There are two ways to get seed starting wrong, and they are opposites. One is sowing too late and never ripening a tomato. The other is sowing in February and standing in May with a tray of pale seedlings flat across the rim.
The difference almost never comes down to talent. It comes down to four numbers per crop: sowing depth, germination temperature, days to germinate and weeks before planting out. Get those right and the seedlings raise themselves. Below are the numbers for 24 crops, dates for USDA zones 3 to 6, and the part no packet mentions – which crops are not worth starting indoors.
Starting seeds indoors at a glance
| Item | Detail |
|---|---|
| What it is | Sowing under cover, planting out a finished transplant |
| Other names | Seed starting, raising transplants |
| Season gained | 3–8 weeks, depending on crop and zone |
| Sowing depth rule | Two to three times the seed's own diameter |
| Germination temperature | 20–25 °C (68–77 °F) in the mix, not the room |
| After germination | 16–18 °C (61–65 °F) by day, cooler at night |
| Light needed | 12–16 hours a day, lamp 5–15 cm (2–6 in) above the leaves |
| Watering | From below, letting the surface dry between times |
| Commonest cause of failure | Sowing too early for the available light |
| Hardening off | 7–14 days outdoors, in longer and longer sessions |
| Crops rarely worth it | Carrots, radishes, dill, beans, peas, spinach |
| Effort level | Low per plant, but daily attention for weeks |
Contents
- Starting seeds indoors at a glance
- Why start seeds indoors at all?
- Which crops are worth starting indoors – and which are not?
- The seed starting chart: depth, temperature, days and weeks
- Counting backwards from your last frost date
- Zone table: sowing dates for USDA zones 3–6
- Mix, containers and the kit you actually need
- Germination: warmth, moisture and patience
- Do seedlings need grow lights?
- Bottom watering and the first feed
- Pricking out and potting on
- Hardening off without losing the plants
- Common problems and how to fix them
- How to start seeds indoors step by step
- Common mistakes
- Frequently asked questions
Why start seeds indoors at all?
A cold-climate season is short. USDA zone 6 gives roughly 180 frost-free days; zone 3 around 90 to 110. A chilli needs 90 to 120 days from transplanting to ripe fruit, so sown outdoors in June it never finishes. Starting seeds indoors moves the plant's first six to twelve weeks into a room where you set the temperature.
Four gains, and only one of them is time
The season stretches. An eight-week-old tomato flowers weeks earlier than a direct-sown one. In zone 5 that is August fruit instead of September; in zone 3 it is fruit instead of none.
Bed space is freed. While transplants grow indoors the bed grows something else. Cabbage sown in late March follows the spring radishes.
Germination rates climb. Cold, wet spring soil is a poor place for a seed. Indoors you get 90–95 percent where outdoors gives 50–60, so you sow exactly what you need.
Pests do not get the first bite. A new cotyledon is defenceless against slugs and flea beetles. A plant with four true leaves survives what a seedling does not.
What it costs
It costs windowsill space, electricity and weeks of daily attention. A tray that dries out on a bright March afternoon is dead in four hours. Transplanting is also root disturbance, and some species resent it badly – hence the list below.
Which crops are worth starting indoors – and which are not?
Seed packets almost always say "can be started indoors". True, in the sense that it is possible. The question is whether you get the windowsill back.
Crops where it is close to compulsory
Anything needing a long warm run: chillies, peppers, aubergine and tomatillo. Without an indoor start there is no crop north of zone 7. Chillies are the extreme at 10–12 weeks; habanero types want 14.
The slow growers belong here too: leeks, celeriac, celery and bulb onions from seed. They sit indoors for 10 to 12 weeks apparently doing nothing. Celeriac sown in March is still small in May, but needs every week.
And tomatoes, the most common of all. Six to eight weeks is right; longer gives a root-bound plant that flowers in the pot. Exact dates are in the guide to when to sow tomato seeds.
Crops where it helps but is not critical
Brassicas gain three to four weeks and dodge the first flea beetle wave. Lettuce gains two to three weeks and slots into gaps. Cucumbers, squash and melons are the most root-sensitive crops here, but gain so much from a warm start that it pays – provided they get three to four weeks and go straight into a large pot.
Sweet corn works for two to three weeks in a deep container. Basil, parsley and thyme pay off because they germinate slowly and weeds beat them outdoors.
Crops to leave alone
- Carrots, parsnips, daikon and turnips. The taproot forks permanently at the smallest disturbance. A transplanted carrot is a forked carrot.
- Radishes. They germinate in three to five days and crop in four weeks – no time to save.
- Dill, coriander and chervil. Umbellifers bolt straight after root disturbance. Direct-sow in succession instead.
- Beans and peas. They germinate fast in 15 °C (59 °F) soil and grow so quickly that a two-week head start vanishes within a month. Peas take frost and go in during April.
- Spinach and mizuna. Both bolt from heat and root disturbance. Direct-sow early, while it is cool.
- Garlic and onion sets. These go in as bulbs, not seed, and garlic is planted in autumn.
- Potatoes. Sprouting in the light is a different process – see the guide to chitting potatoes.
The exception that proves the rule: in zones 3 and 4, beetroot and chard can be started three weeks ahead in multi-sown cells, because the season will not stretch otherwise. Beetroot's taproot is short enough to tolerate it.
The seed starting chart: depth, temperature, days and weeks
Four columns that normally sit in four different places. Germination temperature refers to the mix, not the room – a windowsill tray often runs three to five degrees Celsius below it. Days to germinate apply at the stated temperature; cooler takes twice as long.
| Crop | Sowing depth, cm (in) | Germination temp, °C (°F) | Days to germinate | Weeks before planting out | Worth it? |
|---|---|---|---|---|---|
| Tomato | 0.5–1 (1/4–3/8) | 20–25 (68–77) | 6–10 | 6–8 | Yes, almost always |
| Chilli pepper | 0.5–1 (1/4–3/8) | 25–28 (77–82) | 10–21 | 10–12 | Yes, essential |
| Sweet pepper | 0.5–1 (1/4–3/8) | 24–28 (75–82) | 8–14 | 10–12 | Yes, essential |
| Aubergine | 1 (3/8) | 24–28 (75–82) | 7–14 | 8–10 | Yes, essential |
| Tomatillo | 0.5 (1/4) | 22–26 (72–79) | 7–14 | 8–10 | Yes |
| Cucumber | 1.5–2 (5/8–3/4) | 22–28 (72–82) | 3–6 | 3–4 | Yes, briefly |
| Summer squash | 2–3 (3/4–11/4) | 21–27 (70–81) | 4–7 | 3–4 | Yes, briefly |
| Winter squash and pumpkin | 2–3 (3/4–11/4) | 21–27 (70–81) | 4–8 | 3–4 | Yes, briefly |
| Melon | 1.5–2 (5/8–3/4) | 24–28 (75–82) | 4–8 | 3–4 | Yes |
| Sweet corn | 2–3 (3/4–11/4) | 20–25 (68–77) | 4–8 | 2–3 | Yes, in a deep pot |
| Cabbage | 1 (3/8) | 18–22 (64–72) | 4–7 | 5–6 | Yes |
| Broccoli | 1 (3/8) | 18–22 (64–72) | 4–7 | 5–6 | Yes |
| Cauliflower | 1 (3/8) | 18–22 (64–72) | 5–8 | 5–6 | Yes |
| Kale and sprouts | 1 (3/8) | 18–22 (64–72) | 4–7 | 5–6 | Yes |
| Leek | 1 (3/8) | 18–22 (64–72) | 10–16 | 10–12 | Yes, essential |
| Bulb onion from seed | 1 (3/8) | 18–22 (64–72) | 6–12 | 8–10 | Yes |
| Celeriac and celery | Do not cover | 20–22 (68–72) | 14–21 | 10–12 | Yes, essential |
| Lettuce | 0.5 cm (1/4 in), barely covered | 15–20 (59–68) | 2–5 | 4–5 | Yes, keep it cool |
| Swiss chard | 2 (3/4) | 18–22 (64–72) | 7–12 | 3–4 | Optional |
| Beetroot | 2 (3/4) | 18–22 (64–72) | 7–12 | 3 | Zones 3–4 only |
| Basil | 0.5 (1/4) | 20–25 (68–77) | 5–10 | 6–8 | Yes |
| Parsley | 0.5–1 (1/4–3/8) | 18–22 (64–72) | 14–21 | 6–8 | Yes |
| Marigold and bedding plants | 0.5 (1/4) | 20–22 (68–72) | 5–10 | 6–8 | Yes |
| Carrot | 1–1.5 (3/8–5/8) | 15–25 (59–77) | 10–17 | – | No, direct-sow |
| Radish | 1–1.5 (3/8–5/8) | 15–20 (59–68) | 3–5 | – | No, direct-sow |
| Pea | 3–4 (11/4–11/2) | 15–20 (59–68) | 7–10 | – | No, direct-sow |
| Beans | 3–4 (11/4–11/2) | 20–25 (68–77) | 6–10 | – | Rarely, two weeks max |
| Spinach | 1.5 (5/8) | 10–18 (50–64) | 6–12 | – | No, direct-sow |
| Dill and coriander | 1 (3/8) | 15–20 (59–68) | 10–20 | – | No, direct-sow |
Two rows deserve a note. Lettuce is the only crop here that germinates worse as it warms: above 25 °C (77 °F) the seed enters thermal dormancy, so keep that tray cool. And celeriac needs light – press the seed down and nothing happens.
