Growing Tomatoes Indoors

How to grow tomatoes

Tomatoes are a warm-season crop originating in the New World. In Britain, they are normally started as transplants in the late winter or early spring, after which they are grown in the summer and autumn inside a purpose-built structure such as a greenhouse or polytunnel.

Tomato plants have two very distinct growth habits, indeterminate and determinate. How a plant is managed is fundamentally affected by which growth habit it displays:

These varieties have a bush-like growth habit and do not need to be pruned. They require little or no support, and their fruit ripen over a concentrated time period.  Plants of some determinate varieties are especially small, and these are ideal for growing in hanging baskets as edible ornamentals.

These varieties are just the opposite to determinate types. The plants need to be pruned and trained, and given the right conditions can grow to be very tall ­­­– the Guinness world record is held by one that reached an astonishing 19.8 metres.  Additionally, fruit ripen over an extended period of time, making them perfect for growing over a long season.

Producing transplants

As a general rule of thumb, it takes 7 to 8 weeks from seed sowing to transplanting tomatoes into a tunnel or greenhouse. The seed should be sown from mid-February to mid-March into a fine-textured, lump-free compost and covered to a depth of 6mm. Seeds can be scattered thinly in flat trays or, alternatively, sown individually into the cells of modular trays. The compost should be kept at a temperature of 24˚ to 25˚C, though both higher (up to 29ºC) and lower (down to 20ºC) temperatures should give satisfactory results.

At the higher temperatures, good quality seeds will begin to germinate 3 to 4 days after sowing. Though germination can be started either in the dark or light, the seedlings must be exposed to light as soon as they emerge from the compost to prevent long, spindly growth. Natural light is ideal and can be provided by keeping the seedlings in a propagator in a greenhouse or tunnel. However, if you plan on raising the seedlings in the house, then artificial light should be provided.

After germination is complete, the temperature is managed differently to help seedling growth.  On bright sunny days, air temperature should be kept at about 20˚ to 21˚C, though it can be allowed to go as high as 24˚C. The temperature can be dropped slightly on dull, overcast days and should be reduced to 15˚ or 16˚C at night.

When the first true leaves appear, the seedlings started in flat trays should be pricked out and transplanted to compost-filled pots 7 to 8cm in diameter.  Alternatively, those in modular trays  can be transferred, compost and all, into pots when the roots just fill the cells. The seedlings should be planted deeply, almost up to their cotyledons (seed leaves). The young plants then need to be watered regularly and given a complete liquid feed at least once as they their roots fill the pot and use up the nutrients.  Compost temperature should match the air temperature, so any water and liquid feed given to the plants must not be too cold.



Transplanting into an unheated tunnel or greenhouse can begin in mid-April down south, later up north and at high elevations. Ideally, the transplants will have roots that just fill their pot; straggly, root-bound plants with open flowers or fruit should be avoided.

With its reserves of water and nutrients, the ground inside a tunnel or greenhouse (or ‘border soil’ as it is often called) is the best place to grow tomatoes. Add plenty of organic matter, and if fertility levels are low, incorporate a solid complete fertiliser just before planting. Leave about 45cm between plants and water regularly. As the nutrients in the soil are used up, they should be replenished by regularly fertilising with a liquid tomato feed that is high in potassium.

Tomatoes can also be grown very successfully in containers such as grow bags and compost-filled pots. If pots are used, they should be at least 30 cm in diameter, while grow bags can have two or three plants in each one. Extra attention must be paid to watering and fertilising plants in containers since they will run out of nutrients and dry out quicker than those in the ground. Watering, for example, might have to be done twice a day when the weather is hot and sunny.

Managing growth

Once transplanting is complete, determinate varieties can be left to grow and develop on their own, especially those with a compact growth habit. Indeterminate varieties, however, are another matter and need regular care and attention if they are to produce a decent crop.

Supporting the main stem: The central or main stem of an indeterminate tomato needs to be supported so that it grows upright. One of the easiest ways to provide this support is to push a 60cm piece of bamboo cane into the ground (or compost in the case of pots) next to each plant. String is then suspended from the top of the structure and tied to the cane. As the main stem grows, twist it around the string, being careful not to break off the shoot tip, leaves and trusses. If using grow bags, tie the string around the bag, using one string for each plant.

