How to Grow Tomatoes: A Complete Guide from Seed to Harvest

Choosing tomato varieties for a short season

Variety choice matters more in zone 4 than any other decision you make. A 95-day beefsteak will simply not ripen outdoors in Minnesota, Manitoba or the Scottish Highlands, no matter how well you tend it. Start by deciding where the plant will live, then pick a variety that fits the number of frost-free days you actually get.

Determinate or indeterminate?

Indeterminate varieties grow until frost stops them. They set truss after truss on a single leader and can reach 2.5 m (8 ft) under glass. They give the most fruit per square metre, but they need staking and weekly pruning. Determinate varieties, sometimes sold as bush tomatoes, grow to a fixed size, set most of their fruit within a two to three week window, then stop. That concentrated harvest is an advantage in zone 3 and 4: everything ripens before the first frost instead of half of it hanging green in September.

There is also a semi-determinate group and a set of dwarf and trailing types bred for hanging baskets. They tolerate neglect, need no support, and are the safest choice for a first attempt on a balcony.

Read the days to maturity number

Days to maturity on a seed packet counts from transplant, not from sowing. Under 60 days is very early, 60–70 early, 71–80 mid-season and over 80 late. Match the number to your frost-free season with two weeks of margin, because cool nights slow ripening well beyond what the packet assumes.

Hybrid or open-pollinated?

F1 hybrids are crosses between two parent lines. They tend to be more uniform, more resistant to verticillium and fusarium wilt, and higher yielding. You cannot save usable seed from them. Open-pollinated and heirloom varieties come true from seed year after year and usually taste better, but they are less disease resistant and crop less evenly. A sensible plan in a cold region is one reliable early hybrid as insurance plus two heirlooms for flavour. There is a fuller comparison in the guide to the best tomato varieties for cool climates.

When should you start tomato seeds?

The right sowing date is the one that gives you a transplant-ready plant on the day the weather is ready for it. Work backwards. A tomato seedling needs 6–8 weeks from sowing to transplant size. Find your average last frost date, subtract eight weeks, and that is your earliest sensible sowing day.

In zone 5 with a last frost around 20 April, that means sowing in the last week of February. In zone 4 with a last frost around 10 May, sow in mid-March. In zone 3, where frost can linger into the first week of June, mid to late March is early enough. Growing under cover shifts everything two to three weeks earlier, because a polytunnel or greenhouse buys you that much protection.

The cost of sowing too early

Starting in January without supplemental lighting is the most common mistake in northern gardens. A north-latitude window in midwinter delivers a fraction of the light a tomato wants, so the plant stretches. You end up with pale, floppy stems and a plant that is root-bound in its pot by the time the ground is ready. A seedling sown three weeks later under a decent lamp overtakes it by mid-June. Without grow lights, wait until eight weeks before your last frost and no earlier.

Sow in two batches

Split your sowing into two rounds ten days apart. Damping-off, a cold draught or a missed watering can wipe out a tray, and in a short season there is no time to start again from scratch. The second batch costs fifteen minutes and saves the year more often than you would expect. Timing details by region are covered in the companion guide on when to sow tomato seeds.

Sowing tomato seeds step by step

1. Fill a seed tray or small pots with moist, sterile seed-starting mix. Seed mix is deliberately low in nutrients; rich potting soil produces fast, weak growth. Firm the surface lightly.
2. Space seeds 2 cm (3/4 in) apart. Cover with 0.5–1 cm (1/4–3/8 in) of sifted mix or vermiculite. Deeper than that delays emergence and invites rot.
3. Water from below by standing the tray in lukewarm water until the surface darkens. Watering from above washes seeds out of position.
4. Cover with a humidity dome or clear bag and keep at 20–25 °C (68–77 °F). A heat mat set to 24 °C (75 °F) roughly halves germination time. Light is not needed until the seeds sprout.
5. Check daily. Seed leaves break the surface in 5–10 days. Remove the dome as soon as most seeds are up — a forgotten cover causes damping-off within a day.
6. Move straight under lights and drop the temperature to 18–20 °C (64–68 °F). This is the one step where being a day late shows all summer.
7. Pot on into individual 8–10 cm (3–4 in) pots when the first pair of true leaves is fully open, usually 12–18 days after sowing. Hold a leaf, never the stem, and set each seedling deeper than it was growing.
8. Turn the pots a half turn every second day if they sit in a window, so the plants grow straight rather than leaning.

