How to grow tomatoes in a greenhouse, and fix the common problems
Three of the commonest fruit problems in a greenhouse, blossom end rot, splitting and patchy ripening, trace back to watering that comes and goes, according to the RHS. The rest of the season is simple by comparison: sow in late winter, plant out when the house stays warm, pinch out side-shoots and feed once the fruit swells. Here is the year in order, with a table for the problems you are most likely to meet.
- Sow late February to mid-March at about 18 °C, and plant out once the greenhouse stays reliably above 16 °C.
- Water evenly, feed with potash. Keep the compost evenly moist and, in pots, feed every 10 to 14 days once the first fruits swell.
- Most fruit problems are water, heat and light. The table below gives the cause and the first step for each.
What are the common problems growing tomatoes in a greenhouse?
Blossom end rot, splitting and patchy ripening from uneven watering; hard green shoulders from heat and strong sun; yellow leaves from magnesium shortage; and blight, whitefly and red spider mite. Check what you see against the table.
| What you see | Likely cause | First step | More help |
|---|---|---|---|
| A dark, sunken patch at the flower end of the fruit | Blossom end rot: calcium not reaching the fruit, because the water supply is uneven | Keep the compost evenly moist; in hot weather water twice a day rather than once with double the volume | RHS: blossom end rot |
| Fruit splitting, often in late summer | Moisture that rises and falls, sometimes swings in temperature | Water little and often, never in floods after a dry spell | RHS: splitting |
| Hard green shoulders, or blotchy fruit with white tissue inside | Greenback or whitewall: strong light, high temperatures or too little feed | Ventilate and shade, feed regularly, and next year grow a greenback-resistant variety | RHS: ripening problems |
| Older leaves yellow between green veins | Magnesium shortage, often after heavy potash feeding | On the oldest leaves only, no cause for concern; if it spreads, an Epsom salts spray at 20 g per litre on a dull day | RHS: nutrient deficiencies |
| Leaves curling, often after cold nights | Night temperatures that swing, or aphids | Look for aphids; keep nights steadier. The RHS says it rarely costs crop and needs no spray | RHS: leaf problems |
| Flowers but little fruit | Poor pollination | Open the vents for insects; tap the open flowers | Fruit not setting |
| A fast, watery rot of leaves, brown patches on stems and green fruit | Tomato blight, less likely under glass | Use the fruit quickly; bin or burn infected growth, never compost it | Our blight guide |
| Tiny white insects that fly up, sticky leaves | Glasshouse whitefly | Check the undersides of leaves; act early | Our whitefly guide |
| Fine pale speckles, webbing in bad attacks | Glasshouse red spider mite | Damp down; predatory mites | Our red spider mite guide |
Blossom end rot cannot be undone on a fruit that has it, the RHS says, but steadier watering protects the fruit that follows. The same habit, compost kept evenly moist and never flooded after it has dried out, prevents most splitting and patchy ripening too. Change one thing at a time, so you can tell what made the difference. For a photo guide to more symptoms, see our tomato problem diagnostic.
When should you sow and plant greenhouse tomatoes?
Sow from late February to mid-March for a greenhouse crop, at about 18 °C. Plant out once the temperature is reliably above 16 °C, usually in early summer, 45 to 60 cm apart.
The RHS tomato guide gives those dates for plants that will crop under glass. Seeds need warmth to germinate, so a windowsill indoors or a heated propagator does the early work; the greenhouse takes over when its nights stay warm. Space the plants 45 to 60 cm apart, depending on their eventual size, or plant one in a 30 to 45 cm pot and two in a standard growing bag.
Which kind suits a greenhouse?
Cordons, for most greenhouses. They grow tall on one stem and use the height, while bush types stay compact for small spaces and pots.
Cordons are vigorous and grow very tall, so they need tall supports: canes, or strings tied to the roof. Bush varieties are naturally more compact and suit a small greenhouse or a row of pots along the staging; their side-shoots are left to grow. Set cordons where the roof is highest.
How do you pinch out and stop greenhouse tomatoes?
On cordons, remove the side-shoots that grow between the leaves and the main stem, and stop the plant once it has set seven trusses under glass.
Check for side-shoots each time you water, and pinch them out or snap them off while they are small, so the plant grows on one stem. Once a greenhouse plant has set seven fruit trusses, remove the growing point two leaves above the top truss, the RHS advises.
The RHS keeps removing leaves to a minimum: if you do take any off, go no higher than the lowest truss.