Counting backwards from your last frost date
Every date hangs off one point: the day the plant can safely stand outside. Everything else counts back from there.
Three dates you need
- Your average last frost. The zone table gives a benchmark, but your garden may differ – a south slope runs a week ahead of a frost pocket in the same township.
- Your planting-out date. For frost-tender crops, the last frost plus seven to ten days, once nights hold above 8–10 °C (46–50 °F). For frost-hardy crops such as brassicas, lettuce and leeks, two to three weeks before the last frost.
- Your sowing date. Planting-out date minus the weeks in the chart, plus three to five days for germination.
A greenhouse or tunnel shifts the whole chain
If the tomatoes are going into an unheated greenhouse or polytunnel, plant out when air and soil hold above 10–12 °C (50–54 °F) – three to four weeks before the outdoor date in a sunny structure. Sow three to four weeks earlier to match. An unheated structure gives no frost protection: on a clear May night it is as cold inside at dawn as out.
The mistake almost everyone makes
Sowing by the calendar instead of by the light. In January a windowsill at 50 degrees north gets about six hours of weak daylight. A tomato sown then runs on seed reserves for ten days, then stretches thin toward the glass. Without a lamp, the first half of March is the earliest sensible tomato sowing in zones 3 to 6. With a lamp, three weeks earlier. See the seed sowing calendar.
Zone table: sowing dates for USDA zones 3–6
These dates count backwards from each zone's normal planting-out window. Brassicas and lettuce take light frost and go out about two weeks ahead of everything else. A cold site or frost pocket behaves like the zone below.
| Zone | Typical areas | Average last frost | Plant out frost-tender | Sow chillies and peppers | Sow tomatoes | Sow brassicas and lettuce | Sow cucumbers and squash |
|---|---|---|---|---|---|---|---|
| 3 | Northern Minnesota, North Dakota, interior Alberta, northern Scandinavia | Late May to early June | 1–10 June | 13–27 March | 10–24 April | 10–17 April | 8–15 May |
| 4 | Central Minnesota, Vermont, New Hampshire, southern Quebec | Mid to late May | 20–31 May | 1–15 March | 30 March–13 April | 30 March–6 April | 27 April–4 May |
| 5 | Iowa, southern New England, southern Ontario, upland Scotland | Early to mid-May | 10–20 May | 20 February–6 March | 20 March–3 April | 20–27 March | 17–24 April |
| 6 | Ohio valley, coastal New England, lowland Scotland and Ireland | Mid to late April | 25 April–10 May | 5–20 February | 6–20 March | 6–13 March | 3–10 April |
Leeks, celeriac and bulb onions from seed use the same window as chillies and peppers, but go out earlier because they take frost. Sow them late in the window so they are not left waiting in the pot.
Light does not follow the zone. Further north spring arrives later, but days then lengthen fast and claw back part of the short season. In zone 3 it is usually better to sow late in each window and invest in a tunnel than to fight the March gloom. See also grow lights for seedlings.
Mix, containers and the kit you actually need
Seed-starting mix is not potting soil
Seed compost is lean, fine and open. A seed carries its own food through germination, and high nutrient levels damage new root hairs. Ordinary potting soil is too coarse and too rich: germination turns patchy and salt damage follows.
A mix that works is two parts peat-free seed compost or coir to one part perlite. The perlite keeps the structure open, the single biggest factor against damping off. Sieve out lumps. Never reuse last year's tray compost – damping-off fungi survive in it.