Raising strong seedlings: light, potting on and hardening off

Light is the limiting factor

Tomato seedlings want 14–16 hours of light a day at roughly 15,000–20,000 lux. A bright south-facing window in March at 55° north delivers perhaps a third of that at midday, and far less on a grey week. A 20–30 W LED grow light over a standard tray fixes it. Hang the fixture 15–25 cm (6–10 in) above the leaves and run it on a timer.

Insufficient light shows immediately: long gaps between leaves, pale stems and plants that lean. Sufficient light shows just as clearly — short internodes, dark green leaves and a stem as thick as a pencil by transplant time.

Temperature and air movement

After germination, aim for 18–20 °C (64–68 °F) by day and 15–17 °C (59–63 °F) at night. The night drop keeps plants compact. Run a small fan on low for a few hours a day, or brush your hand across the tops once daily. That mechanical stress thickens stems and makes hardening off far easier.

Potting on

Pot on twice before planting out. First from the tray into an 8–10 cm (3–4 in) pot at the true-leaf stage. Then into a 1–2 L (1–2 quart) pot about three weeks later, when roots show at the drainage holes. Bury the stem deeper each time. Tomatoes root along buried stem tissue, so a seedling set 5 cm (2 in) lower ends up with a noticeably larger root system.

Hardening off

Hardening off is the week that decides whether the transplant sulks or takes off. Put the plants outside in shade for two hours on day one, four on day two, and build up to full days over a week. Keep them out of direct midday sun for the first three days — unadapted leaves scorch in fifteen minutes. After seven to ten days they can stay out overnight as long as it holds above 10 °C (50 °F). Skipping this costs two to three weeks of growth to transplant shock, which in zone 4 is the difference between a crop and a disappointment.

Planting out: ground, raised bed or container

Under cover or in the open?

An unheated greenhouse or polytunnel extends the season by four to six weeks at each end and roughly doubles yield compared with open ground in the same zone. If you have no structure, a cold frame, low tunnel or a south-facing wall does much of the same job in zones 5 and 6. In zone 3 some form of protection is close to essential unless you are growing 55-day cherry tomatoes.

Soil, spacing and depth

Tomatoes want fertile, free-draining soil at pH 6.0–6.8. Dig in 2–3 L (2–3 quarts) of well-rotted manure or mature compost per planting hole. Avoid fresh manure and high-nitrogen fertiliser at planting: too much nitrogen produces lush foliage and very little fruit.

Space indeterminate plants 45–60 cm (18–24 in) apart with 90–120 cm (36–48 in) between rows. Determinate types manage at 45–50 cm (18–20 in). Any tighter and air stops moving between plants, which is the single biggest risk factor for late blight and grey mould in a damp northern summer.

Plant deep. Strip the lower leaves and set the plant so that only the top 15–20 cm (6–8 in) stands above the soil. If a seedling is tall and leggy, lay it almost horizontally in a shallow trench instead and bend the tip upward. The entire buried stem roots within two weeks, which gives a drought-resistant plant.

Warm the soil first

Soil temperature matters as much as air temperature. Tomato roots barely function below 13 °C (55 °F). Lay black plastic or landscape fabric over the bed two weeks before planting to lift soil temperature by 3–5 °C (5–9 °F). A wall of water, a cloche or an upturned bucket on cold nights gives another week or two of head start in zone 3 and 4.

Support from day one

Put the support in before the plant, never after — a stake driven in beside an established plant cuts roots. Indeterminate plants under cover are easiest on a string dropped from an overhead wire; wind the stem clockwise round it once a week. Outdoors use a stake at least 150 cm (5 ft) above ground, or a sturdy cage of 10–15 cm (4–6 in) mesh, and tie with soft ties in a figure of eight every 25 cm (10 in).

Containers need to be larger than most people expect: 20–30 L (5–8 gal) for an indeterminate plant and at least 10–15 L (3–4 gal) for a bush type. Container watering and feeding follow their own rules, set out in the guide to growing tomatoes in pots.

How much water and fertiliser do tomatoes need?

Water: consistency beats volume

A mature plant under cover in July uses 1.5–2.5 L (0.4–0.7 gal) a day. Outdoors in cooler air, 1 L (1 quart) is often enough. The exact volume matters less than the rhythm. Alternating between bone-dry soil and a soaking is the direct cause of both blossom end rot and split fruit.

