Growing Seedlings

Preparing the Substrate (Soil Mix)

Tomatoes need light, loose, and nutritious soil with a neutral or slightly acidic reaction (pH 6.0–6.5) (Autko et al., 2012). In heavy, clayey soil, roots will suffer from a lack of oxygen and stagnant water. Ready-made high-quality soil mix can be bought at a store, or you can prepare it yourself. One proven recipe includes a mix of peat, humus, and sod land in a ratio of 7:2:1 (Gavrish, 2005). To improve the structure, vermiculite or coarse sand (about 1 part to 3–4 parts of the mix) can be added. They will make the soil more breathable and moisture-retentive.

Why this is important: The light structure of the substrate allows young roots to easily penetrate deep, and the air necessary for root respiration circulates freely. The correct pH ensures the availability of nutrients for the plant.

Disinfection of the substrate. Soil, especially garden soil, may contain disease pathogens (damping-off, fusarium) and pest larvae. To protect future seedlings, the soil must be disinfected. The most reliable way is steaming: pour boiling water over the soil or treat it with steam for 30–40 minutes (Autko et al., 2012). You can also water the soil with a weak solution of potassium permanganate or use ready-made disinfected mixes.

Preparing Seeds

Tomato seeds can be carriers of dangerous viral and bacterial infections (e.g., tobacco mosaic virus, bacterial cancer). Pre-sowing treatment helps to disinfect the seeds and increase their germination.

Calibration. This is the selection of the largest and fullest seeds. They can be separated in a 5% solution of table salt. Full seeds will sink to the bottom, while empty ones will float (Gavrish, 2005). The best germination and germination energy are found in large seeds.

Disinfection. The most common method is soaking in a 1% solution of potassium permanganate for 20–30 minutes, after which the seeds are washed with clean water (Autko et al., 2012). Heat treatment in hot water (about 50 °C) for 20–25 minutes is also effective, which allows destroying surface infection (Jones, 2008). It is important to remember that these procedures are not suitable for pelleted (coated with a protective shell) or already treated seeds from the manufacturer.

Stimulation for uniform germination. To improve and accelerate germination, seeds can be soaked in a solution of growth stimulants, for example, in "Epin-Extra", "Zircon", or an infusion of wood ash. Soaking in a solution of micronutrients (boron, manganese, zinc, copper) for 12 hours is also effective (Gavrish, 2005; Autko et al., 2012). Stimulants activate internal processes in the seed, increasing its resistance to diseases and stress.

Important: If you purchased seeds in branded packaging from a well-known manufacturer, most of the pre-sowing treatment (disinfection, pelleting) has already been done (Gavrish, 2005). In this case, additional procedures are not required and may even be harmful, washing away the protective coating. Simply soak them in warm water for swelling before sowing.

Great, let's move on to the most crucial part – sowing. The friendliness and uniformity of the seedlings depend on how correctly and carefully you perform this stage.

Causes of Stretching

1. Lack of light. This is the main reason. Tomato is a light-loving plant. When it lacks photosynthetically active light (less than 4–6 thousand lux), it begins to "look" for it, stretching the stem upward (Gavrish, 2005, p. 18). This is an ancient biological mechanism aimed at outgrowing neighbors and reaching the light. In cloudy weather or in a north-facing room without supplementary lighting, seedlings are simply doomed to stretch.

2. Too high temperature. Especially when combined with a lack of light. At high temperatures (above 22 °C), growth processes (cell division and elongation) proceed very intensively. If there is little light, the plant cannot effectively photosynthesize and store energy, so it simply "stretches" at the expense of stem thickness and root development (Gavrish, 2005; Atherton & Rudich, 1986). This is why it is so important to lower the temperature immediately after emergence.

3. Overcrowding. When seedlings are too close together, they compete with each other for light. To outgrow their neighbors, each of them starts to stretch. This creates a vicious circle: overcrowding leads to stretching, and stretching worsens shading (Gavrish, 2005).

4. Excess nitrogen. Nitrogen is the "macronutrient of growth", stimulating the development of green mass. If at an early stage, especially with a lack of light, you give the plants too much nitrogen, they will start to "fatten" – intensively build up pale, loose greenery, which also leads to stretching (Gavrish, 2005; Welbaum, 2015).

How to Fix the Situation and Prevent Stretching

Fortunately, these causes also give us the keys to the solution:

1. Provide maximum light. This is the most important rule. Ideally, seedlings should stand on a south-facing windowsill and receive 14–16 hours of light per day. If natural light is insufficient (and in March-April it is almost always insufficient), be sure to use supplementary lighting with phytolamps or fluorescent (or LED) lamps with a cool spectrum. Place the lamps at a height of 10–15 cm above the plants (Gavrish, 2005; Autko et al., 2012). Why this works: Supplementary lighting compensates for the lack of sunlight, allowing the plant to photosynthesize normally and build strong, stocky stems, rather than thin, long ones in search of light.

