WildBristol.uk - Discovering Wildlife in Bristol

Rare. The only place it grows naturally in the UK is The Avon gorge in Bristol where it was discovered by Dr H.O. Stephens in 1847. It also occurs on the Channel Islands (Jersey) and is naturalised elsewhere in Britain. Habitat includes: Free-draining, calcareous soils, in full sun (South to South-west facing) rock faces / ledges etc. Two isolated populations on the Bristol side of the Gorge (Goat Gully and St Vincent's Rocks). Other names include: St Vincent's Rock Leek, Round-headed Leek, Round-headed Garlic, Ball-head Onion, Drumstick, Kugellauch (Germany), Allium Sphaerocephalum (old name). It is often cultivated in gardens. Flowers: July to August. Height: Up to 50 cm. Reproduction: Bumblebees pollinate the flowers, dispersal by seed is the most used method (where they grow where they're dropped). They also spread by bulb division and sometimes bulbils (growing on the bottom row of flower head) World distribution: N. Africa to W. Asia (Iran). It is also naturalised in N. America (New York). It grows throughout Europe except for the most Northern regions (Ireland, Netherlands, Scandinavia, Baltic States). World distribution: N. Africa to W. Asia (Iran). It is also naturalised in N. America (New York). It grows throughout Europe except for the most Norther region (Ireland, Netherlands, Scandinavia, Baltic States). 4 subspecies are recognised. Most individuals (Including ours) are ssp. sphaerocephalon. White-flowered plants from Mediterranean islands and the Balkans with smooth flowers are ssp. arvense, while those from Greece and south-west Turkey with papillose flowers (with a roughened surface) are ssp. trachypus and plants restricted to Sicily are ssp. laxiflora. Bulbil-bearing forms belong to subsp. sphaerocephalon, and are sometimes referred to as var. bulbilliferum.

J.W.White:

On some ledges of St. Vincent's Rock's, in small quantity. More plentiful on Durdham Down, nearly a mile from the first station. First found by Dr. H. O. Stephens, as recorded by him in Phytol. II, p. 929, "On steep declivities of the cliffs, St. Vincent's Rocks; 31st July, 1847:" The date on which he forwarded specimens for exhibition to the Botanical Society of London. Dr. Swete's account (Fl. Brist. p. 78), that the species was discovered by Dr. Stephens "on the most inaccessible part of the Great Quarry, below St. Vincent's Rocks," was evidently a misapprehension. When I reached the place for the first time in 1882 there were about 20 specimens, and nearly that number on subsequent visits. In 1910 a plucky lady of my acquaintance climbed the Rocks without shoes and reported the plant to be still in excellent order in two spots. Theloyalty is not in the least danger from quarrying, as was erroneously stated to Dr. Syme (Engl. Bot. ed. 3, pub. 1865), but it suffers occasionally from the pranks of scrambling boys who, in attempting to gather the flowering heads, pull up the root and all from the thin, loose soil. A friend, walking along the riverside road some time ago, picked up four fine plants with their bulbs that had been thrown down from above. We deplore this kind of mischief, of course, but know not how to deal with it. A sympathetic correspondant of The Spectator (July, 1907) suggested that this gem of the Bristol flora might be protected by railings. Although the writer showed himself to be imperfectly acquainted with the facts, his idea was extremely good; and, as regards the Durdham Down localities, could be easily carried out without detriment to the public.

On Durdham Down A. sphaerocephalum occurs in larger number, but the plants are not so fine. Two colonies exist, a short distance apart, and at each one would formerly count at least forty heads. But whilst one of these shows no sign of trouble, the other has been more than decimated during the last decade, not by collectors I believe, so much as by children gathering "bunches" of wild flowers. One girl was seen to pull all the heads she could find at the place; unfortunately, the stems being tough the bulbs came along with them. The original vegetation of our Downs reasserts itself with surprising vigour within the small enclosures that have already been wired off around some ineffectual plantations, and it cannot be doughted that should the Round-headed Garlic be accorded a similar protection it would soon make good its losses. These are the only localities for the species in Great Britain. It is found sparingly in the island of Jersey.

My notes:

There is not much information available about the life story of this plant so I decided to try and find it out for myself. I also decided to grow A. shaerocephalon in my garden (although rare in wild it is commonly grown in gardens and often sold under the name Drumstick).