Water in the morning, water the soil and never the foliage, and give a thorough soak every other day rather than a splash daily. A 5 cm (2 in) mulch of straw, grass clippings or leaf mould halves evaporation and smooths out moisture swings. Sinking an empty pot or a bottomless bottle beside each plant delivers water straight to the root zone instead of letting it run off the surface.

Feeding in three phases

A tomato's nutrient demand changes completely across the season. As a young plant it wants nitrogen to build leaves. From first flower onward it wants potassium to build fruit. Feed the same balanced fertiliser all season and you get a large, handsome, nearly fruitless plant.

Epsom salts are cheap insurance. One tablespoon dissolved in 10 L (2.6 gal) of water once a month keeps magnesium deficiency away, and magnesium deficiency is the usual reason for mottled yellow leaves in midsummer.

Pruning, topping and removing leaves

Pruning means snapping out the side shoots, or suckers, that form in the angle between the main stem and a leaf stalk. The point is to push energy into fruit rather than into new branches. It applies to indeterminate varieties only. Never prune determinate types — the trusses you remove are the crop.

How to do it

Go over the plants once a week from late June. Snap suckers out while they are under 5 cm (2 in) long; at that size they come away cleanly with a thumb and the wound seals in a day. Larger shoots need snips, and the blades should be wiped with rubbing alcohol for at least 30 seconds between plants so you do not spread virus or fungal spores. Prune in the morning when the plant is turgid, and pick a dry day.

The one sucker worth keeping is the one just below the lowest flower truss. Left in place it becomes a second leader and adds trusses, which works well under glass where light is plentiful. Two-leader training and the finer points are covered in the guide to pruning tomato suckers.

Topping at the right time

Topping means cutting the growing tip two leaves above the highest truss you expect to harvest. The arithmetic is simple: a truss needs roughly six to eight weeks from flower to ripe fruit. So top in mid-July in zone 3 and 4, and in early August in zone 5 and 6. Trusses that flower later will never finish, and they steal sugars from the ones that could.

Removing lower leaves

Strip the lowest leaves as the trusses beneath them colour up. It improves airflow, cuts blight risk and makes watering easier. Take no more than three leaves per plant per week and always leave at least twelve working leaves — the foliage is the factory that makes sugar for the fruit.

Flowering, pollination and fruit set

Tomatoes are self-fertile. Each flower carries both anthers and stigma, and the pollen falls when the flower is shaken. Outdoors, wind and bumblebees handle it. Inside a closed greenhouse the air is still, so flowers drop without setting even when everything else looks healthy.

The fix takes ten seconds: flick each truss with a finger, or shake the support string, in the middle of a dry day. Repeat every second day through flowering. An electric toothbrush held against the stake does the same job and is what commercial growers use.

Pollen becomes sterile above 32 °C (90 °F) and stops shedding below 13 °C (55 °F). Blossom drop during a heatwave is not your fault, it is physics. Ventilate from early morning, shade the glass if it runs above 30 °C (86 °F) for several days, and leave the door open at night during hot spells. In zone 3, the opposite problem is more common: cold June nights below 13 °C (55 °F) stall the first truss, which is another argument for a cloche.

How many trusses will actually ripen?

Season length sets the realistic truss count. Under an unheated tunnel in zone 5 or 6, an indeterminate plant finishes six to eight trusses. In zone 4, four to six. Outdoors in zone 5 expect three or four; in zone 3, two or three. Decide the number before the season starts and top the plant when you reach it. A plant carrying ten half-ripe trusses yields less edible fruit than one carrying five finished ones.

Thin the trusses if you want large fruit. On beefsteaks, snip out the three or four smallest fruitlets in each cluster. The remaining tomatoes end up noticeably bigger and ripen sooner, because the same amount of sugar is split between fewer fruits.

Why does my tomato plant have brown spots and yellow leaves?

Almost every tomato problem can be read off the plant. Here are the seven you are most likely to meet in a cool, damp climate, with the symptom and the fix.

Blossom end rot

A dark, sunken, leathery patch at the base of the fruit. It is not a disease but a calcium transport failure caused by uneven watering — the calcium is in the soil but does not reach the fruit when the plant dries out. Fix the watering rhythm, mulch, and cut back on nitrogen, which competes with calcium uptake. Affected fruit never recovers, so remove it.