2. Strictly control the temperature regime. As we have already discussed, immediately after emergence (the first 5–7 days), the temperature should be lowered to 18–20 °C during the day and 15–16 °C at night (Gavrish, 2005; Autko et al., 2012). Subsequently, in sunny weather, maintain 21–23 °C, in cloudy weather – 18–20 °C, and at night – 17–18 °C. It is important that the night temperature is always 4–6 degrees lower than the daytime one – this also prevents stretching (Gavrish, 2005). Why this works: Lowering the temperature (especially at night) slows down the process of cell elongation without interfering with photosynthesis. Thus, the plant uses its energy to thicken the stem and develop roots, rather than useless upward growth.

3. Carry out picking in time and monitor density. If you sowed in a common box and the seedlings are too crowded, transplant them into individual containers as early as possible, giving each enough space (pots 8×8 or 10×10 cm) (Autko et al., 2012). At the final stage, 20–25 days before planting, carry out spacing (move the pots apart) so that there is a small gap between the leaves of neighboring plants (Gavrish, 2005).

4. Balance nutrition. At early stages, avoid nitrogen fertilizing. The first feeding is carried out 8–10 days after picking and contains more phosphorus and potassium for root system development, rather than nitrogen for green growth (Gavrish, 2005; Autko et al., 2012). Phosphorus stimulates root growth, and potassium strengthens cell walls, making plants more resilient.

If the seedlings have already stretched, they can be saved. When picking, bury the stem up to the cotyledon leaves – additional roots will quickly appear on the buried part of the stem, and the plant will become more powerful (Jones, 2008). You can also use the "pinching the growing point" technique – this will stop upward growth and direct energy to the development of side shoots.

Final checklist for preventing stretching:

• Light: Maximum natural light + supplementary lighting for 14–16 hours.
• Temperature: After emergence, sharply reduce to 18–20 °C (day) and 15–16 °C (night). Always keep the night temperature lower than the daytime one.
• Density: Pick and space seedlings in time so that plants do not shade each other.
• Nutrition: Exclude nitrogen fertilizing in the first month. The main fertilizing should be phosphorus-potassium.

Let's continue. Picking is one of the most controversial issues in growing seedlings. Some gardeners consider it mandatory, others – unnecessary stress for the plants. Let's figure out what it is, why it is needed, and whether it is always justified.

Why is Picking Needed?

1. Increasing the feeding area. In a common container, seedlings quickly begin to compete for light, water, and nutrients. Their roots intertwine, which delays growth and leads to stretching. Picking gives each plant its own space for root system development (Nonnecke, 1989).

2. Stimulation of root system development. During picking, the central root is pinched (about a third of its length is removed). This stimulates the growth of lateral and adventitious roots. As a result, a more powerful, fibrous root system is formed, which supplies the plant with water and nutrition much better than one long taproot (Jones, 2008). Why this works: When the apical bud of the main root is removed, apical dominance is lifted, and the plant begins to actively grow lateral roots.

3. Culling of weak plants. Picking is a convenient moment to select only the strongest, healthiest, and properly developed seedlings. All weak, diseased, or ugly specimens are discarded (Autko et al., 2012). This saves your time and resources for the further cultivation of obviously unpromising plants.

4. Control over development. After picking, you can temporarily slow down the growth of seedlings (the seedlings experience a slight shock), which is especially useful if you sowed too early and the weather for planting still does not allow it. This allows you to "delay" overgrown seedlings (Swiader & Ware, 1992).

Is It Needed at All? Weighing the Pros and Cons

When picking is JUSTIFIED:

• You sowed seeds in a common box, and the seedlings emerged densely.
• You are using inexpensive, non-hybrid seeds, and you have a large supply of plants.
• You have the time and desire to work to get the strongest and healthiest seedlings possible.
• You want to delay the development of seedlings if the weather does not allow planting them on time.

When picking is better NOT TO DO:

• You are using expensive hybrid seeds F₁. Every seed counts, and the risk of losing it during picking is too great (Gavrish, 2005, p. 27).
• You sowed seeds immediately in individual containers (cassettes, peat tablets, or pots). In this case, transplanting will only harm.
• You are growing seedlings for early planting in the ground. Picking delays development by 10–14 days, which can shift the timing of the first harvest (Autko et al., 2012).

