The ABCs of Tomatoes

From sowing to harvesting your own seeds: growing in open ground, an unheated tunnel, or a greenhouse, as well as in pots and bags filled with potting mix.

Season-Long Calendar

Before planting, check the forecast: no frost, nighttime temperatures preferably at 10°C, and soil temperature at a depth of 10 cm in the morning at 12°C. In a cold spring, delay planting by 7–14 days and check again. A tunnel does not protect against frost.

Frost after planting: Water the soil the day before, as moist soil retains more heat at night. In the evening, cover the plants with a double layer of agrotextile, even inside the tunnel. This protects against light frost. In the morning, when it gets warmer, uncover the plants.

0. Choose a variety suited to your location

Check three things in the variety description:

• Location: For open ground, choose early and mid-early varieties. Late and large-fruited varieties do best under cover.
• Maturity: In cool regions with a short growing season, choose earlier varieties. A late variety planted in the ground may not have time to ripen.
• Growth habit: Tall varieties require staking and the removal of side shoots. Determinate and dwarf varieties require less work and are best suited for pots and grow bags.

For your first season, choose 2–4 varieties, including one early “cocktail” variety. Sow 30–50% more seeds than you need plants.

1. Prepare the area and sow the seeds

Prepare the sowing medium, clean containers with drainage holes, 8–10 cm pots, labels, and a thermometer. Do not use garden soil or soil that previously held diseased seedlings. Label each variety and the sowing date. Work with one variety at a time.

The simplest method: sow directly into the growing medium. There’s no need to soak the seeds. Do not treat seeds from a reliable source with homemade solutions; do not soak or rinse commercially treated seeds. Do not add fertilizer to the sowing medium.

1. Moisten and mix the growing medium. When squeezed, it should not drip. Fill the containers and level the surface.
2. Sow one seed per cell in the seed tray or every 2–3 cm in the container. Cover with 0.5 cm of growing medium. Dry seeds do not need to be oriented in a specific direction.
3. Water gently, label the variety, and cover with a transparent lid. Maintain a temperature of 23–25°C. Do not place the container on a radiator or under the lid in direct sunlight.
4. Check and ventilate the container daily. When the surface begins to dry out, lightly moisten it. Do not water wet growing medium.
5. Germination usually occurs after 5–10 days. Older seeds may take longer. If no germination occurs after about 14 days, check the temperature and humidity; sow a second small batch.

Germination on lignin—optional method

Spread the seeds individually on moist lignin in the container described above, cover loosely, and maintain a temperature of 23–25°C. The lignin must not dry out, and the seeds must not sit in water. Check daily; once germination▤ begins, check in the morning and evening.

When the white root is 2–3 mm long, transfer the seed to the growing medium. Hold it by the seed coat, never by the root. In the planting hole, position the root pointing downward and cover the seed with 0.5 cm of soil, without pressing down. If it has grown into the lignin, cut out a piece of it and plant it along with the seed. Do not tear off the root. Continue caring for it as you would after a regular sowing.

2. After germination: light, less heat, and moderate watering

When the first seedlings appear, remove the cover and expose them to light immediately. Don’t wait for the last seed to sprout. Place the tray by the brightest window, without a curtain, and away from a radiator. The leaves should not touch the cold windowpane.

Maintain a temperature of 18–21°C during the day and 15–17°C at night. If you don’t have a bright window, use a seedling grow light for 14–16 hours a day, with a break at night. Adjust the distance according to the lamp’s instructions. Thin out the plants before they start to shade each other.

Water in the morning at soil level with room-temperature water when the top layer dries out slightly and the container feels lighter. Pour out any water from the saucer. Do not overwater or let the seedlings dry out.

A limp seedling leaning toward the window needs more light and less heat. Space the plants further apart. Plant a healthy, leggy seedling deeper when transplanting; fertilizer won’t help it recover.

DON’T DO THIS: Do not keep seedlings under airtight plastic wrap, and do not water wet soil. With damping-off▤, the stem near the soil surface darkens, becomes constricted, and the seedling falls over. This is caused by moisture, overcrowding, and lack of air circulation. Remove the diseased seedling along with the surrounding soil, check drainage, and reduce watering. Do not try to save it by planting it deeper.