Late blight

Grey-green patches that turn brown fast on leaves and stems, often with a pale fuzz on the leaf underside in humid weather, and hard brown marks in the fruit. Phytophthora infestans travels on the wind from potato crops and strikes after warm, wet spells in July and August. Prevent it with space between plants, a roof over outdoor plants and soil-level watering only. Infected plants cannot be saved — bag them for municipal waste, not the compost heap.

Magnesium deficiency

Yellow, marbled patches between the veins of the oldest leaves while the veins stay green. Common in July when fruiting pulls magnesium out of the foliage. Treat with Epsom salts, one tablespoon per 10 L (2.6 gal), as a foliar spray or in the watering can.

Leaf roll

Leaves curl upward into tubes. It looks alarming and is usually harmless — a response to wide day-to-night temperature swings or to overfeeding. Ventilate more evenly and water with plain water for a week if you have been feeding hard.

Cracked and split fruit

Rings or vertical splits around the stem end. They happen when a dry spell is followed by heavy rain and the flesh swells faster than the skin. Keep watering steady and pick fruit just short of full colour during wet weeks.

Whitefly

Tiny white insects that rise in a cloud when you brush the plant, sticky leaves and sooty mould. Use yellow sticky traps to monitor, spray weekly with insecticidal soap, and introduce the parasitic wasp Encarsia formosa under cover.

Grey mould

Fuzzy grey-brown growth on stems, pruning wounds and fruit. Botrytis cinerea thrives in high humidity and stagnant air, which is exactly what a closed tunnel gives on a cool northern night. Ventilate daily even when it is chilly, prune on dry days and clear plant debris off the floor. The same logic applies to most greenhouse crops, though anyone who also wants to grow cucumbers in the same house has to compromise, since cucumbers prefer the humidity that tomatoes hate.

Tomato planting calendar for USDA zones 3–6

The dates below are guidelines built on average last and first frost dates. Local variation is large, so check a ten-day forecast before you plant out and keep row cover within reach.

In zone 3 a greenhouse, polytunnel or at least a low tunnel is close to a requirement. Choose cherry varieties under 65 days, plant into black grow bags that store daytime heat, and stand a filled water barrel inside the structure to buffer night temperatures.

Harvesting, ripening and storing

A tomato is ripe when the colour runs evenly down to the shoulders and the fruit yields slightly to gentle pressure. Twist it off with the calyx attached; fruit picked with its stalk keeps several days longer. Harvest every second day at peak. Overripe fruit left hanging tells the plant it can stop producing.

Realistic yields: an indeterminate plant in an unheated tunnel gives 4–6 kg (9–13 lb) over a season, a bush plant in a 15 L (4 gal) pot 1.5–3 kg (3–7 lb), and an outdoor plant in zone 4 or 5 roughly 1–2.5 kg (2–5.5 lb). The gap between a well-tended plant and a neglected one is nearly always watering rather than variety.

Never refrigerate a fresh tomato

Below 12 °C (54 °F) the volatile compounds that carry tomato flavour break down, and the texture turns mealy. The damage is permanent. Keep fruit at room temperature, stem scar down, on a tray with the tomatoes not touching. A ripe tomato holds five to seven days that way.

Ripening green tomatoes indoors

Every northern season ends with green fruit on the vine. Any tomato that has reached full size and started to change colour will finish indoors. Put them in a paper bag or cardboard box with one ripe apple. The apple releases ethylene, which triggers ripening, and the process takes one to three weeks at 18–20 °C (64–68 °F). Small, hard, dark green fruit will never ripen — turn those into chutney or pickles instead.

Freezing and drying

Whole tomatoes freeze as they are. The skins split on thawing and slip off with your fingers, which is the easiest way to peel a tomato there is. They work in sauces and stews but not in salad. Halved cherry tomatoes dried in an oven at 70 °C (160 °F) for six to eight hours keep for months in a jar of olive oil in the fridge.

Saving your own tomato seed

One ripe fruit of an open-pollinated variety yields 50–150 seeds that stay viable for four to six years. It does not work on F1 hybrids, whose offspring split back into the parent traits.

