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Why do indoor tomatoes not set flowers or fruit?

Tomato flowers not setting fruit
Tomato flowers not setting fruit

Blog: From poor pollination to irregular watering – 10 reasons why crops fail!

Tomatoes are my biggest crop of the year – my conservatory’s a working greenhouse and is packed with plants, as is my little greenhouse.

There’s nothing quite like home-grown tomatoes (more on how to grow them here) – they have a zinginess you just can’t get from commercially grown varieties.

However, they need consistency and for a few reasons this year, I’ve taken my eye off the ball and the setting of the fruit has been very poor (hey, we all have lives to deal with).

Last weekend was time for a clean-up – removing leaves below finished trusses, topping up soil, tying in, stopping cordon varieties Rosella, Apero and Reisetomate and staking flopping bush variety Maskotka.

My problems have been caused by poor pollination, fluctuations in temperature and appalling compost (I’ll be having a rant about this later on – there was such a shortage during lockdown half of my bags seemed to be sour).

There are many reasons why fruit won’t set from flowers, or blooms drop off, or you don’t get any at all. These are all for indoor tomatoes:

Partially set Rosella truss
Partially set Rosella truss

Poor light

This can be caused by insufficient light. Plants need six-eight hours of full sun a day to produce blooms and fruit. Any less and you’ll get either plenty of foliage but spindly or leggy growth.

Solution: move plants to a sunnier place.

Watering – too little and too much

Most beginners know that tomatoes need a lot of water, especially in pots. Too little results in poor fruit development – the plant struggles to survive, so drops its fruit and flowers until better times. Too much can result in rotting.

Solution: never let plants dry out, or get flooded – keep them evenly moist. Mulch around the base of the plant.

High, low and/or fluctuating temperatures

Tomatoes need 18-21ºC during the day and at least 13ºC at night to set fruit. However, if the temperature rises above 29ºC and 24ºC at night, flowering will stop, as pollen turns sterile. If you have plenty of blooms but no tomatoes, it may be too cold, resulting in ‘blossom drop’.

Solution: Open windows, doors and vents on hot days and ‘damp down’ the floor (water it – the evaporation brings down the temperature). During a heatwave, keep plants well-watered, so when temperatures drop, they’ll start forming flowers again. Close windows and vents at night during cooler weather.

Conservatory before trimming back the tomatoes
Conservatory before trimming back the tomatoes

Poor pollination

Tomatoes are self-fertile – each flower can pollinate itself BUT bees and/or wind improves pollination by disturbing the flowers and knocking pollen from the stamens. The vibration of bumblebees’ wings loosens the pollen and it falls onto the stigma (the female part of the flower). Cold, windy, or wet weather limits bee activity, too.

Obviously, growing tomatoes under glass means limited access for bees and other pollinators. My greenhouse has double doors that are kept open all summer so I don’t have a problem there, just in the conservatory, which I can’t leave wide open all the time.

Solution: Open up doors and vents, or artificially pollinate tomatoes by lightly shaking the plants yourself.

Humidity

Very high humidity can clog the pollen, so it’s unable to drop and fertilise the stigma, while in low humidity, pollen fails to stick and simply rolls straight off.

Solution: Regular watering can raise humidity around the plants – keep windows and doors open to provide good air circulation.

Planting too close

Crowding in plants will produce fewer fruits and leave them more open to pests and disease.

Solution: Space tomatoes at least 60cm apart.

Conservatory after trimming back the tomatoes
Conservatory after trimming back the tomatoes

Grey mould (botrytis)

Fungal diseases, like botrytis, can cause flowers to drop.

Solution: Pick up any dead plant material, watch out for high humidity and space tomatoes at least 60cm apart.

Not enough potash

Tomatoes are greedy plants but need potash-rich fertiliser (high in potassium) to promote flower and fruit formation.

Solution: When the first flowers appear (some say when the first fruit is pea-sized), feed plants with a specialist tomato fertiliser (Tomorite or similar – usually in a red bottle). You can make your own with comfrey – but it stinks!

Wrong fertiliser

Don’t use an ordinary fertiliser, usually 10-10-10 NPK (nitrogen, phosphorus, potassium) – it has far too much nitrogen in it, which promotes green leaf growth, not flowers or fruit.

Solution: switch fertiliser to a specialist tomato variety (see above).

Is the variety right for you?

There are two main types of tomato – indeterminate (cordon) varieties which grow upwards like a vine and need to have their side shoots nipped out. Their lead shoot is ‘stopped’ when they reach the top of their cane. They will fruit until the first frosts (I’ve had them still bearing fruit in November).

Then there are determinate (or bush) tomatoes, which grow to a determined height and are short, bushy and are often trailing. They tend to fruit early. Once they reach that height, no more fruit or flowers will be produced – that’s it.

Solution: For a long season, choose cordon and bush tomatoes and varieties bred for growing under glass – outdoor varieties will suffer in intense heat.

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