Gadenin日本語Get the appThe site is chosen for what happens in late summer
Every other fruit tree in the garden is sited for spring frost and summer sun. A prune plum has a third requirement that outranks both: the fruit stays on the tree long after a dessert plum would have been picked, right through the wettest part of late summer in many climates, and it has to stay dry while it does. That extended hang is not optional. It is the entire reason the fruit reaches the sugar level that lets it dry whole without fermenting, and it is also the period when the crop is most vulnerable to everything that goes wrong.
So the question to ask about a planting site is not just how much sun it gets at the height of summer, but how fast it dries after rain in the last six weeks of the season. A slope with air moving across it, an open aspect with nothing shading the lower half of the tree in the afternoon, and free-draining ground underfoot all do the same job: they get water off the skin of the fruit within hours rather than leaving it sitting there overnight. A sheltered corner that feels kind to a tree in spring is a brown rot factory in autumn.
Frost still matters, because these trees blossom early, in the same window as any other plum, and a hard frost at full bloom removes the crop entirely. Look for ground that sits above the low point of the garden so cold air drains away downhill past the tree instead of pooling around it. A difference of six or eight feet (2 to 2.5 m) in elevation across a garden is often the difference between a full crop and none.
| Site factor | What you want | Why it matters to a prune specifically |
|---|---|---|
| Sun | Full sun, at least eight hours | Sugar accumulates over the long hang; shade caps how sweet the fruit gets |
| Air movement | Open on at least two sides | Fruit that dries within hours of rain does not start brown rot |
| Drainage | Water gone from a test hole in four hours | Wet roots in late summer push water into ripe fruit and split it |
| Elevation | Above the frost pocket | Early blossom is the crop; a bloom-time frost ends the season |
| Access | Reachable on all sides | You will go over this tree ten or more times in one autumn |
Test drainage before you dig the planting hole, not after. Fill a hole a foot (30 cm) deep with water and let it drain, then fill it again. If the second fill has not gone in four hours, plant on a raised mound 12 to 18 inches (30 to 45 cm) high instead of in a hole.
Rootstock decides how many times you can climb the tree
A prune harvest is not one afternoon. Because the fruit is left to shrivel slightly at the stem and does not do that all at once, you will pick over the same tree every two or three days for three to five weeks, and in between you will be pulling off anything that has started to rot. That workload, repeated, is what should decide the rootstock, and it argues hard for a small tree.
On a vigorous rootstock these trees reach 18 to 22 feet (5.5 to 6.5 m) and hold most of the crop out of reach. On a semi-dwarfing rootstock the tree settles at 10 to 14 feet (3 to 4.5 m), and on a dwarfing one at 8 to 10 feet (2.5 to 3 m). Under about 12 feet (3.5 m) you can reach the whole canopy from the ground and a three-step ladder, which is the size at which the repeated picking rounds actually get done rather than skipped.
Space trees so their canopies never touch at full size: 12 to 15 feet (3.5 to 4.5 m) apart for semi-dwarfing, 10 feet (3 m) for dwarfing. Touching canopies trap humidity in exactly the weeks the fruit needs to be dry, and they shade each other's lower fruit, which is the fruit that ends up least sweet. Against a wall, allow 15 feet (4.5 m) of horizontal run for a fan and pick the sunniest wall you have.
Soil and the planting hole
Prune plums are not fussy about soil type and will crop on anything from sandy loam to moderately heavy clay, provided water moves through it. What they will not tolerate is ground that stays saturated, because roots sitting in water in late summer take up a surge every time it rains and push it straight into fruit whose skin has stopped expanding. Get the drainage right on planting day and you have removed the main cause of split fruit for the life of the tree.
- Dig wide, not deep Make the hole two to three times the width of the root system but no deeper than the roots themselves. A hole dug deep and backfilled with loose soil settles, and the tree sinks with it.
- Break the sides of the hole Run a fork into the walls of the hole in several places. On clay especially, a smooth-sided hole acts as a sump that roots circle inside instead of leaving.
- Set the graft union clear of the soil The union must sit two to four inches (5 to 10 cm) above the finished soil level. Buried, the scion roots and the rootstock stops controlling the size of the tree.
- Backfill with the soil you took out No compost, no fertilizer in the hole. Enriched backfill keeps roots inside the hole. Firm the soil in layers with your heel as you go, then water heavily to settle it.
- Stake low and short Drive a stake in before backfilling, angled away from the prevailing wind, and tie the trunk at about a third of its height. A low tie lets the top flex, which is what thickens the trunk.
- Mulch, but not against the trunk Two to three inches (5 to 8 cm) of mulch out to the edge of the canopy, with a clear ring four inches (10 cm) wide around the trunk. Mulch touching bark keeps it wet and invites canker.
The first three years build the frame that carries the load
A mature prune tree carries a heavy crop and carries it late, which means the branches are still bearing full weight into autumn when storms arrive. The frame has to be built for that, and it is built in the first three seasons. Aim for three to five main limbs leaving the trunk at wide angles, ideally somewhere between forty-five and sixty degrees from vertical, spaced around the trunk and separated vertically by six to eight inches (15 to 20 cm) so no two come off at the same point.
A narrow crotch angle is the single structural weakness that ends these trees. Bark gets pinched between the two limbs as they thicken, the joint never knits properly, and it splits under a wet crop in a wind. Correct the angle while the shoot is young and pliable by wedging a clothes peg or a short stick between it and the trunk in its first summer. After the wood has hardened it is too late to bend and the branch has to come off instead.
Do not let the tree fruit in the first two years even if it tries. Rub off the flowers. Everything the tree makes in that period should go into roots and trunk. A tree allowed to crop early stays small, cropsy and slow to fill its space, and takes several extra years to reach the point where it produces enough fruit at once to be worth setting up a drying run for.
What to write down on planting day
Record the rootstock, the planting position, and the aspect the tree faces. Rootstock is the one piece of information you will need for the next twenty years and the one most likely to be lost, because the label perishes long before the tree matures. It determines the eventual height, the spacing to the next tree, and how hard you can prune without provoking a thicket of watershoots.
Then start a blossom record from the first spring, even before the tree crops. Note the date the first flowers open and the date the last petals fall, every year. Within three or four years that record tells you your own site's bloom window with far more precision than any general advice, and it is what lets you decide whether a forecast cold night is worth going out to cover the tree for. Add the date of the first autumn frost too, because the gap between last picking and first frost is the margin you are working with at the other end of the season.
Also note where the water sits after a heavy autumn downpour. Photograph the puddles. That map of the wet spots is the best guide you will ever have to which parts of the garden can carry a tree whose fruit must stay dry in the last month of the season.