Symphoricarpos albus (L.) S.F. Blake, Snowberry

Account Summary

Growth form and preferred habitats

S. albus was originally planted in gardens, parks and demesnes as a not exactly choice ornamental, but useful for hedging or landscaping ground cover or, more usually, it was planted in woodlands as coverts for game, being especially useful on shooting estates for rearing pheasants. This strongly suckering, thicket-forming, deciduous shrub, native of NW America has become widely naturalised and established in B & I since its first introduction in 1817. The plant introduced to B & I and to Europe is native of the W side of N America and is var. laevigatus (Fern.) S.F. Blake. Its suckering habit, generated as arching, branching, aerial shoots arising from underground, woody, rhizome branches, enables the shrub to form very dense, continuous thickets that can be up to 3 m high, but more usually reach heights of 1.5 to 2 m (Gilbert 1995; Stace & Crawley 2015). Older branches have shredding brown bark, while twigs are slender and brown; young shoots are yellowish-brown in colour and are glabrous.

Leaves are opposite, 2-9 cm, glabrous, mostly simple, elliptic or ovate, obtuse at apex, but conspicuously lobed on strong sterile shoots and on suckers (Gwynn Ellis 1993; Sell & Murrell 2006). Bud burst takes place in early March and the leaf canopy in the English Midlands last for about 190 days. This makes the interior of thickets very deeply shaded and prevents successful flowering. No autumn leaf colouring takes place (Gilbert 1995).

When being used for ground cover for pheasant rearing, Snowberry bushes need to be cut back periodically (ideally every three years), to prevent the growth becoming too thick, so that the birds can more easily be flushed from cover (Anonymous 1981a; Gilbert 1995).

Snowberry tolerates a wide range of soil types but avoids the most acid or the permanently wet, so that it never occurs on deep peat bogland, exposed uplands, or in wet fens. S. albus tolerates both sun and shade and, in open or half-light, the thicket excludes all but a few mosses. In deeper shade, it grows less dense, so that an under-canopy flora of early leafing and flowering woodland herbs manages to persist under it. Alternatively, shade-tolerant evergreen species that also possess vigorous means of vegetative spread may manage to form a ground layer under the shrub (Gilbert 1995).

While the species is gregarious and generally forms a dense stand by self-coppicing, lateral increase and spread by suckering (stems with many adventitious roots) is a relatively slow process, even when carried out in full sun. Gardeners report very little spread of existing patches, even after 50 years of observation (Gilbert 1995). Spread is extremely slow in woodland shade, since S. albus is not all that aggressively competitive with existing vegetation. Thus rapid spread is confined to well-lit, relatively open habitats, such as hedgerows, scrub, riverbanks, railway embankments, derelict sites and waste ground. The current author (RSF) has not been able to locate sample measurements for the rate of Snowberry spread anywhere in B & I.

The suckering and self-coppicing that S. albus habitually carries out makes it unlikely that any planted individual in B & I has died of old age. The wood is ring porous, so the age of stems can be determined: the oldest stem examined in a survey had just 34 growth rings (Gilbert 1995).

Flowering reproduction

S. albus flowers reliably from June to late September. The pink, bell-shaped flowers occur in small clusters and are not very conspicuous, but they do produce copious nectar and the long flowering period means they can attract many visits from short- to medium-tongued insects, including hoverflies, bumble-bees and honey bees, plus wasps amongst other pollen vectors (Gilbert 1995). In the absence of pollinating insects, automatic self-pollination is unlikely, due to the pendulous nature of the flowers and the relative positions of stamens and stigma within them (Gilbert 1995).

Snowberry produces its large, 8-15 mm diameter, pure white, fleshy, berry-like fruits (actually they are 3-layered drupes containing a stone-covered seed) abundantly, making Snowberry a moderately popular decorative garden subject. The large white fruit 'berries' are far more conspicuous to the human eye than the small pink flowers. The large white drupes have a spongy texture and are full of air, making them popular with children as they explode with a loud pop when stamped on or squeezed hard. Only deep shade suppresses fruiting (Gilbert 1995).

Viable seed is set, mature plants producing it abundantly each year, but unlike in Continental Europe, in B & I there does not appear to be any effective method of seed dispersal (Gilbert 1995; Stace & Crawley 2015). The conspicuous white fruits seem strangely unattractive to British birds and so they remain on the bushes until January, when they turn brown, rot and drop to the ground. Instances of birds consuming Snowberries in Britain are sufficiently rare to be reported in the ornithological literature. For instance, single examples of Greenfinches at Bristol pulling at the fruit to expose and then eating the seeds, and of a single occasion when Blackbirds were observed consuming over-ripe Snowberries that had fallen to the ground were reported by Radford (1980). In Germany, however, Snowberries are considered an important winter food for Greenfinches, and S. albus is increasing by seed in some towns there as a result (Eber 1954). Why the spread by birds should operate in Germany, but not in B & I, leads to interesting speculation in the absence of real concrete evidence. Birds in B & I do not meet native species with edible white fruits and thus may avoid them, but German Greenfinches do not seem to have any qualms as to their suitability as food. It may be necessary, or even essential, for the seed to pass through the bird's gut for it to be capable of germination (Stace & Crawley 2015).

