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A plum's calendar is unusually rigid compared with other fruit trees, because two of its main jobs are locked to narrow windows by disease. Pruning cannot move into winter because of silver leaf, and thinning cannot be postponed past early summer because by then the branches are already carrying the load that breaks them. Miss either window and you cannot make it up later in the year.
Against that, the tree flowers earlier than almost anything else in the garden, which means the crop is decided very early, often before you have started thinking about the season at all. The frost that takes a plum crop typically arrives while other fruit trees are still in tight bud.
| Season | Main job | The decision to make |
|---|---|---|
| Late winter | Nothing with pruners | Check the forecast; get row cover ready for blossom |
| Early spring, blossom | Protect from frost, keep insects working | Cover on frost nights only, uncover by morning |
| Late spring, after petal fall | Watch the natural drop | Judge whether a real crop has set |
| Early summer | Thin to one fruit every 3 in (8 cm) | How hard to thin; which limbs need propping |
| Midsummer | Prune; remove rotting fruit weekly | What to cut to keep the tree reachable |
| Late summer | Pick over the tree every two or three days | Ripe by feel and release, not by color |
| Autumn, after leaf fall | Strip every mummified fruit | Cultivate soil under the tree for sawfly |
Late winter into blossom
There is nothing to prune, and that is the discipline of this season. The tree looks bare and accessible and it is exactly the wrong moment to cut it. Confine yourself to looking. Walk round the tree and note any branch with sunken bark or gum, any tear from winter wind, and any mummified fruit that escaped the autumn strip. All of that is a summer job, written down now.
Check the stake and tie on young trees while the ground is soft, because a tie that has tightened over a season will girdle the trunk. Top up the mulch circle if it has thinned, keeping it clear of the bark itself, and pull any grass that has crept in towards the trunk.
Once the flower buds show color, watch the overnight forecast every day. Open blossom is killed at about 28F (-2C). On a clear, still night with frost forecast, throw row cover over a small tree, and take it off in the morning so bees can reach the flowers. This is the difference between a crop and no crop, and it is a decision made on individual nights, not once for the season.
Petal fall to midsummer: the jobs that decide the crop
This is the busiest stretch of the plum year and where the outcome is settled. Everything here happens in a few weeks and none of it can be caught up on afterwards.
- Assess what actually set Two to three weeks after petal fall, look at the small fruit. Blackened centers mean frost; green fruitlets yellowing and dropping evenly mean a pollination failure. Plenty of fruit remaining after a partial drop means a normal set and a thinning job ahead.
- Wait for the natural drop to finish The tree sheds a proportion of its own crop a few weeks after flowering. Thinning before this is finished means removing fruit twice and possibly over-thinning. Wait until the drop stops and the remaining fruit is hazelnut sized.
- Thin hard, to one fruit every 3 inches (8 cm) Remove damaged and clustered fruit first, then space the rest. It will feel like too much. Under-thinning is the most common cause of broken branches, small tasteless fruit, and a tree that takes the next year off.
- Prop the heavy limbs Any branch still bending after thinning gets a forked stick underneath or a rope tied to a higher limb. Do this before the fruit swells, not after a crack appears.
- Prune, in full leaf Late spring through midsummer is the only safe window. Dead and damaged wood, crossing branches, vertical water shoots, then open the center and shorten new growth by a third. Small cuts, every year.
- Water through fruit swelling A dry spell while the fruit is sizing causes drop and later splitting. Give an established tree a thorough soak rather than frequent sprinkles, and keep the mulch topped up.
Late summer: picking, and the weekly bucket
Once fruit begins to color, two routines run alongside each other. The first is picking: go over the tree every two or three days, taking only fruit that gives slightly under the thumb and releases with a gentle lift and twist. Ripeness moves through a tree over a week to ten days, so this is several passes, not one harvest day.
The second is hygiene, and it is easy to neglect because it produces nothing. Every few days, walk round with a bucket and collect fallen fruit off the ground, and pick off any fruit on the tree showing a spreading brown patch or a sticky black hole. All of it leaves the garden. Fruit left lying under the tree feeds next year's sawfly and this week's brown rot at the same time.
If heavy rain is forecast when the fruit is close to ripe, bring forward the picking. Rain at that stage splits the skin, and split fruit rots within a couple of days and then infects everything touching it. Slightly under-ripe fruit picked ahead of a downpour is a better outcome than perfect fruit that bursts.
Autumn and the records that pay off
After the leaves fall, the branches are bare and every mummified fruit is visible. Go over the whole tree and pull off every one, and pick up anything remaining on the ground. This is the most important hour of the plum year for next season's brown rot, and it is the reason for keeping the tree at a height you can actually reach. Then lightly cultivate the top inch or two (3 to 5 cm) of soil under the canopy, which exposes overwintering sawfly to birds and frost.
Do not prune, however tempting the bare structure looks. Note the branches you want to take out and do it next summer. If a limb has broken under snow or wind, tidy the tear to a clean edge rather than leaving splintered wood, and accept that as an unavoidable exception.
Then write the year down while it is fresh. Record the date blossom opened and the date it finished, whether frost hit and on which night, the date you thinned and how hard, the picking dates, and roughly how much fruit was lost and to what. Note how many mummies you stripped in autumn, since that number falling year on year is direct evidence the hygiene is working.
Three years of that record turns plum growing from guesswork into a routine. You will know your own tree's flowering window well enough to choose a pollination partner, know whether your site is genuinely frost prone, and know from your own honest notes that last year you did not thin nearly hard enough.
Keep the notes tied to a specific tree and its position in the garden, not just to the crop. Two plums twenty feet (6 m) apart can behave completely differently if one sits lower in a frost pocket, and only per-tree records reveal it.