The short version: Tomato flowers pollinate themselves, but the pollen has to shake loose to do it. Outdoors, wind and bees handle that. Under cover or inside a net, tap the stem behind each flower truss around midday on a dry day. It will not help when the day is above about 32 °C or the night drops below about 13 °C.
Why tomatoes need a shake
A tomato flower carries both parts, but the pollen sits inside a closed cone of anthers and has to be vibrated loose to drop onto the stigma in the same flower. In an open garden, wind rocks the plant and bees do the rest. Blue-banded bees and other native buzz pollinators grip the flower and vibrate, shaking the pollen out in a way honeybees cannot. Where little of that reaches the flowers, in a greenhouse, on a sheltered balcony, under an exclusion net or in a dead-still week, fewer set: some hang a few days, then drop without fruit.
How to do it
- Choose around midday on a dry, mild day below about 32 °C, when the flowers are fully open and the pollen dry enough to fall; damp pollen clumps.
- Hold the stem just behind a truss (the flower cluster) of open flowers and tap it with a finger for a few seconds. Keep your fingers off the flowers, and do not count on shaking the stake, which moves the plant more than the pollen.
- For a surer touch, hold an electric toothbrush to the stem behind the flowers for a second or two. The vibration is enough to shake the pollen loose.
- Do it every second day while the plant flowers. New flowers keep opening along each truss, so one visit misses the late ones.
- Watch the set. Within a few days a pollinated flower drops its petals and shows a small green ball behind; one that yellows at the stem and falls was not set.
When shaking will not help
Pollen works within a temperature range. Fruit set falls away as days pass about 32 °C, sooner when nights stay warm too, because the pollen turns sterile or sticky, and shaking cannot fix that. Nights below about 13 °C stop it from the other end. Very high humidity clogs it too; under cover, open the vents and water early so the air is drier by midday. In a heatwave, shade the plants and keep the water steady; the flowers opening a week or so after the heat breaks can set again if the plant is otherwise well. Save the shaking for mild days, and do not blame the plant for a week of 38 °C.
A tap on the truss at lunchtime is a small job with a lot of tomatoes behind it.
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