In addition, seeds of Snowberry are difficult to germinate and Gilbert (1995) reported that all attempts to do so with UK collected samples have failed. An appeal to members of the BSBI for records of seedlings confirmed that they are almost, but not absolutely, unknown in these isles. Reports now exist of very, very occasional plants growing on wall tops, or atop a pollarded willow, or in gardens, where they were presumably bird-transported and self-sown (Stace & Crawley 2015). One recent report was of a small but fruiting specimen, growing, of all places, out of an old stone wall on Ammanford High Street, Carmarthenshire, in S Wales (VC 44) (Lewis 2000).

It appears from the general lack of seedlings around the country that almost all stands of S. albus in B & I have arisen either as a result of deliberate planting, or from transported vegetative fragments of its specialised underground stem. The commonly noted close association with sites of habitation supports this view (Gilbert 1995).

Fermanagh occurrence

The first record for Fermanagh dates from around 1949, but as with other garden escapes, weeds and aliens, earlier recorders appear to have simply ignored Snowberry's introduction and spread and the current author (RSF) & RHN can find no mention of it in any of the published county data. While it is impossible to trace the local history of the spread of the species, it is now very common and widespread in Fermanagh, having been recorded in 327 tetrads, 61.9% of the VC total. Indeed, it is possibly the most widespread of all the introduced species found in the county. It is frequent and widespread everywhere in Fermanagh, except in wetlands, on exposed uplands or on deep peat.

In Fermanagh, however, S. albus has colonised somewhat more widely than this, since it is found on cliffs at Knockmore, Bolusty More and Hanging Rock NR, and also along many riverbanks and in several quarries. The latter may perhaps be the result of the common and deplorable habit of fly tipping garden rubbish anywhere off the originator's property. The other sites mentioned are almost certainly the result of bird-sown seedlings and an eye should be kept out for any further evidence of seed fertility, seedling establishment and bird transport.

British and Irish occurrence

S. albus is very common, widespread and naturalised throughout lowland B & I. It reaches its highest altitude (385 m) at Forest-in-Teesdale, Co Durham (VC 66) (G.T.D. Wilmore, in: Preston et al. 2002). As almost all stands of S. albus in B & I have arisen from plantings or have established from vegetative fragments, it is generally found close to habitation and usually within the enclosed landscape. It is therefore probable that that few plantings have been made close to the climatic limit of this species, and its apparent restriction to land below c 400 m is likely imposed by cultural rather than by climatic factors. No topographic limitations of slope or aspect have been observed and the plant grows vigorously in a wide range of open habitats and thrives in full sun and survives in deep woodland shade (Gilbert 1995).

It is quite surprising that an introduced shrub that very rarely reproduces successfully by seed in B & I, and that is not known to be accidentally dispersed by human soil-moving activities in the manner of Japanese Knotweed (Reynoutria japonica), should become the third most common neophyte in the flora of this region, recorded in over 70% of hectads in B & I (New Atlas; Stace & Crawley 2015).

European and world occurrence

S. albus is an endemic species in N America, where there are two geographical races or varieties, the Pacific var. laevigatus and the Atlantic race, var. albus. The horticultural superior var. laevigatus is the form that has been introduced across Europe and elsewhere, including SW Australia and New Zealand. It has also been widely planted across N America (Gilbert 1995).

Toxicity

The berries contain several toxic alkaloids and although grazing animals are unaffected, in humans eating one to four rarely causes symptoms, but more than this will cause vomiting, diarrhoea and dizziness, and in larger quantities delirium and unconsciousness (Cooper & Johnson 1998). There have been very few recent incidents of such poisoning.

Herbivory and control

A number of insect herbivores appear to have transferred to S. albus from native Honeysuckle (Lonicera periclymenum) and other members of the Caprifoliaceae, including several species of leaf miners. Similarly, fungal pathogens that attack the plant family can cause stem die-back, leaf blotch and wilting (Gilbert 1995).

Control of Snowberry coverts requires coppicing every three years but with the reduction of manpower on shooting estates, this is rarely practised nowadays. Foresters spray the shrub with glyphosate to cut it back whenever it threatens their tree plantations.

Names

The genus name 'Symphoricarpos' is from the Greek 'symphoreo' meaning 'to accumulate' or 'to bear together' and 'karpos', 'fruit', and when combined gives us, 'clustered berries', referring to the arrangement of the fruits which are clustered to resemble a compound structure (Johnson & Smith 1946; Gilbert-Carter 1964; Gilbert 1995). The Latin specific epithet 'albus' means 'white' and refers to the white drupe fruits (Gilbert-Carter 1964).

Threats

None.

References

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