Conclusion: Picking is not a mandatory procedure, but an agricultural technique that gives advantages when applied correctly and can be harmful when used thoughtlessly. For most amateur gardeners growing 10–20 tomato bushes, it is still recommended, as it allows you to get more powerful plants.

How to Conduct Picking Correctly

To make picking beneficial rather than harmful, it is important to follow several rules.

1. Timing. The optimal time is the phase of 1–2 true leaves, approximately 10–14 days after emergence (Autko et al., 2012; Gavrish, 2005). At this time, the seedlings are already strong enough, but the root system has not yet grown much. Earlier picking (in the cotyledon phase) injures too delicate seedlings. Later picking (3–4 leaves) is fraught with severe intertwining of roots and their damage.

2. Preparation. A day before picking, water the seedlings well. This will soften the soil, and you will be able to extract the plants with minimal root damage (Gavrish, 2005). Prepare individual pots (diameter 8–10 cm) with moistened nutrient soil in advance. Make a small depression in the center of each.

3. Technique. Carefully pry up the seedling with a small stick (or even a toothpick), trying not to damage the roots. Extract the plant, holding it by the cotyledon leaf, not by the stem. The stem is very fragile and can be easily damaged (Gavrish, 2005). Carefully separate the root from the soil. If the central root is too long (more than 4–5 cm), pinch it by ⅓ of its length.

4. Planting. Place the seedling in the prepared hole up to the cotyledon leaves, spread the roots (they should not bend upwards!) and carefully cover with soil, lightly compacting it around the stem (Autko et al., 2012). Burying up to the cotyledons is a very important technique, as additional roots quickly form on the buried part of the stem, making the plant more powerful (Jones, 2008).

5. Care after picking. After transplanting, carefully water the plants with warm water and shade them for 1–2 days to help them adapt more easily (Gavrish, 2005). The optimal temperature in the first days is 20–22 °C during the day and 17–18 °C at night. After 8–10 days, when the plants have rooted, you can carry out the first fertilizing with a complex fertilizer with a predominance of phosphorus and potassium (Autko et al., 2012).

Final checklist for picking:

• Timing: Phase of 1–2 true leaves.
• Preparation: Abundant watering the day before the procedure.
• Container: Individual pots 8×10 cm.
• Rule: Bury up to the cotyledon leaves.
• Care: Shading, moderate watering, fertilizing after 8–10 days.

Let's continue. Hardening off is the final but extremely important stage in growing seedlings. Without it, even the strongest and stockiest seedlings can die or be sick for a long time after moving to a permanent location.

What Is Hardening Off and Why Is It Needed?

Hardening off is the process of gradually adapting plants to unfavorable environmental conditions: lower temperatures, direct solar radiation, and wind. Its main goal is not just to "accustom" the plant, but to cause physiological changes in it at the cellular level (Swiader & Ware, 1992).

What happens to the plant during hardening off:

• The composition of cell sap changes: Sugars and other substances accumulate in the cells, acting as a natural "antifreeze", lowering the freezing point and helping the plant tolerate short-term frosts (Welbaum, 2015).
• The cuticle (wax coating) thickens: This reduces moisture evaporation (transpiration) and protects leaves from burns and drying out in the wind.
• The work of stomata adapts: Stomata (pores on the leaves) learn to close faster when there is a lack of moisture, preventing dehydration.
• Growth slows down, but does not stop: The plant stops building tender greenery and begins to accumulate a margin of safety for survival in new conditions (Gavrish, 2005,).

Hardening off should begin approximately 10–14 days before the planned planting in open ground (Jones, 2008).

How to Harden Off Seedlings Correctly?

The hardening off process should be gradual and include two main components: temperature and light regimes.

Signs of Properly Hardened Seedlings

Properly hardened seedlings differ noticeably from greenhouse ones:

• Dark green leaf color (instead of pale green).
• A thicker and stronger stem with short internodes.
• Leaves become stiffer and leatherier to the touch.
• The root system is well developed and wraps around the root ball.

Important Precautions

• Do not overdo hardening: Too sharp a drop in temperature or exposure to cold winds can weaken the plants, not strengthen them.
• Consider the weather forecast: If frost or strong winds are forecast, bring the seedlings indoors.
• Do not plant in heat after hardening: If the weather is very hot and dry at the time of planting (above 25–28 °C), even hardened seedlings can get sunburned. In this case, shade them for the first 2–3 days.

Final checklist for hardening off:

• Timing: 10–14 days before planting.
• Temperature: Gradual decrease, taking outside at temperatures from 12 °C.
• Light: Gradual accustoming to direct sunlight, starting with partial shade.
• Watering: Moderate, without drying out.
• Duration: Increase the time spent outdoors every day.

