Tree of the month: Exploring cherry plum and blackthorn

THE Prunus genus is a collection of some 200 trees and shrubs overwhelmingly native to the Northern Hemisphere and commonly characterised by the fruit borne, including cherries, plums, peaches, apricots and almonds. In a truly wild state, across the entirety of the British Isles, the Prunus presence comes down to blackthorn (Prunus spinosa) and gean or wild cherry (Prunus avium). Alongside these are bird cherry (Prunus padus) and cherry plum (Prunus cerasifera).

READ MORE: Exploring the month of April and the behaviour of its trees

Bird cherry is native to northern England, Scotland and parts of Wales, while cherry plum is an alien aberration, now naturalised. Over the centuries, cherry plum has earned its position as a wild tree in southern England, although it is increasingly rare the further north and west you go.

A CONTINUING CASE OF CONFUSION

Blackthorn blooming against a blue April sky and not a leaf in sight. Blackthorn leaves will not start to appear until flowering has finished. (Image: FJ)

While risking the wrath of the native tree purists, I will begin with an exotic Prunus species, because as the very first to flower and refoliate in late winter, cherry plum provides a convenient point at which to start. Cherry plum was introduced into the British Isles as a garden plant some 300 years ago, most likely brought from Persia. 

As the name suggests, cherry plum has caused confusion due to close similarities with native blackthorn, apart from separate discussions around whether it is a plum or a cherry. Close similarities between blackthorn and cherry plum include shrub-like to small-tree canopies and the structure of the white, five-petal flowers. At first sight, individual flowers of the blackthorn look measurably smaller than those of the cherry plum.

However, measurement shows no difference in the length of the individual petals, although petals on cherry plum flowers are broader than those on blackthorn.

The flowering period is close, but not overlapping, with early March for cherry plum and April for blackthorn (that’s for the southern half of England).

Such is the continuing confusion between cherry plum and blackthorn during the late winter/early spring period that otherwise respectable books on British wild trees and flowering shrubs frequently fail to mention Prunus cerasifera. Northern-based authors are excused because they may well never have encountered Prunus cerasifera as a truly wild tree in the wider environment, but not authors in the south, where naturalised white-flowering cherry plum is as frequent as native blackthorn in urban, suburban and peri-urban areas.

DIFFERENCES BETWEEN CHERRY PLUM AND BLACKTHORN

Fruits of the gean are true cherries though not offering much in the way of sustenance or sweetness for human consumption, but highly sought after by soft-fruit-eating wild birds and small mammals. (Image: FJ)

For most of the year, blackthorn and cherry plum are nondescript shrubs or small trees lost in the general scrub colonising railway embankments and other unmanaged land, especially close to human settlements. However, as winter lingers on through late February into early March, cherry plum is the only tree lighting up the landscape, especially the built environment, with bountiful white blossoms. As early as February, cherry plum is blooming when all else still slumbers. When cherry plum flowers fade away and the petals fall to the ground, the blackthorn comes into bloom with the advent of April. 

At the end of the day, distinguishing blackthorn and cherry plum hinges on the synchrony of flowering and refoliation. Pure-line cherry plum flowers and refoliates at the same time, but not so the pure-line blackthorn, which flowers before the first green shoots show. But even this difference does not always help due to hybridisation, which produces a whole range of intermediate types in flower shape and structure as well as synchrony of flowering with refoliation. 

Despite the high degree of confusion surrounding the flowers of native blackthorn and exotic cherry plum, there is absolutely no danger of confusing the fruits. Cherry plum fruits are as big as domestic cherries (2–3 cm diameter), which they resemble in shape and form, whereas sloes are miniature plums (1 cm diameter) complete with the groove down one side – a classic character of true plums. 

The presence of a thick layer of surface wax on the sloe appears to change the bandwidth of light spectrum reflected, with observers perceiving a bright blue fruit.

However, when you rub off the wax, the skin beneath appears dark blue bordering on black. The colour of the cherry plum depends on biotype, with wild, white-flowering trees carrying yellow/orange coloured fruit and the pink-flowered red-leaved trees bearing fruits which are red to purple. As a very early flowerer, cherry plums are ripe in June, whereas sloes are not fully mature until August/September.

DIFFERENT DATES FOR THE FRUITS

Fruits of Prunus cerasifera are called cherry plums. Though the same size and shape as a domestic cherry, they are botanically much closer to plums. (Image: FJ)

Come autumn, true connoisseurs of home-made gin are getting ready to seek out the sloe, although the small, blue, miniature plums may well be mature by September but unfit for fermentation into sloe gin until softened up (bletted) by a substantial autumn frost.

However, a completely different story unfolds for fruits of the cherry plum tree, which are traditionally ignored as edible fresh fruit or for use in British cuisine and winemaking, even though they are substantial in size and sweet when ripe (unlike the astringent sloes of the blackthorn). Come June, trees are laden with cherry plums that nobody seems to want (except for blackbirds and starlings), but only if conditions prevailing in March were good for pollination. As an extremely early flowerer, cherry plum blossom is at high risk from wind, rain, low temperature and frost. Cherry plum bears fruit from the third year onwards, attaining normal yield after 10 years and maintaining production for a further 40 years or more.