3. Pricking Out and Healthy Seedlings

The first two leaves are cotyledons▤; the next ones are true leaves. Once two true leaves have developed, it’s time to transplant▤: move each seedling into a separate pot.

Use an 8–10 cm pot with drainage holes and seedling potting mix. Lightly water the seedlings, then gently loosen the roots with a teaspoon or a stick. Hold the seedling by a leaf; do not squeeze the stem. Plant it almost up to the cotyledons, keeping them above the soil. Do not trim the roots. Gently press down the potting mix, water, and pour out any excess water from the saucer.

Protect from direct sunlight for 1–2 days, then return them to the brightest spot. Space the pots apart and maintain the temperatures specified earlier.

Schedule the first feeding 10–14 days after transplanting, once growth has resumed. If the potting mix manufacturer specifies that the starter fertilizer has a longer-lasting effect, wait. Choose a complete fertilizer with micronutrients▤ for seedlings. Fertilize every 7–10 days, but adjust the dose and frequency according to the label. Do not use the concentration intended for fruiting bushes.

When the roots have filled the pot, the soil dries out quickly, and planting is still more than a week away, repot into a 1-liter container. A good seedling▤ is stocky, has a strong stem, several leaves, and roots that hold the soil together. It does not need to be flowering.

4. Hardening off for 10–14 days

Hardening off▤ acclimates the plant to sun, wind, and temperature changes. Begin 10–14 days before planting, on a day when the temperature is above about 12°C.

• Days 1–2: Place the seedlings in bright shade for 1–2 hours, away from strong winds.
• Days 3–5: Extend the time to 3–5 hours. Add gentle sun in the morning or late afternoon.
• Days 6–10: Leave them outside for most of the day, gradually increasing sun exposure and exposure to light winds.
• Final days: Leave them out all day in conditions similar to their planting site. Bring them indoors at night if temperatures drop below 10°C. Protect them from heavy rain, strong winds, and frost.

Check the water more frequently; do not let the seedlings wilt. Hardening off is not about starving or drying them out. Also harden off seedlings intended for protected growing—a closed tunnel does not replace acclimation to the sun.

Store-bought or mail-order seedlings

Unpack them immediately upon receipt, stand them upright, and water them if the soil is dry. Keep them in a bright spot out of direct sunlight for 1–2 days, then harden them off according to the plan above. Seedlings that are slightly wilted and pale after transport usually recover quickly. Do not fertilize them right away.

5. Prepare the soil and plant the tomatoes

Choose a spot that gets 6–8 hours of direct sun, is well-ventilated, sheltered from strong winds, and free of standing water. Rotate your garden plots: after tomatoes, potatoes, peppers, and eggplants, it’s best to wait 3–4 years before planting them again. Don’t plant them right next to potatoes.

It’s difficult to rotate crops in a tunnel. Remove plant debris and add compost every year. If plants wilt or die every year despite proper watering, replace the top 20–30 cm of soil or grow them in large pots or bags filled with potting mix (see description at the end of this section).

Remove weeds and loosen the soil to the depth of a spade. In a bed that has not been previously fertilized, mix 3–5 liters of mature compost▤ per 1 m² into the topsoil. Do not add compost to soil that has already been fertilized. Do not add fresh manure, fertilizer, or lime to the planting holes.

Plant tall single-stem varieties 50–60 cm apart, with 80–100 cm between rows. Spreading, determinate varieties▤ usually need 60–70 cm between plants. In a tunnel, leave a comfortable passageway; do not overcrowd the planting for the last seedlings.

Water the seedlings before removing them. Plant on a cloudy day or in the late afternoon, 5–10 cm deeper than they were in the pot. Remove any leaves that would end up underground. Place a healthy, elongated seedling diagonally in a shallow furrow, covering part of the stem. Leave the top exposed. Do not bend the stem or bury the roots in deep, cold soil.

When planting, drive in a sturdy stake—it’s easier to damage the roots later. Water slowly, usually with 2–3 liters of water, so the soil clings to the root ball. For the first week, check the moisture level near the roots daily and water when the soil begins to dry out.

Cover the warm, moist soil with 5 cm of straw, leaving a gap around the stem. Mulch▤ reduces moisture loss, suppresses weeds, and prevents soil from splashing. In a tunnel, rain won’t water the beds.