The main goal is not just to survive, but to start actively growing in the new place within the first days after transplanting. Properly hardened seedlings adapt to open ground conditions within 3–5 days, while unhardened ones can be sick for up to 1–2 weeks.

Here we come to the final part. Now our seedlings are hardened and ready for planting. It remains to figure out how to determine this moment – when are they really ready to move to the garden bed? Shall we continue?

Great, we are on the home stretch. All the previous work was aimed at getting seedlings ready for the "move". Now it is important not to miss this moment.

1. Age of Seedlings (Calendar)

This is the simplest and most reliable guide. For most tomato varieties and hybrids, the optimal age of seedlings for planting in open ground is 50–60 days from the appearance of full shoots (Autko et al., 2012). For early-ripening determinate varieties intended for open ground, it may be slightly less – 45–50 days (Gavrish, 2005). Why this works: By this age, the plant manages to go through all the key stages of vegetation: form a sufficient number of leaves, develop a root system, and accumulate a supply of nutrients for successful adaptation.

2. Appearance of the Plant (Physiological Criteria)

These are the main visual signs to focus on:

• Height: 20–30 cm (Autko et al., 2012). Not higher than 35 cm. If the seedlings are taller, they are overgrown.
• Number of true leaves: 6–8 pieces, well developed, rich green in color (Autko et al., 2012). This indicates that the plant already has a sufficiently developed photosynthetic apparatus for life outdoors.
• Stem thickness: At least 0.6–0.8 cm at the root collar (Autko et al., 2012). A thick stem is a sign that the plant has not stretched and has accumulated enough nutrients.
• Presence of buds: In early and mid-season varieties, by the time of planting, budding (the first buds on the first inflorescence) or even the beginning of flowering should already be observed (Autko et al., 2012). This indicates that the plant has entered the generative phase and is ready for fruiting. Important: Seedlings should not be blooming profusely – this is a sign of overgrowth (Jones, 2008).
• Root system: When extracted from the pot, the roots should tightly, but not too tightly, wrap around the root ball. This indicates that the root system is fully formed and ready for active growth in the new place (Jones, 2008). White, healthy roots without signs of rot are a good sign.

3. Readiness of Seedlings for Planting (Test)

Experienced gardeners use a simple test: if, when lightly squeezing the stem between fingers, it is elastic and does not dent, and the leaves, after hardening, do not lose turgor (elasticity), even after being taken out into the sun for several hours – the seedlings are ready.

4. Weather Conditions (External Factors)

Even ideal seedlings by all parameters should not be planted in cold or wet weather. The following conditions must be met:

• Soil temperature: At a depth of 10–15 cm, it should be not lower than 14–16 °C (Autko et al., 2012). In cold soil, tomato roots practically do not work, and the plant may die of starvation. Folk method: the soil is ready when you can sit on it with bare skin and not get cold.
• Air temperature: The daytime air temperature should stably stay within 18–20 °C and above. Nighttime – not drop below 10–12 °C (Gavrish, 2005).
• End of frost. This is the most important criterion. Planting is done only after the threat of return frosts has passed. In the middle zone, this is usually the period from May 25 to June 5 (Gavrish, 2005, p. 101). In southern regions – earlier, in northern ones – later.
• Hardening off. Seedlings must undergo a full hardening cycle (at least 7–10 days) before planting.

Final checklist for seedling readiness for planting:

• Age: 50–60 days.
• Height: 20–30 cm.
• Leaves: 6–8 true, dark green.
• Stem: Thickness 0.6–0.8 cm, elastic.
• Buds: The first buds have appeared.
• Roots: Well wrapped around the root ball.
• Hardening: Conducted for at least 7–10 days.
• Weather: Night temperature not below 10 °C, soil warmed to 15 °C, frost excluded.

Main idea: Do not rush to plant, even if the seedlings look great. It is better to wait 3–5 days for consistently warm weather to establish than to lose the plants due to return cold. And remember: overgrown seedlings take root worse than young and strong ones.

Mistake 1: Sowing Too Early

This is the most common mistake. The desire to get a harvest as early as possible forces sowing seeds already in February. As a result, by the time of planting, the seedlings outgrow, stretch, begin to flower, and after transplanting, they are sick for a long time.

How to fix: Strictly observe sowing dates, based on the date of planting in the ground (see chapter 3). In the middle zone, the optimal period is the end of March – the first decade of April. If the seedlings have already outgrown, you can deepen the stem when planting in the ground up to the first true leaves or use the "topping" technique (pinching the top).

