiness rating: H6. Allium thunbergii AGM 2002 Erect to spreading hemispherical umbels to 3.5cm (1½in) diameter. Each has 18 violet, bell- shaped flowerheads, 5cm (2in) long, with elongated stamens and yellow anthers. To 18cm (7in). Hardiness rating: H5. Allium unifolium AGM 2002 Nodding umbels of small, bell-shaped, magenta-rose flowers, followed by attractive seed heads. To 25cm (10in). Hardiness rating: H5. RHS Grower Guide 33 COTSWOLD GARDEN FLOWERS AGMs rescinded All plants that are awarded the RHS Award of Garden Merit (AGM) are periodically reassessed to ensure that current stock is consistent in all aspects with the standard required for the AGM. An AGM can be rescinded if, under assessment, a selection is deemed to have lost those characteristics for which it earned the award. Allium caeruleum AGM 1993 Reason: not persistent, not as vigorous after first year, lacks perenniality. Allium giganteum AGM 1995 Reason: it did not repeat-flower. In fact, it failed to appear in the second year of the trial. Superseded by A. ‘Ambassador’ (p8). Allium hollandicum AGM 1995 Reason: the colour is pale and fades fast. Superseded by many cultivars. Allium hollandicum ‘Purple Surprise’ Reason: No longer available. 34 RHS Grower Guide CROCUS Allium karataviense GAP RHOTOS / CLIVE NICHOLS AGM 1993 Reason: superseded, needs rockery setting to look its best; poor foliage; lacks perenniality. Allium moly AGM 1993 Reason: the named cultivar ‘Jeannine’ (p30) is preferred. Allium oreophilum AGM 1993 Reason: invasive, short flowering season, dirty pink blooms; A. oreophilum ‘Zwanenburg’ (p31) is preferred. Allium schoenoprasum ‘Black Isle Blush’ AGM 1995 Reason: not widely available. Allium schoenoprasum ‘Pink Perfection’ AGM 1995 Reason: not widely available. Allium schubertii Reason: stunning in first year but not reliably perennial, only useful as a one-season plant. RHS Grower Guide 35 How to grow Allium is a variable genus of about 700 species, native to dry or mountainous areas of the northern hemisphere. All alliums produce their flowers in an umbel, which can be few- or many-flowered, ball-like or nodding. Alliums are either bulbous with a single stem and strap-like basal leaves, or have clump-forming rhizomes with clusters of narrow leaves, which may be stem-clasping and evergreen. Each increases by offsets. Every part of the plant has an onion smell when crushed. As well as the many ornamental alliums discussed here, the genus also contains edible plants such as chives, garlic, leeks, onions, scallions (spring onions) and shallots. 36 RHS Grower Guide Allium Ornamental alliums are freely available to gardeners, either to be planted as bulbs in the autumn, or as pot-grown plants in spring. Tall border or drumstick alliums, as shown in the image above of the Main Borders at RHS Garden Harlow Carr, are easy to recognise. Their colourful orbs atop slender, sturdy stems take up little garden space and seemingly float above flower borders plants. These are dynamic plants with a multitude of uses in the garden. Many are also deliciously attractive to pollinators, while their onion ancestry often renders them undesirable to pests looking for an easy meal. Shorter ones are equally at home in a rock garden or at the front of a sunny border. Plant in the right conditions and alliums will reward and delight you with blooms for years to come. Choose a well- drained site – the huge bulbs rot easily in waterlogged or heavier soils – and an aspect that receives day-long sunshine. Provide a little protection from strong wind so that long- stemmed selections will thrive, and choose a spot among low-growing plants that are timed to come into growth to hide decaying allium foliage. They can sometimes be shy to flower when grown in containers. RHS Grower Guide 37 Allium at a ♦ Overview Grown for their striking flowerheads, and available in a range of sizes and colours including blue, purple, mauve, lilac, white and yellow. Alliums flower from late spring to late summer. The seedheads of many also look attractive in the summer garden. ♦ Soil Bulbous alliums