Gadenin日本語Get the appRootstock sets the height, and height sets the workload
A plum on a vigorous rootstock will reach 20 to 25 feet (6 to 7.5 m) and hold most of its crop above the reach of any ladder you are willing to stand on. That matters more with plums than with apples, because plum work is not optional. You have to thin the fruit by hand every year to stop the branches breaking, and you have to pick off rotting fruit as it appears. Both jobs need you to reach every branch. A tree you cannot reach is a tree that breaks itself and then reinfects itself.
A semi-dwarfing rootstock holds a plum at 10 to 14 feet (3 to 4.5 m) with routine summer pruning, and a dwarfing one keeps it at 8 to 10 feet (2.5 to 3 m). Those are the sizes where thinning takes an afternoon rather than a weekend. Under 12 feet (3.5 m) you can also strip the tree of mummified fruit in autumn from the ground and a short stepladder, which is the single most effective brown rot control there is.
The compromise is anchorage. Dwarfing plum stocks are shallow rooted, and a plum in full crop is heavy and catches wind. Stake at planting and keep the stake for at least five years, not the usual two. Keep a circle of bare soil or mulch three feet (90 cm) across at the trunk, because grass at the base of a dwarf plum measurably slows it in its first seasons.
| Rootstock vigor | Mature height | Spacing between trees | Years to first real crop |
|---|---|---|---|
| Dwarfing | 8-10 ft (2.5-3 m) | 10 ft (3 m) | 3 to 4 |
| Semi-dwarfing | 10-14 ft (3-4.5 m) | 14 ft (4.5 m) | 4 to 5 |
| Semi-vigorous | 14-18 ft (4.5-5.5 m) | 18 ft (5.5 m) | 5 to 6 |
| Vigorous | 20-25 ft (6-7.5 m) | 25 ft (7.5 m) | 6 to 8 |
If your garden only has room for one tree, check whether the type you want is self-fertile before you buy. Some plums set a full crop alone and some set nothing without a partner nearby. The pollination guide covers how to tell which you are dealing with.
Plums blossom early, so the site has to dodge frost
Plums open their flowers in early spring, often two to three weeks ahead of apples and while frost is still routine. Open blossom is killed at about 28F (-2C), and the tiny set fruit just after petal fall is killed at around 30F (-1C). A single clear, still night at the wrong moment removes the entire year's crop, and no amount of feeding or watering later will bring it back.
Cold air behaves like water. It runs downhill and pools at the bottom of a slope, against a solid fence, or in a dip. A tree planted in that pool sits two to four degrees colder on a frost night than one planted twenty feet (6 m) up the slope. Walk the garden on a cold, clear morning before you dig, and look at where the frost lies longest after sunrise. That is the ground to avoid.
Aim instead for a spot on a slight slope with somewhere lower for the cold air to drain away to, in full sun, and sheltered from the prevailing wind without being enclosed. A wall that faces the morning sun is a mixed blessing: it warms the tree into flowering even earlier, which brings the blossom further into the frost window. If you have a choice, a position that stays cool a little longer in late winter is worth more than one that starts early.
Soil and the planting hole
Plums want more moisture than most tree fruit and tolerate heavier ground than cherries do, but they will not sit in water. What they genuinely dislike is thin, sharply drained soil that dries out in summer, because drought stress during fruit swelling causes fruit drop and splitting. If your soil is light, plan on mulching every year rather than trying to change the soil once.
Dig the hole no deeper than the root system, and at least twice as wide. The most common planting mistake with all fruit trees is a hole that is deep and narrow: the tree settles, the graft union ends up at or below soil level, and the scion roots into the ground and escapes the rootstock's size control entirely. Then you have the 25 foot (7.5 m) tree you were trying not to plant.
- Check the union height before you dig Find the graft union, the swollen kink low on the trunk. It must finish 4 inches (10 cm) clear of the soil surface once the tree has settled. Lay a cane across the hole to sight the final soil line against the trunk.
- Dig wide, loosen the sides Make the hole twice the width of the roots and only as deep. Scratch the sides of the hole with a fork so the roots are not meeting a smooth, compacted wall. Do not add fertilizer to the hole.
- Drive the stake first Put a stake in before the tree goes in, so you do not drive it through the roots. A short stake angled at 45 degrees, or a vertical one to about a third of the trunk height, both work. The point is to hold the root ball still, not the whole trunk rigid.
- Backfill in layers and firm Return the same soil in three or four layers, firming each with your heel. Plums are wind-rocked easily and a loose root ball never establishes. Water in with two full cans even if the ground is wet, to settle soil into the root spaces.
- Mulch, but keep it off the trunk Spread 2 to 3 inches (5 to 8 cm) of compost or bark in a circle three feet (90 cm) across, leaving a hand's width of bare soil around the trunk itself. Mulch against bark holds moisture there and invites rot at the base.
The first two years decide the frame
A young plum will try to fruit before it has the wood to carry it. Remove the fruit in the first two seasons after planting, and take off most of it in the third. It feels wasteful and it is not: fruit produced on a two year old tree is small, and the energy spent on it comes directly out of the branch structure that has to carry a hundred pounds (45 kg) of crop later.
Water is the other first-year job. A newly planted plum needs a thorough soak, four to five gallons (15 to 20 liters), once a week through its first two summers whenever there has been no significant rain. Frequent light watering is worse than useless, because it keeps roots shallow. Stop watering once the leaves fall.
Prune young plums only in summer, never in winter. This is not a stylistic preference: winter cuts on any plum are an open door to silver leaf, a fungal disease that kills branches and eventually whole trees. The pruning guide explains the reasoning and the timing in full, but the rule starts the day you plant.
What to write down on planting day
Record the rootstock, whether the type is self-fertile, and where in the garden the tree went, with a note on how the ground sits relative to the rest of the plot. Rootstock in particular is impossible to recover later from looking at the tree, and it is the number that tells you how hard to prune in five years' time.
From the first spring, note the date the blossom opens and the date any frost damage shows. Two or three years of those dates tell you whether your site is genuinely in a frost pocket or whether you were unlucky once. If the blossom opens at the same point every year and gets hit in two years out of three, the answer is a different position, not a different spray.
Also note the date of the first hard frost after blossom each year. A tree that loses its crop to a specific late frost, year after year, is telling you something about the site that no amount of care will fix.