Disorder
Blossom drop
Also called Flower drop, Poor fruit set, Failure to set
Tomatoes, beans and courgettes flowering freely and setting nothing. The temperature limits pollen works within, why courgettes drop their first flowers, and what to do at midday.
At a glance
- Type
- Disorder
- Cause
- Growing conditions
- Severity
- Destroys the crop or display
- Affects
- Herbs and edibles
- Active
- April to August
What you are seeing
- Flowers opening normally and then yellowing and falling from the stalk
- A clean scar where the flower was joined, with no sign of a swelling fruit
- Whole trusses of tomato flowers shed while the plant grows strongly
- Runner beans flowering heavily over weeks with almost no pods
- Courgettes with tiny fruit behind the flower turning yellow and rotting at the tip
- Fruit trees blossoming well and carrying almost nothing by June
- Plants that are lush, dark green and vigorous with no crop on them
Before you treat anything
Nothing here is harmful. Take care using a ladder or reaching over glass when hand-pollinating or fleecing blossom.
What you are looking at
The plant flowers well. Then the flowers yellow at the stalk and drop, leaving a clean scar and no swelling behind them. Trusses of tomato blossom come off a plant that is otherwise growing hard. Runner beans flower for three weeks and produce a handful of pods. A courgette makes flower after flower and no courgettes. An apple tree is white with blossom in April and carries a dozen fruit in July.
Very often the plant looks superb. Dark green, vigorous, well-fed, and completely unproductive.
Pollen has a narrow working range
Tomato and pepper pollen is only viable within a band of temperatures. Below about 13 degrees at night it does not develop properly, and above about 30 degrees by day it fails outright. A cold May night and a hot July afternoon both produce the same empty truss two weeks later, which is why blossom drop turns up at both ends of the season.
Humidity narrows the range further. Pollen has to shake free of the anthers and land on the stigma, and it stays stuck in very dry air and clumps into useless lumps in very wet air. A greenhouse that has been shut for three days in a heatwave manages to be both hot and humid, and it sets nothing at all.
Water supply is the third factor. A plant swinging between bone dry and saturated sheds flowers deliberately, because a plant that cannot rely on its water supply will not commit to fruit.
Nitrogen is the fourth, and it is self-inflicted. A tomato fed a general growth feed through flowering makes magnificent leaves and drops its blossom. Switching to a high-potassium feed at the first truss is what turns that around.
Some of it is normal
A courgette or a squash produces a run of male flowers before the first female one appears. Male flowers have a plain thin stalk, no fruit behind them, and they always drop. Nothing has gone wrong.
Female flowers have a miniature courgette behind the petals. Those need pollen from a male flower open on the same morning, and in a cold wet spell with no insects flying they yellow and rot from the tip instead. That is worth hand-pollinating for, and it corrects itself once the weather warms.
Fruit trees shed heavily in June as a matter of course, dropping the fruitlets they cannot carry. A tree that sets a hundred apples and keeps thirty is behaving normally.
Pollination partners, for anyone whose tree never fruits
Most apples, pears, plums and sweet cherries cannot set fruit from their own pollen. They need a different cultivar of the same species, flowering at the same time, close enough for a bee to fly between them.
A tree that blossoms magnificently every year and has never carried a crop, in a garden with nothing else of its kind nearby, has this and nothing else. No feeding, watering or pruning will address it. Plant a compatible partner, choose a self-fertile cultivar next time, or graft a branch of a pollinator into the existing tree.
Cold wet weather during blossom does the same thing temporarily by keeping the insects grounded, and a frost while the flowers are open ends the crop for that year outright.
What to do about it
Water on a rhythm and mulch, so the plant never swings between dry and soaked.
Ventilate a greenhouse from mid-morning, shade the glass in a heatwave, and damp the floor down.
Switch to a high-potassium feed as soon as the first truss sets, and stop the growth feed.
Tap the tomato trusses at midday, every day, through flowering.
Hand-pollinate courgettes in a cold spell, male flower onto female, in the morning.
