Most greenhouse tomato failures trace back to two jobs done at the wrong moment, not to the variety on the seed packet. Feed too early and you grow a magnificent leafy plant with little fruit. Miss the side shoots for a fortnight in June and the plant pours its energy into stems instead of trusses. Get the timing of a handful of jobs right, and a single cordon plant under glass will crop from July well into October.
Here is the full run from seed to harvest for a UK greenhouse: which type of tomato to grow, when to sow, when to plant out, the two pruning jobs that decide your crop, and the feeding and watering rhythm that keeps fruit setting. Every date and figure here follows the RHS guidance for growing under glass.
Cordon or bush: choose the plant to match your space
Tomatoes split into two growth habits, and the choice shapes every job that follows. Cordon (also called indeterminate) varieties grow tall on a single stem, need tying to a support, and want their side shoots removed all season. They take up almost no floor space, which is why they suit a greenhouse border or a row of pots. Bush (determinate) varieties sprawl, need no side-shooting, and crop in a shorter burst, better for a grow bag near the door than a tall border.
For a greenhouse the cordon earns its keep: more fruit from less floor, over a longer season. Among the varieties the RHS rates for growing under glass are 'Shirley' and 'Cristal', both holders of its Award of Garden Merit, with 'Costoluto Fiorentino' for anyone after a ribbed Italian beefsteak. If this is your first year, the cherry varieties 'Gardener's Delight' and 'Sungold' forgive the most mistakes and still taste worlds away from a supermarket punnet.
When and how to sow
Sow from late February to mid-March if the plants will live in a greenhouse. Tomato seed germinates at around 18°C, so a heated propagator or a warm windowsill earns its place in a cold February. Sow thinly, cover lightly, and keep the compost just moist rather than wet.
Once each seedling has a couple of true leaves, after roughly a fortnight, move it into its own pot of peat-free multi-purpose compost. Tomatoes root from their stems, so each time you pot on you can bury a little more of the stalk to build a stronger root system. Keep them in the brightest spot you have; a seedling stretching towards a grey window is telling you it wants more light, not more heat.
Planting into the greenhouse
Move plants to their final position in early summer, once the greenhouse holds reliably above 16°C, into a border, a large pot, or a grow bag. Plant deeply, with the first set of leaves just above the soil, and that buried stem will throw out extra roots within a fortnight. Space cordons about 45cm apart and put the support in now, before the roots are in the way: a cane, a string run to the roof, or a spiral support all work.
The two jobs that decide your crop: side shoots and trusses
This is where greenhouse tomatoes are won or lost, and both jobs take seconds once you can see what you are looking at. A side shoot sprouts at the joint between a leaf and the main stem, at roughly 45 degrees. Left alone it becomes a whole second plant competing for food. Pinch it out or snap it off while it is small, every time you water. It takes a second. A truss, the flowering and fruiting arm, comes straight off the main stem. That one you keep.
Keep tying the main stem to its support as it climbs, and when the plant has set seven trusses indoors, stop it: pinch out the growing tip two leaves above the top truss. That tells the plant to ripen what it has rather than chase height it cannot fill before the autumn light fades. Seven trusses is about what a cordon ripens in a UK greenhouse season.
Feeding and watering: the rhythm that sets fruit
Hold off feeding until the first fruits start to swell, then feed every 10 to 14 days with a high-potassium liquid tomato feed. Earlier feeding pushes leaf at the expense of fruit, which is the leafy-plant-no-tomatoes trap. A fruiting plant wants potassium, not the nitrogen that drives leaf.
Water to keep the compost evenly moist, which in a hot spell means a container plant may need watering daily. Consistency matters more than volume: a plant that swings between bone dry and flooded gets blossom end rot, a sunken brown patch on the base of the fruit, and splits its skins. Water at the base in the morning, never a deluge after a drought.
Pollination and the problems to watch
Tomatoes pollinate themselves, but the pollen still has to move. Open the vents on warm days so pollinating insects can reach the flowers, and on a still day tap or gently shake each truss when the flowers are fully open to shake pollen onto the stigma. Flowers that open and drop without setting usually mean heat: above roughly 30°C the pollen loses viability, which is why a greenhouse that runs too hot stops setting fruit just when summer should be delivering it.
That heat problem, and the watering one, are the two that catch most growers, so they each have their own guide: see keeping a greenhouse cool for the venting routine, and tomato blight for the late-summer disease that a dry, well-aired greenhouse largely keeps out.
Border, pots or grow bags: where to plant
Where the roots go changes how often you water and how big the plant gets. A greenhouse border gives the most root room, the steadiest moisture and the strongest plants, as long as you have not grown tomatoes in the same soil for several years running, which builds up disease. Large pots, 30cm or more across, give you control and let you move or replace a plant, at the cost of daily watering in summer. Grow bags sit somewhere between: cheap and convenient, but shallow, so two plants per standard bag is the honest limit and three is a recipe for thirsty, hungry plants.
For a first greenhouse year, large pots are the forgiving choice. You can stand them where the light is best, top up water without flooding a whole border, and start with fresh compost that carries no soil-borne trouble from last season.
Ripening and harvest
Tomatoes ripen best between about 21 and 24°C, and the colour change runs on warmth rather than direct sun, so a hot, well-fed plant in August does most of the work itself. Pick each fruit when it has coloured fully and comes away with a gentle twist, leaving the green calyx on the vine. Picking little and often through August and September tells the plant to keep filling the trusses above.
As the light fades in October the last green fruit will not ripen on the plant. Pick them and bring them indoors, into a drawer or a paper bag with one ripe banana or tomato: the ethylene the ripe fruit gives off pulls the green ones round over a week or two. A whole truss hung in a warm kitchen does the same and looks better doing it.
Common tomato problems, and what each one means
Most greenhouse tomato trouble comes back to water and heat, the same two levers as fruit set. Here are the ones you are most likely to meet.
A sunken brown patch on the bottom of the fruit
Blossom end rot causes that, and it comes down to watering, not disease. Irregular water stops the plant moving calcium to the fruit, so the base collapses. Nothing saves an affected tomato once the patch appears. Remove it, then water evenly from then on. Container plants suffer most because they dry out fastest.
Split skins
Splitting follows a heavy water after a dry spell, when the fruit swells faster than its skin can stretch. Even, consistent watering prevents it, the same habit that stops blossom end rot. Split fruit eats fine straight away, just does not store.
Fruit that will not turn red
Look to the season for the cause. In a midsummer heatwave, ripening actually stalls above roughly 30°C, so the cure is shade and ventilation, not patience. Late in the year it is simply too cold and dim, which is when you switch to ripening indoors. Hard yellow-green shoulders on otherwise ripe fruit point to heat and strong direct sun, eased by light shading.
Leaves curling upward
Usually harmless. Tomato leaves roll in response to big day-to-night temperature swings and very vigorous growth, and the plant crops normally regardless. Curling with distortion and stunting is a different matter and can mean weedkiller contamination in compost or manure, so trace your inputs if the whole plant looks wrong.
From Waldenhaus: tomatoes set fruit when the air keeps moving and the heat stays off the flowers. Our NORDIC wooden greenhouse has a rear opening window opposite the door as standard, giving the through-draught that helps both pollination and cool. The growing advice here works whatever greenhouse you have.
Your next move
If you are sowing, late February to mid-March at 18°C is the window; if your plants are already in, the jobs this week are pinching every side shoot and putting the support in before the roots spread. Start the high-potassium feed only when the first fruits swell, water evenly at the base each morning, and stop each cordon at seven trusses. Do that and the plant that looked like a thread of stem in March will be handing you 'Sungold' by the bowlful in August.
