Plum

About

Plums have one rule that overrides everything else. Never prune them between autumn and early spring. Silver leaf spores are airborne through the winter and enter through fresh cuts, and the disease kills branches and eventually the tree.

Prune in June or July instead, when the wound seals fast and the spores are scarce. That single habit prevents most plum tree deaths in British gardens.

The other thing to expect is generosity. A good year on an unthinned plum tree produces so much fruit that the branches break under it, so thinning in June is work that protects the tree as much as it improves the crop.

Appearance and character

A round-headed deciduous tree with dark grey bark, generally smaller and shorter-lived than an apple, and inclined to throw suckers from the base.

Leaves are oval, toothed and slightly downy beneath, dull mid-green.

Blossom is white and opens in late March and April, usually before the leaves, covering the bare branches. It is among the earliest fruit blossom and therefore the most frost-vulnerable.

Fruit ripens from late July to September, oval or round, in purple, blue-black, red, yellow or green, usually with a waxy bloom on the skin.

Three groups matter in the kitchen. Dessert plums such as ‘Victoria’ are eaten fresh, gages such as ‘Cambridge Gage’ are smaller, greener and far sweeter, and damsons are small, tart, and made for cooking and gin.

Where it is from

The Caucasus and eastern Europe, where Prunus domestica arose as a natural hybrid between the sloe, Prunus spinosa, and the cherry plum, Prunus cerasifera.

That hybrid origin left it hexaploid, carrying six sets of chromosomes, which is part of why it is so variable and why so many local landrace varieties exist across Europe.

The Romans grew and spread plums, and by the Middle Ages Britain had damsons, bullaces and gages established.

‘Victoria’, the variety that dominates British gardens, was found as a chance seedling in a Sussex garden around 1840. It is self-fertile, heavy cropping and generous, and it is also the variety most prone to breaking under its own weight and to silver leaf.

The natural habitat is woodland edge and scrub in a continental climate, which is why the trees are hardy in wood while their early blossom struggles in a maritime spring.

How it grows

Buy a grafted tree. Pixy rootstock gives a small tree of 2.5 to 3 m suitable for a small garden, St Julien A gives 4 to 5 m and is the standard choice, and Brompton gives a full-sized tree.

Pollination is simpler than for apples. Many plums, including ‘Victoria’, ‘Czar’ and most damsons, are self-fertile and crop alone. Gages generally are not and need a partner.

Plant bare-root from November to March, keeping the graft union clear of the soil, and stake for the first few years.

Fruit forms on both one-year shoots and older spurs, which is why plums need less formative pruning than apples once a framework is established.

Thin the fruitlets in June, after the natural drop, to about 5 to 8 cm apart. This prevents branch breakage, improves size, reduces brown rot and prevents biennial cropping.

Support heavily laden branches with props if thinning has been insufficient.

Suckers arise from the rootstock and should be pulled off at the point of origin rather than cut, since cutting encourages more.

Through the seasons

Winter is for planting only. Plant bare-root trees, and do not prune anything.

Spring brings very early blossom. Protect small trees against frost where possible, and feed and mulch in February.

Summer is pruning and thinning. Prune in June or July, thin the fruitlets, and prop any branch carrying too much.

Late summer and autumn is the harvest, from late July to September. Pick when the fruit parts easily and yields slightly. Remove any mummified fruit before winter.

Care difficulty

Moderate, and the difficulty is disease management rather than horticulture.

Silver leaf is the defining risk and it is largely preventable by pruning only in summer.

Bacterial canker is the other serious disease and it is worse on wet heavy ground and in exposed sites.

Frost at blossom time removes the crop in some years and cannot be prevented on a large tree.

Cropping is famously irregular. Plums swing between gluts and nothing, and thinning hard in a heavy year is the best correction available.

Wasps and birds take a share of ripe fruit and there is little to be done about it.

Watering

Water newly planted trees weekly through their first three summers.

Established trees need watering during prolonged drought, particularly while the fruit is swelling in July. Dry roots at that point cause heavy fruit drop.

Plums are less drought-tolerant than apples and prefer moisture-retentive soil.

Mulch each spring with well-rotted manure or compost, kept clear of the trunk.

Avoid waterlogged ground, which encourages bacterial canker.

Warning signs, diseases and pests

Silver leaf gives the leaves a distinct silvery sheen, usually on one branch first, and the wood shows a dark stain when cut across. The fungus enters through wounds in autumn and winter. Cut affected branches back to 15 cm beyond the stain and burn them. Prevention is the only reliable approach, and it means pruning only between June and August.

Bacterial canker causes sunken patches on the bark that ooze amber gum, along with small round holes in the leaves as infected tissue drops out. Cut out affected wood in summer, and avoid wet exposed sites.

Brown rot turns fruit brown with concentric rings of buff pustules, spreading rapidly through touching fruit. Remove and destroy every affected and mummified fruit, since they carry it into the next year.

Plum moth larvae tunnel into the fruit around the stone. Pheromone traps hung in May reduce the numbers.

Gum oozing from the trunk without canker symptoms is often a stress response to drought or damage rather than disease.

Branches snapping in August mean the crop was not thinned.

Suckers from the base are rootstock growth. Pull rather than cut them.

Growing environment

Full sun, with as much shelter as possible for the early blossom.

Deep, fertile, moisture-retentive soil. Plums tolerate heavier ground than apples and want good drainage nonetheless.

Avoid frost pockets, since blossom in late March is exposed.

Choose Pixy rootstock for a small garden and St Julien A for a standard tree.

A fan against a warm wall suits gages, which need more heat to ripen properly.

Allow room. Even on Pixy the tree spreads, and the branches will be heavy.

Feeding

Apply a nitrogen-rich fertiliser in late winter. Plums are hungrier for nitrogen than apples and pears.

Add sulphate of potash in early spring to support fruiting.

Mulch generously with well-rotted manure in spring.

Do not overfeed a young tree, since soft sappy growth is more vulnerable to bacterial canker.

Container-grown trees on Pixy need regular liquid feeding and annual top-dressing.

Troubleshooting

Silvery sheen on the leaves of one branch. Silver leaf. Cut back to 15 cm beyond any dark staining in the wood and burn the prunings. Never prune plums in winter.

When can I prune? June to August only. Winter and autumn cuts invite silver leaf.

A branch snapped off under the fruit. The crop was not thinned. Thin the fruitlets to 5 to 8 cm apart in June, and prop heavy branches.

Amber gum oozing from the bark with small holes in the leaves. Bacterial canker. Cut out affected wood in summer and improve drainage.

Fruit going brown with rings of pale spots. Brown rot. Remove every affected fruit, including mummified ones still hanging in winter.

Masses of fruit one year and none the next. Typical plum behaviour. Thin hard in the heavy year to even it out.

Shoots coming up from the ground around the trunk. Rootstock suckers. Pull them off where they emerge rather than cutting them.

Blossom killed by frost. Plums flower in late March and are exposed. Fleece a small tree overnight, and avoid planting in a hollow.