Mistake 2: Incorrect Sowing Depth

Too deep sowing (more than 1.5 cm) leads to the sprout not having enough strength to break through to the surface; it emerges late and weakened. Too shallow sowing (less than 0.5 cm) – the seeds dry out, and in emerged seedlings, the seed coat often does not shed, which delays their development (Gavrish, 2005, pp. 93, 95).

How to fix: Observe the sowing depth of 0.5–1.0 cm. If the seed coat does not shed, carefully spray it with warm water several times – it will soften and fall off more easily (Gavrish, 2005, p. 95).

Mistake 3: Failure to Observe the Temperature Regime After Emergence

Leaving boxes with seedlings in warmth (+24 °C and above) after emergence is a direct path to stretching. In warmth, but with a lack of light (and in March-April there is always little of it), the stem begins to grow intensively upwards, thinning and weakening.

How to fix: Immediately after emergence, lower the temperature to 18–20 °C during the day and 15–16 °C at night for 5–7 days. Then maintain a moderate regime with a night temperature 4–6 degrees lower than the daytime one (Gavrish, 2005).

Mistake 4: Lack of Light

Even on a south-facing window in late winter – early spring, natural lighting is catastrophically insufficient. As a result, seedlings stretch, become pale and weak.

How to fix: Organize supplementary lighting with phytolamps or fluorescent lamps of a cold spectrum for 14–16 hours a day. Place the lamps at a height of 10–15 cm above the plants, raising them as they grow (Gavrish, 2005; Autko et al., 2012).

Mistake 5: Overwatering

Excess moisture, especially combined with coolness, is an ideal condition for the development of "damping-off", which can destroy all seedlings in a few days. Overwatering also contributes to stretching.

How to fix: Water the seedlings only as the top layer of soil dries out, with warm water, preferably in the morning. Avoid stagnant water in the tray (Gavrish, 2005).

Mistake 6: Excess Nitrogen Fertilizers

Fertilizing seedlings with nitrogen at an early stage (especially with a lack of light) leads to "fattening" – excessive growth of pale, loose greenery at the expense of the root system and the setting of flower buds.

How to fix: Carry out the first feeding 8–10 days after picking with a fertilizer containing a predominance of phosphorus and potassium (e.g., monopotassium phosphate or a special fertilizer for seedlings) (Gavrish, 2005). Apply nitrogen fertilizers only when there is a clear nitrogen deficiency (pale lower leaves).

Mistake 7: Picking Too Early or Too Late

Picking too early (in the cotyledon phase) leads to the death of immature seedlings. Picking too late (more than 3–4 leaves) – to trauma to intertwined roots and long recovery.

How to fix: Pick seedlings in the phase of 1–2 true leaves (Autko et al., 2012). If you are late, try to transplant the plants with a large clod of earth to minimize root trauma.

Mistake 8: Planting in Unwarmed Soil

Planting seedlings in cold soil (below 14–16 °C) is stress that leads to growth arrest, and with prolonged exposure – to the death of the root system. Roots in cold soil do not absorb nutrients, especially phosphorus, which is manifested by a purple tint to the leaves (Autko et al., 2012).

How to fix: Wait until the soil warms up to 14–16 °C. If frost threatens already planted seedlings, use covers (film, agrofabric, cut plastic bottles).

Mistake 9: Skipping Hardening Off

Skipping the hardening off stage or its insufficient duration is a common cause of death or severe weakening of seedlings after planting. Delicate leaves accustomed to greenhouse conditions get sunburned, and the plant experiences shock from a sharp temperature change.

How to fix: Start hardening off 10–14 days before planting, gradually accustoming the seedlings to the open air (see chapter 9) (Jones, 2008).

Mistake 10: Ignoring Signs of Disease

Many diseases are easier to prevent than to treat. The appearance of yellow or brown spots, wilting, blackening of the stem – these are signals of problems that cannot be ignored.

How to fix: Regularly inspect the seedlings. If signs of disease are found, remove the diseased plants and treat the healthy ones with biofungicides (e.g., "Fitosporin-M"). The main prevention is the observance of agricultural techniques (proper watering, ventilation, cleanliness).

Final checklist: what NOT TO DO when growing seedlings:

• ❌ Sow too early.
• ❌ Bury seeds more than 1 cm.
• ❌ Keep seedlings warm after emergence.
• ❌ Leave without supplementary lighting.
• ❌ Overwater.
• ❌ Overfeed with nitrogen.
• ❌ Pick too early or too late.
• ❌ Plant in cold soil.
• ❌ Skip hardening off.
• ❌ Ignore signs of disease.

This concludes our guide to growing tomato seedlings. Remember: every mistake is an experience. The main thing is to understand its cause and do everything right next time. Good luck and bountiful harvests!