Why are my tomato flowers falling off without setting fruit?

Tomato flowers abort at both ends of the temperature range and when pollen never moves. Which of the four causes you have, and what actually shifts it.

Difficulty: beginner
Time: 15 min
Read: 4 min

Short answer

Flowers drop when it is too hot, too cold, too humid for pollen to release, or the plant is running on too much nitrogen. Under glass the commonest fix is ventilation plus a daily tap on the flower truss — tomatoes are self-fertile but need vibration to shed pollen, and still greenhouse air gives them none.

A tomato flower that is not pollinated within a few days develops a clean joint at the base of its stalk and drops off. The flower itself looks perfect right up to the moment it falls, which is what makes this confusing. Nothing has attacked it; it simply never got fertilised, and the plant is cutting its losses.

Step by step

Check the temperature range the plants are actually in.Pollen becomes unviable above roughly 30°C, and fails to release properly when nights drop below about 12°C. A greenhouse can be in both ranges in the same 24 hours. A cheap max-min thermometer settles the question in a day.
Ventilate hard before you do anything else.Door and vents open from mid-morning in summer, and left open overnight in a heatwave. Most greenhouse flower drop in a British summer is heat, and shading plus ventilation fixes it.
Tap or shake the trusses daily in still air.Tomatoes are self-fertile, but pollen has to be physically shaken from the anthers onto the stigma. Outdoors, wind and bumblebees do it. In a closed greenhouse nothing does. Flick each open truss with a finger, or tap the supporting cane, mid-morning on a dry day.
Watch humidity at both extremes.Very humid air makes the pollen clump so it will not shed; very dry air makes the stigma unreceptive. Damping the greenhouse floor in a dry heatwave, and ventilating in muggy weather, keeps it between the two.
Stop high-nitrogen feeding.A plant in lush green growth with thick stems and huge leaves is being told to build foliage, not fruit. Switch to a high-potash tomato feed as soon as the first truss opens, and if you have been using a general feed, stop.
Check the plant is not dry at the roots.Water stress at flowering aborts flowers as reliably as heat does. Steady moisture matters more than volume.
Be patient with the first truss on an early plant.A plant put out too early, into cold nights, will commonly drop its first truss and set perfectly well on the second and third once the weather turns. That is not a fault to fix.
Look at the light level if the plants are indoors.A windowsill plant in a north or east-facing room often flowers and never sets, simply because there is not enough light to support fruit. Moving it outside after the frosts usually solves it in a fortnight.

Tips

A bumblebee nest or an open door is worth more than any hand pollination — bumblebees buzz-pollinate tomatoes far more effectively than a human with a paintbrush.
Very large beefsteak varieties set less reliably in cool British summers than small-fruited types. If you get flower drop every year, a cherry or plum variety will crop where a beefsteak will not.
Tapping works best in the middle of a dry morning, when the pollen is loose and the stigma receptive.
One or two dropped flowers per truss is normal, particularly on the largest flower. Only a whole truss dropping is a problem.

Common mistakes

Feeding more when flowers drop — Most feeding responses make it worse, because excess nitrogen pushes the plant further towards leaf. Fix temperature and pollination first and only then look at feed.
Keeping the greenhouse closed to keep it warm — In a British summer the daytime problem under glass is nearly always too hot, not too cold. A closed greenhouse in July can pass 40°C, which sterilises pollen outright.
Using a paintbrush the way you would on a courgette — Tomato pollen sits inside a closed cone of anthers and is released by vibration. Brushing the outside of the flower does very little; tapping the truss does the job.

If it doesn't work

Whole trusses dropping in July under glass

Cause: Daytime temperature above the pollen viability limit — Fix: Open all vents and the door by mid-morning, apply shading, damp the floor, and leave ventilation open overnight during a heatwave.

First truss drops on a plant put out in May

Cause: Cold nights below about 12°C — Fix: Fleece the plants at night until the weather settles, or wait for the second truss — it usually sets normally.

Plants huge, dark green, few flowers and those drop

Cause: Too much nitrogen — Fix: Stop general-purpose feed, switch to high potash, and water plain for a fortnight.

Flowers hang on but shrivel without swelling behind

Cause: Pollen not shed in still, humid air — Fix: Tap the trusses daily mid-morning and increase ventilation to move air through the house.

Questions people ask

Do I need a second tomato plant for pollination?

No. Tomatoes are self-fertile — each flower contains both parts and pollinates itself. What they need is movement to shake the pollen loose, not a partner plant.

Is there any product that makes tomatoes set?

Setting sprays exist and can force fruit in cold conditions, but they often produce hollow or seedless fruit and they do nothing about the underlying temperature problem. Ventilation, shading and tapping are more reliable and free.

My plant is flowering but the flowers look tiny and pale. Why?

Small, pale flowers usually mean the plant is short of light or root room rather than pollination. Check the pot size and the light level before looking at temperature.