1. Choose a fully ripe, blemish-free fruit from the plant that cropped best and tasted best.
2. Cut it open and squeeze the seeds and surrounding gel into a jar. Add two tablespoons of water.
3. Leave at room temperature for three to five days. A white mould film forms on the surface. That is intended — fermentation dissolves the germination-inhibiting gel coat and kills several seed-borne diseases.
4. Top up with water and stir. Viable seed sinks; empty hulls and pulp float. Pour off everything that floats.
5. Rinse in a sieve and spread the seed on a ceramic plate or baking parchment. Do not use paper towel — the seeds glue themselves to it.
6. Dry at room temperature for one to two weeks, until a seed snaps rather than bends. Store cool, dark and dry in labelled paper envelopes with the variety and year.

Common mistakes

• Sowing in January with no grow light. The seedling stretches before the daylight arrives. Either buy a lamp or wait until eight weeks before your last frost. There is no third option in the north.
• Undersized containers. An indeterminate plant in a 10 L (3 gal) pot dries out every afternoon in July and gets blossom end rot. Use 20–30 L (5–8 gal).
• Erratic watering. Two dry days followed by a flood gives you split fruit and rot at the same time. Mulch and water to a schedule, not to a hunch.
• Too much nitrogen. Big leaves, thick stems, almost no tomatoes. Switch to a high-potassium feed as soon as the first truss flowers.
• Planting out too early. A night at 4 °C (39 °F) does no visible damage but sets the plant back two weeks. Wait for stable nights above 10 °C (50 °F), and warm the soil with black plastic first.
• Letting suckers run for three weeks. The plant turns into a thicket, air stops moving and yield halves. One weekly pass is enough.
• Overhead watering. Wet foliage in the evening is an invitation to blight and grey mould. Water the soil, in the morning.
• Refrigerating the harvest. Flavour disappears below 12 °C (54 °F) and never returns. Counter, not fridge.

Frequently asked questions

How many tomato plants does one household need?
Allow two to three plants per person for fresh eating. A well-grown indeterminate plant under cover gives 4–6 kg (9–13 lb) a season, and a bush plant in a pot 1.5–3 kg (3–7 lb). For sauce and canning you need far more: roughly eight to ten paste-tomato plants to put up 10 L (2.6 gal) of passata.

Can you grow tomatoes indoors all year?
Yes, but it takes equipment. A tomato needs 14–16 hours at around 15,000 lux, which in practice means a 50–100 W LED fixture per plant, and you have to hand-pollinate. Choose a compact determinate variety and expect a smaller crop than outdoors. Most people find lettuce and herbs a better use of winter light.

Why is my tomato plant flowering but not setting fruit?
Three causes cover almost every case. Temperature above 32 °C (90 °F) or below 13 °C (55 °F) makes the pollen unusable. Still air in a closed greenhouse means the pollen never falls. Or excess nitrogen is pushing the plant into leaf. Ventilate, flick the trusses every second day and switch to a high-potassium feed.

Do all tomato varieties need pruning?
No. Only indeterminate varieties should be pruned. Determinate and trailing types set their fruit on side shoots, so removing suckers removes the harvest. If the packet says determinate, bush, patio or dwarf, leave the plant alone and simply support branches that get heavy with fruit.

How often should I water tomatoes in containers?
Through July and August, usually daily and sometimes twice a day in heat. An indeterminate plant in a 25 L (6.5 gal) pot drinks 1.5–2.5 L (0.4–0.7 gal) a day. Push a finger 5 cm (2 in) into the compost: if it feels dry, water. A saucer that is allowed to dry out between waterings evens out the swings.

Can tomatoes grow in a north-facing spot?
It is difficult. Tomatoes need six hours of direct sun and a true north aspect rarely provides two. If the site catches morning or evening sun you can crop an early cherry variety, but expect roughly half the fruit and two to three weeks later ripening than a south-facing site.

Can I plant tomatoes in the same place every year?
Many greenhouse growers do, but the risk of soil-borne problems such as verticillium wilt and root-knot nematode climbs each season. Replace the top 20 cm (8 in) of soil every third year, or grow in bags and containers you empty annually. Outdoors, rotate on a three-year cycle and never follow potatoes with tomatoes.

When should I stop feeding tomato plants?
Cut back to half strength from late August and stop entirely two to three weeks before you plan to clear the plants. Late feeding pushes new shoots that cannot ripen anything before frost, and it can delay the trusses that still have a chance of finishing.

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