Growing in a Pot

Choose a pot size appropriate for the variety: tall varieties need at least 20–30 liters of potting mix; determinate and dwarf varieties need 15–20 liters; and micro balcony varieties need 5–10 liters. The pot must have drainage holes. Fill it with fresh potting mix for vegetables or tomatoes. Garden soil tends to compact in a pot.

Place the pot in the sunniest spot, sheltered from the wind. Insert a stake when planting, and secure tall plants to prevent them from toppling over. A dark pot in full sun can overheat the roots, so shade the sides of the pot.

There isn’t much soil in the pot, so it dries out quickly. In the summer, check the moisture level daily; during hot weather, water it up to twice a day until water flows out of the drainage holes. Pour out any water from the saucer. Fertilize regularly starting 3–4 weeks after planting, following the fertilizer label’s instructions, as the nutrients in the potting mix run out quickly.

Growing in a Bag of Potting Mix

This is a simple way to grow plants without relying on garden soil: on poor, weedy, or depleted soil; in a tunnel without changing locations; or even on paved surfaces.

1. Buy a bag of potting mix for vegetables or tomatoes. For two plants, you’ll need at least 40–50 liters; a smaller bag is enough for one plant. Before placing it, knead the bag to break up any clumps.
2. Make 6–8 small slits in the bottom of the bag. Without them, water will pool and the roots will rot.
3. Lay the bag flat in full sun, with the drainage holes facing the ground.
4. Cut two holes about 10 × 10 cm in the top, spaced 40–50 cm apart and not right at the edges. Plant one seedling in each hole. The bag is shallow, so position the elongated stem at an angle rather than planting it deeply.
5. A stake won’t stay upright in the soil alone. Drive it through the bag into the ground beneath, or guide the plants along a string to a support structure.

Water slowly, directly into the holes. Water runs off the plastic easily, so it helps to place a bottomless pot pressed into the hole next to the plant: pour water into it. Monitor moisture and fertilization as you would in a pot: check daily in the summer, and fertilize 3–4 weeks after planting.

After the season, remove the plants along with their roots, cut open the bag, and spread the growing medium over your garden or add it to the compost. This will enrich the soil. Discard the plastic sheeting. Do not spread the growing medium from plants that wilted or were diseased at the base where you plan to plant tomatoes, potatoes, peppers, or eggplants. Use a fresh bag for your next tomato crop.

6. Training: First, check the variety type

A side shoot is a shoot that grows▤ between the main stem and a leaf stalk, with its own tip and leaves. A cluster▤ is a group of flowers, which later become fruits. Do not confuse side shoots with leaves or clusters.

For tall, indeterminate▤ varieties, train them to grow on a single main stem. Remove suckers every 5–7 days before they exceed 5 cm. Snap off small suckers; cut larger ones with a clean tool without tearing the stem. Work on dry plants. For two shoots, space them more widely, every 70–80 cm; leave a strong sucker below the first cluster and provide it with a separate support.

Determinate and dwarf varieties

For determinate varieties, leave the side shoots: they end their growth with a cluster and bear fruit on the branches. Provide support. “Dwarf”▤ refers to a compact growth habit, not determinate growth. Unless otherwise specified in the variety description, allow the plant to grow as a natural bush; do not remove all the side shoots.

Tie them up▤ with soft tape every 20–30 cm, using a loose figure-eight knot between the stake and the stem. Check the ties weekly; support heavy clusters separately. Attach the string to a sturdy structure and run it loosely, preferably using clips. Do not tighten it against the stem.

Remove diseased leaves and those touching the ground. Remove healthy lower leaves starting when the lowest cluster begins to ripen, no more than 1–2 large leaves at a time. After harvesting that cluster, continue higher up. Do not strip the bush bare or suddenly expose the fruit to the heat. Healthy leaves above the fruiting clusters nourish the plant and protect the fruit.

7. The tunnel should be well-ventilated, and the flowers should be pollinated

Open the doors and vents in the morning before the interior heats up; on warm days, open both ends of the tunnel. On nights when the temperature is above about 12°C and there is no strong wind, leave the vents open. On cool nights, limit ventilation; in the morning, quickly remove moisture by ventilating▤. Droplets lingering on the leaves indicate insufficient airflow.

At 30°C, ventilate as much as possible; in extreme heat, provide light shade from the outside. Do not spray the leaves to cool them down. Tomatoes are self-pollinating▤, but the pollen must be released. Under cover, every 2–3 days in the late morning when the flowers are dry, gently shake the stake or the cluster. In open ground, wind and insects are usually sufficient. If flowers fall off in hot weather, first lower the temperature and check the water levels; do not increase fertilization.

8. Water the soil, not the leaves

Water in the morning at the base of the plant, slowly, over a wider area around the plant. A daily cup of water placed at the base of the stem mainly moistens the surface. Move the mulch aside and check the soil to a depth of 5–10 cm: if it’s moist, wait; when it starts to dry out, water it. Don’t wait for the plant to wilt regularly.

For a mature plant, start with 3–5 liters, usually every 2–3 days on warm days. In hot weather, with light soil, and under cover, watering may be daily; after rain and in cool weather, less frequently. Moisture levels determine the frequency.

Use a trowel to check from the side to see if the water has reached a depth of 15–20 cm. If it has only moistened the surface, water more slowly, in two portions. If the soil remains soggy for a long time, reduce the amount and improve drainage. In the event of a severe drought, water immediately, without waiting until morning and without wetting the leaves.

Maintain consistent moisture levels around the roots. Overdrying and sudden overwatering can cause fruit to crack▤. Irregular watering, root damage, and excess fertilizer can disrupt the supply of calcium to the fruit. This results in blossom end rot▤: a dry, dark spot on the bottom of the fruit. This is not blight. First, correct your watering; simply adding calcium will not solve the problem.

9. One Simple Fertilization Plan

Enrich the soil with compost. After planting, wait about 10–14 days for new growth to appear. If you’ve added a slow-release fertilizer, take its effects into account and don’t apply a second round of fertilizer.

Choose a single complete tomato fertilizer containing magnesium and micronutrients. Apply it once a week as a general rule, but follow the dosage and schedule indicated on the label. Water with plain water between applications. Do not fertilize dry soil or add liquid manure, farmyard manure, or other fertilizers.

Feed young plants with a balanced fertilizer. Once flowering begins, choose a formula for fruiting tomatoes with a higher potassium content than nitrogen. Nitrogen▤ promotes stem and leaf growth, potassium▤ supports fruit development and water management, calcium▤ promotes healthy tissues, and magnesium▤ supports leaf function. You don’t need four separate fertilizers. The absence of calcium in the fertilizer does not mean it is absent from the soil.

If you have a dark, lush bush with no fruit, stop fertilizing and check the light and temperature. Do not try to balance out excess nitrogen with another fertilizer. If problems persist, test the soil.

10. Protect plants before disease sets in

Inspect the leaves on both sides, the stems, and the fruit twice a week; daily after prolonged rain. The basics are healthy seedlings, proper spacing, dry foliage, and good air circulation. After pruning diseased plants, clean and disinfect your tools.

The simplest protection against late blight in open ground: make a plastic sheet canopy over the row, with the sides open. Late blight thrives on wet leaves, and under the canopy, the leaves stay dry. The canopy is meant to protect against rain, not to enclose the plants, so leave the sides open.

• Late blight▤: rapidly spreading brown spots, sometimes with a light coating on the underside of the leaf; dark lesions on the stems; hard brown spots on the fruit. Remove diseased parts; discard heavily infected plants entirely. On that day, establish protection for the remaining plants.
• Alternaria▤ leaf spot: spots mainly on older leaves, often with concentric rings and a yellowing border. Remove diseased leaves, improve air circulation, and monitor for new spots.
• Gray mold▤: a gray, powdery coating on dying flowers, leaves, and wounds. Remove diseased parts and debris; reduce moisture; and provide good air circulation.

If you’re unsure of the diagnosis, consult photos of the entire plant, both sides of the leaf, and the stem. Do not put diseased parts into a cold compost bin. Spraying will not repair damaged leaves.

Ecological and biological protection: focus on prevention; choose varieties resistant▤ to specific diseases. Select a biological product appropriate for the problem and apply it according to the label, often before symptoms appear. A “natural” product or biostimulant▤ does not necessarily combat diseases. Milk, yeast, and manure will not stop a full-blown blight. Severe infestation may require removing the plants and ending the harvest.

Chemical protection▤: In the event of a local disease threat and prolonged damp weather, consider preventive treatment. Use the MRiRW search tool to select a product for tomatoes, specific diseases, open ground or protected cultivation that is approved for non-professional use. Follow the current label instructions regarding dosage, the pre-harvest interval▤ (the time from the last application to harvest), application intervals, the number of applications, and personal protective equipment. Do not mix products or increase dosages. Be aware of changes in the mode of action—a change in the trade name alone is not sufficient.

Small white insects flying up when a leaf is moved indicate whiteflies▤; small dots and a cobweb-like pattern on the underside indicate spider mites▤; clusters on new growth indicate aphids▤. Act early by selecting a biological control agent or a registered pesticide for the specific pest. A fungicide will not control it.

11. Ten Problems—What to Do

Still need help? Describe the problem to Tomato Help.

12. Topping and Final Harvest

Topping▤ is the removal of the growing tip to prevent the plant from setting more fruit that won’t have time to ripen. This applies to tall, indeterminate varieties; do not routinely top determinate varieties.

Do this in the ground around August 1–10, and under cover August 15–25: 6–8 weeks before the end of the harvest. Leave two leaves above the highest remaining fruit cluster, and cut off the growing tip above them. Remove any subsequent flowers and new side shoots. Do not strip off all the leaves or cut off the water supply.

Harvest fruits that are the color typical of the variety and yield slightly to pressure. Green-fleshed varieties remain green but soften and often turn yellowish. For indigo varieties, a dark skin isn’t enough: check the lighter part and the softness.

Before heavy rain or cold weather, harvest fruits that are beginning to change color. Before the first frost, also harvest fully grown green fruits that are of typical size and have a lighter shade of green. Small, hard, dark-green fruitlets usually won’t produce good tomatoes.

To ripen them, arrange healthy fruits in a single layer in a dry, well-ventilated place at 18–22°C, out of direct sunlight. Remove any spoiled ones daily. Store ripe tomatoes briefly at room temperature, and wash them before eating. If you won’t eat them right away, refrigerate them; always keep sliced tomatoes in the refrigerator. Do not seal wet tomatoes in a bag.

13. Save Your Own Seeds

A stable variety▤ usually comes true from seed, provided there has been no cross-pollination. F1▤ refers to the first generation of a hybrid: the seeds may germinate, but the offspring may not resemble the parent plant. To preserve the variety, choose a healthy tomato of a stable variety that matches the description.

Tomatoes are usually self-pollinating, but insects can also transfer foreign pollen. Limit cross-pollination: place a breathable isolation▤ bag over the closed flower buds of the cluster. Gently shake the plant during flowering. After the fruit sets, remove the bag and label the cluster. Do not use plastic wrap.

In July or August, select a typical, fully ripe fruit from a healthy plant. Discard rotting fruit and material from diseased plants. Fermentation▤ does not guarantee the removal of pathogens.

1. Label the cup with the variety name. Squeeze out the seeds along with the jelly and juice. Work with one variety at a time, then wash your tools and the strainer.
2. Cover loosely and leave in the shade at 20–24°C, usually for 2–3 days. Stir daily. If the mixture is too thick, add a little water.
3. Check starting on the second day. Stop when the jelly separates easily. A film on the surface does not require extending the fermentation. Do not wait a week—the seeds may sprout.
4. Add water, stir, and let the seeds settle. Carefully pour off the residue and rinse the seeds in a fine-mesh strainer. Whether the seeds float is not a reliable test of viability.
5. Spread them out thinly on a labeled, smooth plate or sieve. Dry for 7–14 days in a well-ventilated, shaded area at room temperature; break up any clumps. Avoid ovens, radiators, and direct sunlight.
6. Once the seeds are completely dry and loose, pour them into a paper envelope. Include the variety, year, and information about flower isolation. Seal the envelopes in a container and store in a dry, cool, dark place. Do not seal damp seeds.

Properly stored seeds remain viable for about 4 years, often longer. Test older seeds on moist lignin. Record the results; a small test does not guarantee that the entire packet will germinate.

After the season, remove the plants, roots, and fallen fruit, and wash the supports and containers. Make a note of the best varieties and what to improve for next year.