require well-drained soil. Boost poor soils with a fertiliser in autumn. Add plenty of grit to heavy soils. Dwarf rhizomatous alliums (see p28) are summer-growing and like a much cooler position with plenty of moisture in a well-drained location. ♦ Size Height from 18cm (7in) to 1.8m (6ft). Rhizomatous and small bulbous types have small flowerheads no more than 3cm (1¼in) across, while larger drumstick ones may have flowerheads to 35cm (14in) in diameter. ♦ Garden position Beds and borders, rock gardens and deep containers. Bulbous alliums like to be sited in full sun and bulbs will rot in damp soil; they do not appreciate borders where there is summer irrigation. They are good companions for grasses. Avoid areas regularly cultivated because it is easy to damage the bulbs. The leaves of bulbous allium start growing in early spring and wither as or before the flowers appear. Often the dying leaves are rather unattractive. To mask their appearance, site alliums so that low-growing shrubs and flowers cover unsightly foliage. As leaves start to die off once flowering starts, they can be removed with no ill effect. The smaller late-summer-flowering rhizomatous ones prefer a moister situation and may take shade. A purple border of Allium ‘Globemaster’ and Salvia pratensis in The Queen Mother’s Garden at RHS Garden Hyde Hall, Essex. 38 RHS Grower Guide glance ♦ Planting Plant bulbs in early autumn 10–15cm (4–6in) deep. Plant smaller-growing alliums 8–10cm (3–4in) apart, taller species need 20cm (8in) between the bulbs. Do not plant where edible onions have been grown because of the risk of pest and disease transfer. Rhizomatous alliums are usually planted in early spring normally as growing plants. ♦ Pollinators Favoured by bumblebees and other pollinators. ♦ Water requirements Alliums produce their new bulbs before flowering and therefore there is no need to water them unless there is an exceptionally dry, early spring, before the buds have elongated. ♦ Aftercare Alliums are mostly low maintenance. Lift and divide border bulbs if they become overcrowded once the flowers and foliage have died down. Once the leaves have started to turn brown and wither it is safe to remove them as the new bulb is already formed. Allium leaves contain a strong dye which may stain yellow. If displaying alliums as cut flowers, replace the water in the vase regularly to dull the onion scent. Many allium heads can be cut and dried. ♦ Propagation New offsets may appear around the base of the original bulb. These can be removed and replanted once the plant has flowered. Simply lift the bulb and break off the offsets, then replant as desired. Alternatively, seed can be collected in summer once flowering is over and the bulb is dormant. Seed will not come true from named cultivars and many of these are sterile. It will take up to seven years to produce a flowering bulb from seed. ♦ Pests and diseases Onion white rot, downy mildew, onion fly, allium leaf miner, slugs and snails. Allium in the Country Garden at RHS Garden Wisley, Surrey. RHS Grower Guide 39 Where to use Middle of dry, Front of sunny sunny border border ♦ Any taller drumstick type such as ♦ A. cristophii A. moly ‘Jeannine’ A. ‘Ambassador’, ♦ A. ‘Powder Puff’ A. ‘Gladiator’, ♦ A. ‘Universe’ A. ‘Globemaster’, ♦ A. oreophilum ♦ A. ‘Mount Everest’, ‘Zwanenburg’ ♦ A. ‘Purple Rain’ Rock garden Naturalistic plantings ♦ A. flavum ♦ A. senescens ♦ A. cristophii ♦ A. oreophilum ♦ A. moly ‘Jeannine’ ‘Zwanenburg’ ♦ A. hollandicum ‘Purple Sensation’ ♦ A. sphaerocephalon ♦ A. stipitatum ‘Violet Beauty’ 40 RHS Grower Guide A mix of purple and white alliums provide early floral interest in the Main Borders at RHS Garden Harlow Carr, North Yorkshire. Allium For cut flowers Light shade ♦ Any drumstick ♦ A. beesianum alliums ♦ A. carinatum ♦ A. moly ‘Jeannine’ ♦ A. hollandicum ‘Purple Sensation’ ♦ A. unifolium ♦ A. stipitatum ‘Violet Beauty’ For dried seedheads ♦ A. cristophii ♦ A. ‘Purple Rain’ ♦ A. hollandicum ‘Purple Sensation’ ♦ A. schubertii RHS Grower Guide 41 Planting partners With their distinct shape and