Pinching out side shoots:  Side shoots naturally develop on the plant at the point where the leaves are attached to the main stem. These side shoots should be pinched out by hand when they are no bigger than 4 to 5cm long.

 Removing lower leaves: As leaves get old they contribute less and less to the growth of the plant. In addition, their physical presence reduces air movement, which increases the likelihood of blight and botrytis. Therefore, we recommend regularly – and ruthlessly – removing the lower leaves from the main stem.

Stopping the main stem: ‘Stopping’ is the practice of removing the growing tip of the tomato’s main stem. Apparently, its purpose is to stop further trusses from developing while giving those already formed the best opportunity to ripen before the end of the season. Mainstream gardening advice recommends stopping in mid-July to mid-August when the plants have 6 to 8 trusses. If you decide to do this, cut the main stem two leaves above the uppermost truss you are leaving.

To be honest, we do not ‘stop’ our tomato plants, and have seen no evidence that allowing new trusses to develop hinders the ripening of older ones. In addition, it is the younger leaves that conduct most of the photosynthesis, and taking out the tip means that no new leaves are produced. If you feel a need to limit the number of trusses on a plant, a less drastic option might be to cut off those you don’t want, while allowing the main stem to continue growing.

Venting greenhouses and tunnels

 Daytime temperatures over 30ºC reduce plant vigour, yield and fruit quality. It is, therefore, important to vent on hot days, and if that isn’t enough, coat the inside glass of a greenhouse with whitewash to help shade the plants.



Harvesting will begin more or less in July. For the best flavour, allow the fruits to fully ripen on the plant before picking, then keep them at room temperature rather than in the refrigerator.


Pests, diseases and other problems

Tomatoes are subject to dozens of problems that can affect plant growth and fruit yields. Fortunately, only a relatively small number cause any serious problems, and these are easily prevented or cured:

Blossom end rot: Symptom is a black sunken patch on the fruit caused by a calcium deficiency in the fruit. Exacerbated by insufficient watering, it occurs mostly in early fruit and should eventually disappear in those maturing later in the season.     

Greenback: A physiological disorder that causes fruit to remain hard and unripe around the stem. It is caused by exposure to excessive heat, too much sunlight or an inadequate supply of potassium. Older varieties such as Ailsa Craig are especially susceptible. Grow resistant varieties, vent to keep to the temperature down, and whitewash the inside of greenhouse in the summer. Feeding with a high potassium tomato fertiliser might also help control greenback.

Cracking or splitting fruit: Longitudinal cracks on the ripening fruit, due to uneven watering, though some varieties seem more prone to cracking than others.  

Soil-borne diseases: Diseases in the soil can build up from growing members of the Solanacae family (aubergines, peppers, tomatoes and potatoes) in the same spot year after year. It might be worth using a four-year rotation, though this might be difficult in a small greenhouse or tunnel. You could also change the soil periodically, but that would be expensive and time consuming. Alternatively, the use of containers would avoid the problem altogether.

Late blight: Late blight is a fungal disease that affects both tomatoes and potatoes. The early symptoms are brownish spots on the leaves and stems and brown areas developing on the fruit. It is more of a problem on outdoor tomatoes, though those inside can be infected if the weather is cool and cloudy. The varieties Ferline and Red Alert may have some resistance. Good air movement within a tunnel or greenhouse will reduce risk infection, and if symptoms occur the removal of all but the healthiest leaves will help.  

Botrytis or grey mould: A fungus disease causing grey fuzzy patches on the plant and small white circles on the fruit, It often appears in the autumn, usually starting in a damaged part of the stem. Prevention is the best cure, so keep the structure clear of dead and dying plant material. Ventilate to promote air movement and reduce humidity, and be sure to make a neat, clean break when pruning out side shoots and taking off leaves.

Aphids, spider mites and whiteflies: These are all sucking pests that can cause considerable damage to the tomato plant. If aphids or red spider mite occur on just a few leaves simply cut these out. Alternatively, they can all be controlled biologically, and for excellent information about these pests and their ‘natural’ control, go to: