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.
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
Indeterminate / vine / cordon varieties require support and training and
this needs to be in place before the plants are placed in their final
WHEN TO SOW TOMATO SEED FOR GREENHOUSES
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.
TIMINGS FOR SOWING GREENHOUSE TOMATO SEED
WHEN / HOW LONG?
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.
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
Watering should occur when
needed to keep the compost slightly moist.
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
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
Note 3 - Pricking out greenhouse tomato
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:
Don't let the temperature fall below 4°C / 41°F. Tomatoes plants
are killed by the cold.
keep greenhouse glass clean to allow maximum light transmission.
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
temperatures are kept low, then reduce feeding and watering.
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