What pollinates it ?

• In the wild: Red-Tailed Bumblebee, Common Yellow-faced Bee.
• In my garden: Honey Bee, Red-tailed Bumblebee, Buff-tailed Bumblebee, Marmalade Hoverfly

I would guess that other short-mouthed bees and other insects probably visit as well but I'll have to wait and see.

Dispersal method:

Seeds are heavy so can't travel far from the parent plant. This is what leads to the formation of colonies along with bulb division; but how do they disperse further afield to form new populations? Certainly some will get blown around a bit by the wind or the rain might wash them along and of course anything that steps on them will achieve dispersal, however I've noticed a lot of Ants around on the limestone rocks throughout the Gorge. I believe that this species (Dusky Ant, Formica fusca) is the main 'spreader' of the Bristol Onion. Members of the Amaryllidaceae family all have heavy seeds. Examples such as Three-cornered Garlic (Allium triquetrum) are well known for being dispersed by Ants. Although I am certain Ants are the dispersers I am yet to see it in action.

This Ant doesn't just have an important relationship with Bristol Onion but to the Avon Gorge ecology as a whole. I would go as far as saying that this Ant is the most important species in the Avon Gorge! It's role in habitat creation, physically landscaping and helping to create the important soil composition for all of the Gorge's flora, through making nests; provides a perfect well-drained seed mix for germination and the formation of islands of vegetation, among the exposed rock faces. They clean up and collect dead matter controlling the balance and spread of pests and diseases, They are pollinators for so many plants and finally they are a vital food source, especially for birds such as Green Woodpecker and in the Summer; Gulls when whole colonies upon millions take to the skies to breed. This key stone species is vital to the health of the Avon Gorge.

They also spread by bulb division and sometimes bulbils / bulbules / bulbits (sometimes growing on the bottom row of flower head)

Threats:

Trampling (especially in Goat Gully where it is not safe from the public), encroaching scrub, habitat loss, climate change (generally warmer wetter winters and hotter, drier summers) although this onion is pretty hardy, heavy downpours of rain in winter when there is less vegetation cover could wash away the thin dust-like soil overlaying the steep rock faces in which it grows; along with the bulbs themselves. The problem is made even worse in the event of trampling, meaning there is no support from the surrounding vegetation to hold the soil together and protect the bulbs from being washed away. I have also observed Magpies searching the area for food and found a bulb that had been pulled up, possibly by birds (I of course re-planted it). Gardeners are often in conflict with birds for pulling up Onion and Shallot sets. So it is likely that with climate change bringing generally warmer wetter conditions locally and increased visitor numbers will see this plant in further decline.

What can be done:

Goat Gully's population is most at risk. Currently there is a fence to keep people out but people just go through a gap that has been created. The area is a main spot for ignorant gangs of teenagers and party going, social gathering young adults who often jump the fence. Maybe the entrance to the gap should be blocked (I don't think this would do much though, someone will just come along with wire cutters or jump the fence). A temporary fence could potentially be erected around the immediate site during the growing season and then removed in the autumn. More signs could be put up but I fear this would encourage more plant pickers! But I think the best option would be for there to be a guard watching over during the flowering period; just like what they have for the Lady's Slipper Orchid at the site of the last remaining wild plant. Although the Bristol Onion is not quite as rare. It should still be treated with just as much protection for the fact that the Avon Gorge houses the only population native to Britain!

St Vincent's Rocks population is not threatened by the same risks as Goat Gully's however invasive Non-natives and the effects of climate change will test it.

Population size:

Goat Gully: The population here is limited to two sites, one (site A) is on the grassy ledge behind the fence and the other (site B) is on an island held precariously attached to a steeply angled exposed rock face not far from site A. With all the trampling and heavy downpours it is slowly eroding away and is literally sliding off.

2021 : about 149 individual plants made up of 84 full flowers (70 at site A, 14 at site B), 7 picked flowers, 6 trampled flowers, 46 with bulbits and 6 bulbs (one was a double so you could say 7).

2022 : about 44 individuals (Just 1 at site A - a massive decline, 43 at site B - quite an increase) 1 (at site B) was found pulled out and lying on the surface.

St Vincent's Rocks:

Don't know yet but I imagine that they are more numerous and healthier than in the Goat Gully.

Useful Links:

Kew

Herbaria United - the original specimen