Plum and Sloe

Calling all vc36 field botanists! Please help improve our understanding of these familiar but difficult plants by taking a close look at those that grow in your area!

Difficult (some would say impossible) because there is a combination of species hybridisation and the involvement of cultivars.

The BSBI which informally governs flora recording works to Clive Stace’s Flora1. An alternative, more extensive treatment of the group is by Peter Sell2. This complements rather than contradicts Stace, with one exception, discussed more below. Sell’s extra detail may be necessary if we are to do this group justice in Herefordshire.

In our county, a fruit-growing region, there is an important question to be answered, which is the degree of introgression of the (cultivated) plum into the (wild) sloe. We can’t answer it until we all get better at examining and understanding these shrubs. The make-up of the plums themselves is less important, but still interesting. Amongst their multitude are there untold stories that might tell us more about our social history?

One thing going for Prunus is that it needs to be inspected at the very start of the field season, when there’s not much else to divert the attention, and again at the end of it, when the opportunity to really look closely at fruit characters can be dressed up as a foray for sloe gin.

Below I itemise the taxa one by one, starting with what I see as the easiest and ending with the hardest. Bear in mind, however these are all inter-fertile and admixed in large amounts in the Herefordshire countryside, so it will not be uncommon to find puzzling plants. Trying to put a name to shrubs in regularly-trimmed hedges is often not possible as typical diagnostic growth forms and leading shoots will be missing, but often in hedges there are one or two helpful bushes or trees which ‘get away’.

Measurements in a key are off-putting, but important. It is much easier to appreciate the difference between small (c13-14mm), medium (c17mm) and large flowers (c22mm) in life, and the same is true for fruit. These size differences are important for identification.

Cherry Plum Prunus cerasifera

Initial recognition: This is the first Prunus to flower by 2 to 4 weeks, now flowering in vc36 as early as late February, though see the modern caveat at the end of this account. You can spot it from a vehicle, one of your first forays into field botany of the year. Indeed, as it’s rather a local plant in Herefordshire, spotting it in the course of road journeys can be an efficient means of survey. Mentally note the plants you see for revisiting to check.

DO NOT SUBMIT RECORDS for plants in garden hedges whether in town or country (though these are sometimes useful for brushing up identification skills). The difficulty Cherry Plum has in naturalising – especially in the West of Britain – is part of its story, and so the Herefordshire record should not be muddied by the inclusion of plantings made in a garden. Most plants seen from a passing car in vc36 fall into this category.

DO SUBMIT RECORDS for plants in agricultural hedges (because they potentially illustrate trends in the past management of the countryside), and for plantings en masse in public places with a semi-natural character (because they may influence the ‘wild’ vegetation, and because such places may offer evidence of naturalisation). This is a plant beloved of landscape architects as well as gardeners on account of its very early blossom, and is indeed occasionally planted en masse in wild-like public places, as at Aylestone Park at Hereford. They also frequently bung it in along the cuttings of major trunk-roads and motorways, which now makes it locally frequent in some parts of Britain, though not in Herefordshire where we do not have many roads of this type. Watching out for these places elsewhere can give you a feel for the plant, though. A few years ago I was travelling along the M23 in Surrey exactly at the right time of year to see the tall canopies of thin cherry-plum blossom just passing their best alongside the denser blossom on low, thicket-forming shrubs just starting to appear.

Identification tips: Usually with a discernible trunk, and scarcely suckering. Spine-free, with last year’s shoots a bright, glossy green all over (may be scabrous but not hairy), and long-pedicelled flowers with leaves appearing at the same time. Where hedges are trimmed and the green shoots removed the plant can be very hard to find.

The undersides of the leaves have quite a distinctive pubescence: densely hairy either side of the vein, but only as far as the mid-point or less. Poland & Clement3 also give an ostensibly useful character based on number of leaf teeth, but I have not got this to work in my local material.

Has distinctive red, glossy fruits, but these seldom form. Perhaps this is because the tree is self-incompatible, but probably also because the species is far from home in the UK, especially in vc36.

Cherry Plum (L) has dense hairs flanking the lower half of the vein, often visibly bushier at the petiole end. Plum (R) variably hairy but not in this distinctive pattern.

Beware: Potentially confused with plum (to which it is originally one parent), some plants of which also have the odd green leading shoot, no doubt because of the residual Cherry Plum genome. Check leaf undersides for the distinctive pubescence; plum may be hairy (or not), but not in this distinctive pattern. Or a late visit may enable plants to be named where fruits form.

Much nursery stock has come in from Europe over the last thirty years and it seems likely that with it has come some exceptionally early-flowering Prunus spinosa / x fruticans. These plants now vie with Cherry Plum for the earliest flowering slot. Often they are found along roadside plantings (suggesting a large volume of stock had been required by the landscapers) and contain tell-tale tree tubes at their base. They are easily told from Cherry Plum close up, but not necessarily from a speeding car.