Disinterest in cherry plum fruits could be about to change on a significant scale, especially in the metropolitan areas of southern England, where trees are common and towns and cities are recently enriched by people and communities with ancestral roots in south-east Europe and western Asia. In such regions cherry plums are used in their immature condition to flavour meat dishes and to make condiments. Ripe fruit is used for making tarts, pies, jams and wines.

The cherry plum has travelled much farther afield than Britain from its south-east European/western Asian homeland. I have seen the trees flowering across the Adelaide Hills in South Australia where the tree yields more consistently and heavily than it does in the UK.

CHERRY PLUM IN THE URBAN ENVIRONMENT

Sloes, fruit of the blackthorn, have a waxy covering which imparts a bright blue colour but when rubbed off reveals a very dark blue fruit. (Image: FJ)

Cherry plum is closely associated with human settlement, but why is this so? The tree was brought to Britain for its ornamental attributes rather than its timber traits, and has subsequently invaded the wider environment as a gradual garden escape over the centuries. 

However, despite having been in Britain for a long period of time, cherry plum remains a tree of the wayside rather than the woodland. I have never found cherry plum in well-established woodland, so in the same way that blackthorn is rapidly shaded out and relegated to the margins as the woodland canopy closes, perhaps cherry plum cannot cope with low light conditions.

Fruit-eating garden birds are central to the long-term dispersal of the cherry plum. I have a cherry plum tree in my garden which appeared from nowhere three years ago and has flowered for the first time this year. I suspect the seed came from the pink-flowered, red-leaved cultivar, Prunus cerasifera nigra, as a mature tree growing in my neighbour’s garden. However, by not breeding true I now have white-flowered tree bearing leaves slightly tinged with red, clearly on the way back to a wild type but not quite there. 

The pink-flowered, red-leaved varieties are traditionally planted as grafted trees using a true, white-flowered wild type rootstock. Being much more vigorous, the white-flowering rootstock sends up fast-growing suckers which can overwhelm the tree in just a few years – another way in which the wild, white-flowered cherry plum can establish and spread in the wider environment. 

PRUNUS CERASIFERA – A CHERRY OR A PLUM TREE?

The million-dollar question: is Prunus cerasifera a cherry or a plum?  Leaf shape and structure and flower form and floral formula point towards a plum, but the fruit closely resembles a cherry in both size and shape. The internet is littered with off-the-cuff remarks that cherry plum is a hybrid of a cherry and a plum but this is only true when the word hybrid is used in the generic (general) context and not the genetic one. 

Blackthorn and cherry plum blossom are difficult to distinguish from a distance but the urban/suburban setting and the date (March 2009) tells you the trees are Prunus cerasifera (cherry plum). (Image: FJ)

Waters are muddied even more by the plum-cherry, a genetic hybrid of Chinese or Japanese plum (Prunus salicina) and the domesticated sweet cherry (Prunus avium), of which gean (wild cherry) is the British wild native equivalent. Fortunately, there is one single and salient fact that cuts through all the clouds surrounding the genetic status of Prunus cerasifera. The introduced species has freely hybridised with British native blackthorn (Punus spinosa) but not with British native Prunus avium. The verdict therefore must be that the fruits of Prunus cerasifera may well look like authentic cherries but are genetically closer to proper plums. 

CHERRIES – THE REAL DEAL

The word avium, which is the species name of the gean or wild cherry, conjures up a close association with birds through the word ‘aves’ – an enigma during my formative years. I always believed ‘bird cherry’ was an alternative common name for Prunus avium because the seeds contained in the bright-red, juicy (though extremely tart) and small cherries were spread around the landscape by soft-fruit eating birds such as blackbird and starling. But this is not so, because ‘avium’ is derived from the Latin avius meaning ‘remote’ or ‘out of the way’. Indeed there is a completely different tree (Prunus padus), native only to northern England, Scotland and parts of Wales that is rightly called ‘bird cherry’. 

I can well understand why northerners and Scots might be thrown by Prunus cersifera, because it took me a long while to get my head around bird cherry with its flowers and fruit borne on upright panicles and not in clusters or bunches. Prunus padus is present in southern England, but only as a planted amenity tree with good specimens in both St James’s Park and Green Park in central London. I recently discovered a good number planted 20 years ago as street trees in Welwyn Garden City, Hertfordshire, covered by one of the more progressive local authorities in my neck of the woods when it comes to trees.

However, my closest encounter with bird cherry was a bit of a mixed blessing. Prunus padus is the main food plant of the bird-cherry ermine moth (Yponomeuta evonymella).

The caterpillars strip the trees of leaves while spinning extensive and sinister webs as they feed, leaving the bare trees with a ghostly appearance. 

Some years ago there was an outbreak in an Essex graveyard which left the cemetery looking like a Hammer House of Horror set. The story made the national headlines and I followed it up for essentialARB. The local tree officer and entomologist in attendance were extremely helpful with information and pictures. All seemed to be going well until I was ambushed by a real-life vampire from the PR department at the local authority who proceeded to tear into me for contacting the tree officer directly which apparently you are not supposed to do – long live the democratic local state. Suffice to say I told her to disappear to a place where the sun doesn’t shine and where plums, cherries and cherry plums can’t ripen.

On a happier note, Prunus padus (bird cherry) is starting to establish in woodlands and hedgerows in southern England, the seeds having been carried from parks and gardens into the wider and wilder environment.