colourful flowerheads, ornamental alliums offer irresistible opportunities to create a planting design statement as well as attract masses of pollinating insects into the garden. With their clean stems and large round heads, drumstick alliums make a dynamic impact in any garden, drawing the eye with their attractive colours. To make the most of their shape, size and colour, plant them in the middle of the border so they can rise above lower neighbours. This will also help disguise the basal leaves that are usually fading as they come into flower. Surrounded by masses of fine foliage such as that of grass Anemanthele lessoniana, for example, the round heads offer intrigue, and appear to be floating above them. Grasses of all types are often perfect partners, both revelling in similar, well-drained situations. Tall border alliums have an architectural quality, a characteristic to be maximised. The size of the flowerhead may help determine whether you choose to plant them in groups that are repeated around the garden, or singly but at 42 RHS Grower Guide regular intervals, effectively creating a river of round flowerheads providing rhythm and regularity. Shorter kinds are perfect for planting in drifts along the front of a border. For dramatic effect, place alliums next to colours that provide sharp contrast. Orange, bronze, yellow and soft lime flowers or foliage add zingy tones to accent deep purple, lilac and blue blooms. Orange geums and oriental poppies with their intense colours and simple, open flowers add drama. For softer contrast choose flowering plants in complementary tones such as silver-hued Artemisia ‘Powis Castle’or purple fennel to create a more restful effect. The smaller-flowered alliums such as A. sphaerocephalon are best planted in tight groups for maximum impact. Some of the smaller flowered ones are suitable for clumps on a rock garden. Others will tolerate part-shade among shrubs, such as roses. For progression of flowering season follow A. jesdianum, A. hollandicum ‘Purple Sensation’, A. ‘Purple Rain’ and A. stipitatum ‘Mount Everest’ with A. ‘Globemaster’, A. cristophii, A. ‘Ambassador’, and A. moly ‘Jeannine’ , then late flowering A.sphaerocephalum and A. ‘Summer Drummer’. ABOVE Border allium with Anthriscus sylvestris ‘Ravenswing’ (cow parsley) in the Hilltop Garden at RHS Garden Hyde Hall, Essex. LEFT A wide range of Allium selections are available to suit different garden situations. RHS Grower Guide 43 Pests and diseases Ornamental alliums are remarkably resilient. All allium bulbs are susceptible to onion white rot. To avoid this, be sure to plant bulbs in well-drained ground. Allium rust is caused by a fungus that results in orange spots on leaf surfaces in mild, damp weather. Downy mildew, which looks like mould, is another fungal disease that may affect bulbs. Spray with any treatment for Botrytis. New growth is always attractive to slugs and snails, though not rodents or deer. Allium leaf miner may affect leaves, though foliage often dies back before the flowers bloom. Pets may be poisoned if they consume certain species. Rust Allium leaf miner 44 RHS Grower Guide Ornamental Allium Trial 2013–16 List of plants trialled ♦ A . aflatunense ♦ A . altissimum ♦ A. altissimum ‘Goliath’ ♦ A. ‘Ambassador’ ♦ A. amethystinum ‘Red Mohican’ Trials field ♦ A. ampeloprasum ♦ A . ampeloprasum ‘Hairy Friend’ ♦ A . ampeloprasum ‘Purple Mystery’ ♦ A . ampeloprasum ‘Rose Picture’ ♦ A . aschersonianum ♦ A . ‘Atlas’ ♦ A. atropurpureum ♦ A. backhousianum ‘Green Craze’ ♦ A. ‘Beau Regard’ ♦ A. ‘Bolero’ ♦ A. caeruleum ♦ A. ‘Caméléon’ ♦ A. carolinianum ‘Rosy Dream’ ♦ A. ‘Crans’ ♦ A. ‘Creta’ ♦ A. cristophii ♦ A. cupuliferum ♦ A. decipiens ♦ A. ‘Eros’ ♦ A. ‘Firmament’ A. ‘Crans’ ♦ A. ‘Forelock’ ♦ A . giganteum ♦ A. giganteum ‘Twinkling Stars’ ♦ A . ‘GK6’ ♦ A . ‘Gladiator’ ♦ A . ‘Globemaster’ ♦ A . ‘Globus’ ♦ A . ‘Graceful Beauty’ ♦ A . ‘Guna’ ♦ A . hollandicum ♦ A . hollandicum ‘Purple Sensation’ ♦ A . ‘Jackpot’ ♦ A . jesdianum ♦ A . jesdianum ‘Akbulak’ ♦ A . jesdianum ‘Early Emperor’ ♦ A . jesdianum ‘Pendjikent’ ♦ A . jesdianum ‘Per Wendelbo’ ♦ A. jesdianum ‘Purple King’ ♦ A. jesdianum ‘Shing’ ♦ A. jesdianum ‘White Empress’ A. ‘Eros’ ♦ A . karataviense ♦ A . karataviense ‘Ivory Queen’ ♦ A. karataviense (red- and pink-flowered) RHS Grower Guide 45 ♦ A. kwakense A. ‘Mercurius’ ♦ A . ‘Long Beauty’ A. ‘Pinball Wizard’ ♦ A. loratum ‘Sweet Candy’ A. scorodoprasum ♦ A. ‘Lucy Ball’ ♦ A. macleanii ‘Art’ ♦ A . macleanii ‘His Excellency’ A. ‘Summer ♦ A. ‘Mercurius’ Drummer’ ♦ A. ‘Metallic Shine’ ♦ A. ‘Miami’ ♦ A. nigrum ♦ A . nigrum (pink-flowered) ♦ A . obliquum ♦ A. ‘Ostara’ ♦ A . ‘Pinball Wizard’ ♦ A. ‘Ping Pong’ ♦ A. ‘Pink Jewel’ ♦ A. ‘Powder Puff’ ♦ A . ‘Purple Rain’ ♦ A . ‘Purple Suze’ ♦ A. ‘Red Eye’ ♦ A. rosenbachianum ♦ A. rosenbachianum ‘Album’ ♦ A. rosenbachianum ‘Stam II’ ♦ A. rosenorum ‘Michael H. Hoog’ ♦ A . ‘Round ’n’ Purple’ ♦ A . sarawschanicum ♦ A . sativum var. ophioscorodon ♦ A. schubertii ♦ A. scorodoprasum ‘Art’ ♦ A. scorodoprasum ‘Bizar’ ♦ A. scorodoprasum ‘Passion’ ♦ A. ‘Silver Spring’ ♦ A. sphaerocephalon ♦ A . ‘Spider’ ♦ A . stipitatum ♦ A . stipitatum ‘Album’ ♦ A . stipitatum ‘Mars’ ♦ A . stipitatum ‘Mont Blanc’ ♦ A . stipitatum ‘Mount Everest’ ♦ A . stipitatum ‘Pink Globe’ ♦ A . stipitatum ‘Violet Beauty’ ♦ A . stipitatum ‘White Giant’ ♦ A . stipitatum (white-flowered) ♦ A . ‘Stratos’ ♦ A . ‘Summer Drummer’ ♦ A . suworowii ♦ A. ‘Sweet Discovery’ ♦ A. texanum ♦ A. tschimganicum ♦ A. ‘Universe’ ♦ A . ‘Venus’ ♦ A . vineale ♦ A . vineale ‘Dready’ ♦ A . vineale ‘Hair’ ♦ A . ‘White Cloud’ ♦ A . ‘World Cup’ ♦ N ectaroscordum meliophyllum ♦ N . siculum ♦ N . tripedale 46 RHS Grower Guide Suppliers to the RHS Plant Trial of ornamental Allium 2013–16 Christine Skelmersdale broadleighbulbs.co.uk Jac. Uittenbogaard jubholland.nl Jacques Amand jacquesamandintl.com PW – Peter Warmenhoven c/o the Trials office Wietse Mellema wmel.nl Wim de Goede c/o the Trials office For more information Details of current and past RHS Plant Trials, including Trial Reports, can be found online at rhs.org.uk/trials-awards There are also a limited number of earlier Plant Trial Bulletins available to download, or as printed copies. Printed copies can be obtained from The Wisley Plant Centre at RHS Garden Wisley, or by contacting the Trials Office at the address below (donations are requested to cover the cost of these bulletins). Horticultural Trials and Relations office, RHS Garden Wisley, Woking, Surrey GU23 6QB Email: [email protected] In 2018 the following trials will be taking place at RHS Gardens and Partner Gardens: Aubergine Hibiscus syriacus Chinese cabbage Hydrangea macrophylla Chrysanthemum Hypericum (in containers) Iris reticulata Colchicum Lilium (Asiatic hybrids) Cornus Paeonia Cypripedium Physocarpus Echinacea Rosmarinus Eucomis Sanguisorba Erica (winter-flowering) Sarracenia (outdoors) Gentiana Shallots Geum Viburnum plicatum Hemerocallis Winter cabbage This Grower Guide was compiled with information from the RHS Plant Trial notes 2013–2016, written by RHS Trials Officer Karen Robbirt. With thanks to Lady Christine Skelmersdale, Brian Mathew and Avon Bulbs. For more information about RHS Plant Trials visit rhs.org.uk/trials-awards RHS Grower Guide 47 About the RHS The Royal Horticultural Society was founded in 1804 by Sir Joseph Banks and John Wedgwood to inspire passion and excellence in the science, art and practice of horticulture. Our vision is to enrich everyone’s life through plants, and make the UK a greener, healthier, happier and more beautiful place. This is the third of a new series of Grower Guides from the RHS. It contains a wealth of information about ornamental Allium, as well as highlighting the important role of Plant Trials in the work of the RHS to better inform and give confidence to gardeners about the plants they purchase. For more information about the RHS, visit rhs.org.uk The list of ornamental Allium with an RHS Award of Garden Merit (AGM) listed within this guide are correct as of February 2018. The Award of Garden Merit is reviewed according to a rolling programme to ensure the standard of garden-worthiness is maintained.
