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Supporting and Training Greenhouse Tomatoes

It's crucial to plan and prepare in advance if you are growing tomatoes in a greenhouse. Using the previous page, you will already have decided the method for growing tomatoes.

The next decision is when you want your tomatoes to be ready for eating. That, and the type of greenhouse you have, will determine when you should sow the seeds.
Indeterminate / vine / cordon varieties require support and training and this needs to be in place before the plants are placed in their final position.



Many factors influence the growing of tomatoes in a greenhouse. These include the weather conditions in your area, how much light gets to the greenhouse and how much heat (if any) is available.

This article will assume (see assumptions page for this entire article) that you have a greenhouse with sufficient heat to avoid any frost from December onwards. If your greenhouse is "cold" (i.e. no heat is available) or "warm" (a minimum temperature of 16°C / 60°F can be maintained) then you will need to adjust the dates given below. Bad light availability will also affect the dates given below.

If you have a "warm" greenhouse then all activities can occur one month earlier than stated below. If you have a cold greenhouse then all activities should occur one two months later. Tomatoes will not tolerate any frost whatsoever.


as it grows.



Sow tomato seed in trays (see note 1 below). The best temperature for germination tomato seed is 18°C /65°F. See note 2 below. Mid mid-February Indoors or wherever a constant temperature can be maintained.

Prick off into 12cm / 4½in pots filled with good quality multipurpose compost. Lightly water the pots. See note 3 below. This should occur about 12 days after sowing seed when the seedlings are just large enough to handle. Pricked off seedlings should be moved to a frost free greenhouse.

Watering should occur when needed to keep the compost slightly moist. Whenever needed.  

Feed with general purpose liquid fertiliser. Start feeding four weeks after pricking off. Little and often is the best feeding regime. The cooler the temperature, the less feed is required.  

Temperature control from pricking off up to planting out. The reasonable, economical temperature range is from 14°C /57°F at night to 18°C /64°F at night. See note 4.  


Note 1 - Sowing greenhouse tomato seeds

Ensure the compost is moist then sow tomato seed on the surface of the lightly firmed down compost. Sprinkle on the compost so that the seeds are just covered then lightly firm down the surface. Cover the trays with paper or similar to block out the light.

Note 2 - Germination of  greenhouse tomato seeds

Where sowing lots of tomato seeds, they are best sown in trays because this reduces the amount of heated space they occupy. They need a temperature of about 18°C /65°F and this cannot be maintained in an unheated greenhouse during winter and early spring. If only sowing a few seeds then they can be sown directly into 12cm / 4½in pots.
Plastic pots are best. They can easily be cleaned after use, ready for next year.

Note 3 - Pricking out greenhouse tomato seedlings

To prick out seedlings, gently ease up the soil directly below them with something like a small plant marker. Gently ease the seedling (with any roots) out of the tray holding the leaves but not the stem which can very easily be damaged.

Note 4 - Greenhouse temperatures from pricking out to planting out.

The economical, ideal temperatures for growing tomato seedlings after pricking out is given in the table above. However, many, many factors affect what temperatures you grow your own tomatoes at between pricking out and planting in their final position. Factors such as available heating, levels of sunlight, what temperature zone you are in all have an influence on temperatures.

However, there are some definite "dos" and "donts" at this stage of growing tomatoes. These are:

1 Don't let the temperature fall below 4°C / 41°F. Tomatoes plants are killed by the cold.

2 Do keep greenhouse glass clean to allow maximum light transmission.

3 Do ventilate the greenhouse immediately temperatures reach 22°C / 72°F. These temperatures can be reached surprisingly early in the year. High temperatures often occur at the same time as high levels of humidity, both can cause damage to young tomato plants.

4 If temperatures are kept low, then reduce feeding and watering.