How often should you water and feed greenhouse tomatoes?
Keep the compost evenly moist, daily for pots in hot weather. In containers, feed every 10 to 14 days with a high-potassium liquid feed once the first fruits start to swell.
Even moisture is the single habit that matters most. Plants in pots or grow bags have little root room, which is why the RHS puts them most at risk of blossom end rot; in a hot spell they may need water twice a day. Feed only onto moist compost, because feed added to dry soil restricts the water the plant can take up.
Potassium helps the fruit colour evenly, but too much of it can bring on magnesium deficiency and blossom end rot, so follow the label rather than doubling the dose. Plants in rich border soil may need a feed only every fortnight.
How do you get greenhouse tomatoes to set fruit?
Let the insects in and help the pollen move. Open the vents regularly, tap or shake the flowers when they are fully open, and mist them lightly.
A greenhouse kept shut keeps out the insects that pollinate tomatoes. The RHS suggests opening the vents regularly, tapping the open flowers to shed pollen and misting them with water. Our guide to fruit that isn’t setting covers the other causes.
What do hot days do to greenhouse tomatoes?
Strong sun and high temperatures cause greenback and blotchy ripening. Ventilate, use shade netting or white greenhouse paint, and avoid extremes.
Tomatoes need warmth and will not thrive below 12 °C, the RHS notes, but too much heat and light causes its own problems. It links greenback and whitewall to excess light, high temperatures and too little feed, and advises controlling temperature and sunlight to avoid extremes: ventilation, shade netting and white greenhouse paint in summer. It names ‘Alicante’, ‘Shirley’ and ‘Craigella’ among greenback-resistant varieties. A maximum-minimum thermometer among the plants shows the extremes they actually meet. Our guide to keeping a greenhouse cool goes through shading and airflow.
Both our greenhouses let air through from end to end. Plot 3.14 has a door at each end whose top half opens on its own, with optional roof vents; NORDIC has an opening upper door section and a rear window, with an optional automatic vent that opens at 18 °C and is fully open by 25 °C.
What do you do with green tomatoes in autumn?
Pick them with the stalk on and ripen them indoors: a drawer with a banana helps, the RHS says.
Tomatoes ripen from midsummer onwards; pick each one as soon as it is fully coloured. When the nights turn cold, bring the unripe fruit indoors to finish. Our guide to ripening end-of-season tomatoes has the timing and the tricks.
Quick answers about greenhouse tomatoes
Usually because the moisture rises and falls, often in late summer. Water little and often, and avoid flooding a dry pot.
Calcium cannot reach the fruit when watering is uneven, the RHS explains. It is rarely missing from the compost; steady watering protects the fruit that follows.
The RHS stops greenhouse cordons once they have set seven trusses, removing the tip two leaves above the top one.
Not always. Open the vents for insects; tapping or misting the open flowers helps when the greenhouse is kept closed.
What should you do each week in summer?
Water to keep the compost evenly moist, pinch out side-shoots, feed on schedule, check the undersides of leaves and ventilate on warm days.
Five jobs that prevent most problems
Tick them off as you go. Your ticks stay on this device.
Room for a summer of tomatoes
Both let air through from end to end: NORDIC through its opening door section and rear window, Plot 3.14 through two stable doors.
- Width
- 2.6 m walk-in
- Lengths
- 2 to 6 m, 8 × 6 to 8 × 20 ft
- Ridge
- 2.3 m, eaves 1.51 m
- Frame
- 45 × 45 mm FSC pine, galvanised brackets
- Glazing
- 4 mm twin-wall, 6 mm option
- Air
- opening door top and rear window
- Width
- 3.00 m on the ground, 3.14 m at the widest
- Lengths
- 4, 6, 8, 10 or 12 m
- Ridge
- 2.02 m
- Glazing
- 4 mm twin-wall, 6 mm option
- Air
- two gable doors, upper halves open
- Roof vents
- optional, automatic openers
From the product pages, 20 October 2026. Staging, watering and the base are arranged separately.
See the greenhouse built for airflow
NORDIC has an opening upper door section and a rear window as standard, and an automatic vent if you want one. An expert can help you choose the size.
What we checked for this guide
Every date, dose and cause above links to the RHS page it comes from. Checked 20 October 2026.
Related guides
Written by the Waldenhaus team. We sell outdoor saunas and greenhouses and write these guides from manufacturer manuals, official guidance and our own product pages. Where your equipment manual says something different, follow the manual.