Gadenin日本語Get the appSelf-fertile and self-sterile are two different trees
A self-fertile plum can fertilize its own flowers with its own pollen. Plant one on its own and it will crop, sometimes heavily enough that thinning becomes the main job of the summer. A self-sterile plum cannot. Its flowers open, look perfect, get visited by bees, and drop within a fortnight, because the pollen landing on them is genetically its own and the flower rejects it.
Both kinds are sold in the same nurseries and look identical as young trees. There is no way to tell them apart from bark, leaf or habit. The information comes from the label or from asking, and if you have inherited an unlabelled tree, the way to find out is to watch what it does: a mature, healthy, unfrosted tree that flowers well every year and never sets is almost always self-sterile with nothing to pollinate it.
There is a middle category worth knowing about. Some plums are described as partially self-fertile. They will set a light crop alone, perhaps a third of what they are capable of, and a full one with a partner. If you have one of these and you have been getting thin crops without knowing why, the answer may simply be that there is nothing else in flower nearby.
| Situation | What you see | What to do |
|---|---|---|
| Self-fertile, alone | Full set, needs heavy thinning | Nothing; just thin the fruit |
| Partially self-fertile, alone | Light but real crop most years | Add a partner or accept the reduced crop |
| Self-sterile, alone | Heavy blossom, no fruit at all, every year | Plant a compatible partner or graft a branch on |
| Any type, partner flowering too late | Blossom drops within two weeks | Match flowering times, not just species |
A partner only counts if it flowers at the same time
This is where most attempts at fixing a pollination problem fail. Buying a second plum is not enough. The two trees have to have overlapping bloom, meaning open flowers on both at the same time for a week or more. Plums vary by two to three weeks in when they open, which is easily enough for two trees in the same garden to miss each other completely.
Nurseries group plums into flowering periods, usually numbered or lettered from earliest to latest. A tree in one group will overlap with its own group and usually the group either side. Two groups apart is a gamble that depends on the spring. Ask for the flowering group when you buy, and match it, rather than assuming any plum will do.
Distance matters much less than people expect. Bees carry plum pollen readily and a partner tree anywhere within about 100 feet (30 m) is effective, including one in a neighboring garden. If plums grow nearby and yours still does not set, the problem is timing or frost, not distance.
Where there is genuinely no room for a second tree, a branch of a compatible type can be grafted onto the existing tree. One grafted limb is enough to pollinate the whole tree, and it costs you nothing in space. It takes a couple of years to flower, so it is a fix to start now rather than one that helps this season.
If a neighboring plum has been pollinating yours and it is felled or heavily cut back, expect your crop to collapse the following spring. It is a common and completely invisible cause of a tree suddenly stopping fruiting.
Getting the insects to do the work
Plums flower early, when the weather is cold and flying insects are scarce. Even a fully compatible pair fails if nothing is moving between them. Cold, wet, windy blossom weather is the commonest cause of a poor set in an otherwise healthy pair of trees, and it is partly within your control.
- Have something else in flower nearby Early flowering perennials and bulbs under and around the tree give bees a reason to be in that part of the garden in the first place. A plum flowering in isolation in a bare lawn gets fewer visits than one surrounded by other early flowers.
- Do not spray insecticide during blossom Nothing at all goes on an open plum flower, including products aimed at aphids. If aphids need dealing with, do it before the buds show color or after petal fall.
- Hand pollinate in a cold spring If the blossom week is cold and you see no insects at all, take a soft brush and move pollen from the flowers of one tree to the other, working at midday when any pollen is dry and loose. Do it on two or three separate days, since flowers open in waves.
- Protect the blossom from frost, not from rain Throw row cover over a small tree on a clear, still night when frost is forecast during flowering, and take it off in the morning so insects can get in. Leaving it on for days does more harm than the frost would.
Telling a pollination failure from a frost
The two look similar from a distance, but the flowers themselves tell you clearly. Cut open a few blossoms or examine ones that have just dropped. Frosted flowers have a blackened or brown center, and the tiny developing fruit at the base is dark and dead even while the petals still look white. It happens overnight, and it usually affects the lower and inner parts of the tree worst, because that is where cold air settles.
A pollination failure looks different. The flower center stays green and healthy looking, the petals fall normally, and the small green fruitlet sits there for one to three weeks before yellowing and dropping. The tree looks like it has set, briefly, and then quietly gives up. It happens evenly across the whole tree rather than at the bottom.
There is a third thing that gets mistaken for both. A tree that sets far more fruit than it can carry sheds a proportion of it a few weeks after flowering. This natural drop is normal and healthy, and you can tell it apart because plenty of fruit remains behind. If you can still see a fruit every few inches after the drop, nothing has gone wrong, and thinning is your next job.
Keep the blossom record
Write down the date the first flowers open and the date the last petals fall, every year, for each plum you own. Do this for three years and you will know your trees' flowering groups directly, without relying on a label. If you are choosing a partner tree, that record is more reliable than any catalogue.
Alongside it, note the overnight low on the coldest night of the blossom period and whether damage followed. Over a few years this shows whether you are losing crops to frost, to a missing partner, or to bad luck. Those three problems have completely different fixes, and guessing wrong costs you several seasons.
If your tree flowers well for three consecutive years, is never frosted during blossom, and sets no fruit at all, treat it as self-sterile and act on it. Waiting a fourth year rarely changes the answer.