Leave the greenhouse open so bumblebees can get in.
Fleece fruit blossom on a frosty night and uncover it the next morning.
Check whether your fruit tree has a pollination partner before you change anything else.
What it gets mistaken for
Blossom wilt browns and kills the flowers where they stand and the dead petals stay attached to a dying spur. Blossom drop sheds clean flowers from healthy wood.
Blossom end rot affects fruit that has already set, marking the base of a swelling tomato with a black leathery patch.
Frost damage blackens the centre of an open flower, and cutting one open shows a dead brown core where the seed should form.
Pollen beetles crowd into open flowers in spring and do almost no harm. Their presence in a blossom that later drops is coincidence.
Capsid bug and other sap-suckers distort developing fruitlets rather than causing flowers to fall cleanly.
What to do, in order
-
Start here
Prevention, and the changes that stop the problem returning. Most of these pages should end at this tier.
- Keep the soil moisture even
- A plant that swings between dry and drenched sheds flowers, because aborting them is how it protects itself when supply looks unreliable. Water thoroughly and on a rhythm rather than heavily after a drought. Mulch outdoor crops and use large containers under glass.
- Manage the temperature under cover
- Tomato pollen fails below about 13 degrees at night and above about 30 by day, and peppers and aubergines are similar. Ventilate a greenhouse hard from mid-morning in summer, shade the glass, damp the floor down, and close it in the evening in early summer to hold the night temperature up. Both ends of the range cost you the same trusses.
- Get the humidity into the middle
- Pollen will not shake free of the anthers in very dry air and it clumps in very wet air. Damping down a greenhouse floor in the morning fixes the dry end. Ventilating fixes the wet end. Aim for a greenhouse that feels fresh rather than either arid or muggy.
- Stop feeding high nitrogen once flowering starts
- A plant given growth feed while in flower puts everything into leaf and sheds the flowers. Switch to a high-potassium tomato feed at the first truss and stay on it, and expect a very lush dark green plant with no fruit to be a feeding decision rather than a mystery.
- Keep runner bean roots wet
- Runner beans set badly in a dry hot spell and the cause is at the root rather than at the flower. A trench filled with organic matter, a deep soak twice a week and a thick mulch do more than anything sprayed on the flowers. Spraying the flowers with water is traditional and there is little behind it.
- Plant a pollination partner for fruit trees
- Most apples, pears, plums and sweet cherries need pollen from a different compatible cultivar flowering at the same time. A tree blossoming heavily and setting nothing, year after year, in a garden with no other fruit tree nearby, has this problem and no other. Nursery catalogues list flowering groups for exactly this reason.
- Expect the first courgette flowers to come to nothing
- A courgette or squash usually produces several male flowers before the first female one, and males have no fruit behind them and always drop. A female flower has a miniature courgette behind it, and if no male is open on the same day it yellows and rots from the tip. This is normal for the first fortnight of flowering.
-
Then reach for a barrier
Netting, hand removal and water. Nothing here harms anything except the target.
- Tap the tomato trusses at midday
- Tomatoes and peppers pollinate themselves, and the pollen needs shaking loose. Flick each open truss with a finger, or tap the supporting cane, around midday when the air is warmest and driest. A minute a day through the flowering weeks measurably improves set under glass, where there is no wind to do it.
- Hand-pollinate courgettes and squash
- Pick a male flower, strip its petals back, and brush the pollen onto the centre of a female flower in the morning while both are open. This rescues a crop in a cold wet June when no insects are flying.
- Open the greenhouse to the insects
- A closed greenhouse has no pollinators in it. Leave a door or vent open through the day once the weather allows, and grow a few flowers among the tomatoes to bring bumblebees in.
- Fleece fruit blossom against frost
- A frost while a fruit tree is in flower kills the blossom outright and there is no second chance that season. Throw fleece over a small tree or a trained wall fruit on a clear still night in April, and take it off in the morning so the insects can work.
Plants this turns up on
Guides that name it among the problems to watch for. The list is the plants it is characteristic of rather than everything it will feed on.