Plum Prunus domestica ssp. domestica

Initial recognition: Size. All authorities agree this is a tree (ie with a discernible trunk). To boot it has large flowers, leaves and fruit (see fruit chart image at foot of page).

DO NOT SUBMIT RECORDS for plants in garden hedges whether in town or country, or for orchards.

DO SUBMIT RECORDS for plants in agricultural hedges or for old and interesting survivors. If you like fruit and are able, add a cultivar name (‘cv’).

Identification tips: The large size and parts should be accompanied by sparsely hairy glabrescent twigs in contrast to ssp insititia which has hairy twigs.

(Top) year old twigs of plum Prunus domestica ssp domestica. (Bottom) year old twigs of damson Prunus domestica ssp insititia. Plum may be sparsely hairy. The two trees from which these were taken look very similar.

Beware. We are lulled into a false sense of security by the three commonly recognised subspecies of plum viz. domestica, insititia and italica, when the reality is a fruit-bowl of taxa where appearance may be no guide to relatedness. Italica (the greengage) is likely to be rare or nonexistent as a naturalised tree – even the Flora of Worcester only came up with one specimen, and we can thus probably disregard it. Insitia is problematic though in that some forms, notably the Shropshire Prune Damson, may approach plums in size.

Blackthorn Prunus spinosa

Initial recognition: We can all recognise Blackthorn, but I’m asking botanists across Herefordshire to take a closer look at this familiar suckering shrub. It is perhaps worth remembering that this is the only truly wild plum of Western Europe, and yet we have very little wild-type habitat left in vc36, places like unenclosed grassland, ‘soft’ woodland edge, and cliffs. How is it faring in our heavily-managed landscape, given the propensity of the plant to hybridise?

Identification tips: Good material is relatively short, with abundant blossom appearing before the leaves. It may straggle upwards where there is competition but the given value of 4m seems a good cut-off and vigorous suckering stands in open county more commonly only attain around 3m. Good material has robust but relatively short spines (< c 8cm in my estimation) on year-old shoots which are sharp and painful to even a light touch. Abundance of flowers varies but is often profuse, and often especially on the laterals. Good material has flowers of small diameter (c.13mm), all (or almost all) of them with five petals, with sloes that are spherical and on shorter, rigid stalks (type ‘A’ in the fruit chart at the bottom of the page).

If your nearest ‘blackthorn’ looks a bit strange, try benchmarking plants in more remote places, such as the bigger commons, edges of large woods, or steep slopes further from habitation, where its genetic integrity is more likely to have been maintained.

Beware Prunus x fruticans which can be very similar…

The hybrid between plum and blackthorn, Prunus x fruticans Weihe

Initial recognition: As for blackthorn, but pay close attention to plants in and around villages (much of Herefordshire!), especially old settlements with small field patterns and commons and village greens nearby, where there is a strong possibility of hybridisation and introgression. The hedges further away from such settlements may also contain the plant, having been bird-sown there. Birds like to perch in hedges.

Also take a close look at new landscape plantings! Prunus x fruticans appears to have been put in in clumps along the A4103 (Roman Road) north of Hereford, for example.

Identification tips: In winter, suspect spiny bushes which have unusual forms of spininess, such as extra long spines (10-15cm or more) on last year’s leaders, or where spines are weak-tipped and grade into short laterals without spiny tips at all (see photos below). Some plants seem to have comal tufts on their spiny leaders (though might this be related to damage, rather than parentage?). At blossom time, check such plants for larger flowers (perhaps up to 17mm or more) with a proportion of six, seven or eight-petalled forms (which may be profuse on the laterals like spinosa or may be quite sparse on the main shoot axis like the domestica parent). In autumn go back again to see if these plants have correspondingly larger, ovoid fruit on longer, arching stalks (type ‘B’ in the fruit chart at the bottom of the page).

Spines in Prunus spinosa are formed from short lateral shoots the tips of which are decisively modified into points sharp enough to cause pain with only gentle pressure. It is interesting to speculate how these form – perhaps by selective cell death or reduced cell expansion coupled with great strengthening of the epidermis. This spine formation does not occur in the equivalent position on first year shoots of domestica where branching is unspecialised (see Prunus domestica ssp insititia below). Not uncommonly in bushes that may belong to Prunus x fruticans, therefore, there is a halfway house, with longer-than-normal laterals that do not atrophy so effectively, making the spines weaker, and having forms which intergrade between spine and a ‘normal’ short branch.

For a short period in spring just after the flower buds have broken but before many of the flowers are fully open it is also sometimes possible to find more than one flower emerging per bud (a domestica character) on bushes which have general spinosa-like appearance (see below). However, these bud scales rapidly fall and it is no longer possible to tell two flowers emerging from one bud from two singletons from adjacent buds which have shed their scales.

Odd spininess: long spines in tandem with un-spined short laterals
Odd spininess: long spines, some rather weak, with a tuft of normal shoots at the apex
Odd spininess: long, strong spines on abundant suckering shoots combined with large flowers characterise a distinctive hybrid at Vowchurch
This part-trimmed hedgerow Prunus caught the eye by virtue of exceptionally long spines (not especially visible in this photo). On examination several of its flowers were emerging two to a bud, suggesting some domestica genome.
(L) detail of a shoot from the plant above showing two pairs of flowers each emerging from one bud. In a few days’ time the bud scales will have fallen, making it very difficult to appreciate this, as the two flowers might as easily have emerged from a pair of adjacent buds. (R) Pairs of flowers from a single bud are more clearly seen in this larger shoot of Prunus domestica ssp domestica.

That the domestica – spinosa cross continues to take place is nicely illustrated by an extraordinary plant at Vowchurch, where a very spiny parent has crossed with a robust taxon of Prunus, presumably either plum or Shropshire Prune Damson (see below), to yield an exceptionally thorny and heavily-suckering large-flowered taxon unlike any other I have seen (photo above). Such a plant falls into Prunus x fruticans Weihe, but interestingly would not equate to Sell’s conception of this taxon (Prunus domestica ssp insititia var fruticans Weihe).

I have not found stone shape in Prunus to be all that helpful. Plum does indeed have a flattened stone which parts relatively easily from the flesh, and sloe does indeed have an almost globose stone which does not. But telling those two apart is not a problem. I can’t see that stone shape is reliable help with any of the other taxa, though.

Beware The easy and widespread pitfall of mistaking it for Prunus domestica ssp insititia, as many have probably done in the past. The problem is that almost no Floras or field guides (including Stace) include x fruticans in their keys, so that any shrub that doesn’t quite seem to fit spinosa on account of size and patchy spine development will come out as that plant.

Damson and Bullace Prunus domestica ssp insititia

Initial recognition: This is the most variable taxon and consequently there is no overall ‘jizz’ character. In theory they are all shrubs with a main trunk or small trees with no or modest spininess (spines usually restricted to low down on the main trunk), with distinctly hairy shoots.

Identification tips: You cannot really separate this taxon from the last without spring and autumn visits to the bush. This is a pain, I know, but console yourself with the thought that with a little patience there may be an interesting botanical discovery to be made.

It is possible that, as in Worcestershire, most of the representation of this taxon in vc36 is in the form of the Shropshire Prune Damson (Type ‘D’ in the fruit chart below). This has fruit so large that a casual observer might think it a cultivated plum, and flowers of the same, large size. One of my most exciting botanical finds of 2022 was however the true (black) Bullace in a hedgerow beside the A438 west of Letton (see pic below). This is Prunus domestica ssp insititia var nigra Asch. & Graebn. I am sure there are many botanists out there who understand this plant (you may be one of them), but in nearly 40 years of field botany it has eluded me. Maybe I’ve simply never worked where it has grown, though it is interesting that Purchas & Ley considered it very rare in the 1880s, and in the unlikely event that there has been massed planting of it in the meantime, that can only be very locally. In the spring my A438 plant presented as a small tree with a trunk (albeit collapsed), with large flowers with a low % of multi-petalled flowers. Going back in November, the boughs were hung with the remains of clearly spherical fruit, distinctly bigger than a sloe, about the size of a cherry. I would be really grateful to have further news of plants outside of gardens and orchards with spherical fruits like these in order to work out how common or rare the bullace really is in Herefordshire.

The trickiest call? Bullace (L), Prunus domestica ssp insititia var nigra. Although the English name endures, and there are no shortage of records, this may be a very uncommon tree in VC36. (R) is Prunus x fruticans, correspondingly under-recorded. Note fruit (larger and spherical in Bullace (cherry-sized), smaller and distinctly ovoid in x fruticans); absence of spines on leading shoots of Bullace, but prominent spines on x fruticans (although these look a little odd for spinosa itself, which should also have globose fruit).
Prunus fruits from Vowchurch Common. (A) small, spherical sloes contribute to the evidence for a few good Prunus spinosa bushes on relict patches of common and beside a disused railway; (B) the vast majority of local shrubby Prunus however produce fruit of this type, larger, ovoid on a longer, often arching stalk; this is thought most likely to be Prunus x fruticans; (C) larger still, olive-sized fruits occur on some small trees bordering smallholders’ fields, including one row of 17m; clearly planted, they seem most likely to be damsons of a cultivar unknown to me given given the fact that the (Black) Bullace Prunus domestica ssp insititia var nigra is spherical; (D) is the Shropshire Prune Damson Prunus domestica ssp insititia var damascena with ovoid fruit tapering to the stalk often with a vivid blue bloom; (E) is a plum Prunus domestica ssp domestica, cultivar unknown, but which moves from smallholder’s field to hedge and vice versa by occasional suckering.

1 Stace, Clive (2019) New Flora of the British Isles (Fourth Edition) C & M Floristics

2 A readable and accessible account predating Peter Sell’s detailed but expensive flora is Sell, Peter, The Cherries and Plums of Cambridgeshire (1991) in Nature in Cambridgeshire 33 pp 29-39

3 Poland, J & Clement, E (2009) The Vegetative Key to the British Flora BSBI

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