Containers: depth before width
Anything with drainage holes works; depth separates them. A plug tray with 3 cm (11/4 in) cells suits lettuce and brassicas but is too shallow for tomatoes, which root along the whole stem. Those want 6–7 cm (21/2–3 in) at pricking out and 9–10 cm (31/2–4 in) at the end.
- Plug trays use windowsill space best, but dry out fast and need pricking out.
- Small plastic pots are tough, stackable, easy to wash and dry more slowly.
- Coir pellets need no mix but dry out in the middle without showing it. The netting must come off at planting.
- Newspaper pots work but often grow mould outside. Use them for the last four weeks, not the first.
- Deep tubes or root trainers suit corn, beans and peas, which want to go down.
Wash used containers in hot soapy water. The point is removing old compost, not sterilising.
What you do not need
No heat mat is needed for tomatoes, brassicas or lettuce if the room holds 20 °C (68 °F), and no propagator either – a clear bag over the tray does the same job. You do need a lamp if you sow before mid-March, and a marker pen: unlabelled trays become a guessing game within two weeks.
Germination: warmth, moisture and patience
Between sowing and emergence a seed needs three things and not a fourth: moisture, warmth and oxygen. Most seeds need no light at all in that phase.
Wet the mix before you sow
Fill the containers, water through and drain before sowing. The mix should feel like a wrung-out cloth. Sowing dry and watering from above shifts the seed and caps the surface.
The right depth, and the exceptions
The rule is two to three times the seed's diameter. A 3 mm tomato seed sits just under a centimetre down. The two usual errors are sowing too deep – the seed germinates but cannot reach the surface – and forgetting which species need light. Celeriac, many lettuces and several bedding annuals are pressed onto the surface only.
Cover for moisture, but not for long
A lid or clear bag keeps humidity even until emergence. Lift it briefly each day to change the air. The moment the first cotyledons appear, take it off completely – still, humid air over new seedlings is exactly what damping off wants.
Drop the temperature immediately
This step is skipped more than any other, and it matters most. After germination the temperature should come down to 16–18 °C (61–65 °F). Warm air plus weak light is what produces leggy seedlings; cool and bright gives short, thick stems. Chillies and peppers want 20–22 °C (68–72 °F) throughout.
Do seedlings need grow lights?
It depends entirely on when you sow. In one sentence: before mid-March you need a lamp; after the first week of April a south window will do.
Why a window fails in February
Light is measured in both intensity and hours. A south window on a clear March day gives perhaps 10,000–20,000 lux at midday, but only for a few hours – on an overcast day a tenth of that. It also arrives from one side, so the plant leans and stretches. In February both intensity and day length are too low whatever the weather.
What works
- Hours: 12 to 16 a day. Beyond 16 there is no gain for most vegetables, and seedlings need darkness to respire.
- Distance: 5–15 cm (2–6 in) between tube or LED bar and leaves, raised as they grow. Too great a distance is the commonest reason a lamp seems not to help.
- Type: a plain high-output white LED batten works well. Pink-purple grow lamps are no better, only harder to judge leaf colour under.
- Timer: essential. An irregular day length stresses the plants.
Turn them and touch them
Rotate the tray half a turn every other day so the seedlings grow straight. Brush a palm across them daily, or run a small fan on low for a few hours. Mechanical stress makes stems shorter and thicker, exactly as wind does outdoors.
Bottom watering and the first feed
Bottom watering means standing the containers in a couple of centimetres of water and letting the mix draw it up through the drainage holes. When the surface darkens, lift them out to drain – ten to twenty minutes.
Three reasons. The surface stays drier, cutting the risk of damping off and mould. Seeds are not washed out of place. And roots grow down toward the water rather than gathering at the top.
How often
Let the surface dry between waterings. Lift the container – if it feels light, it is time. In practice, every second or third day in March and daily by May. Never leave containers in water more than half an hour; a wet base suffocates roots as efficiently as drought kills them.
When feeding starts
Seed compost holds almost no nutrients, which is correct until the first true leaves appear and the seed's reserve is spent. Pricking out into a richer mix covers another two or three weeks. After that: a weak liquid feed at a quarter of the bottle strength, once a week. Yellowing older leaves mean nitrogen shortage; dark green plants growing tall mean too much nitrogen for the light.
Pricking out and potting on
Pricking out is moving seedlings from a dense sowing into their own containers. Potting on is moving a plant into a larger pot once it outgrows the old one. Same handling, different moments.
When
Prick out once the first pair of true leaves has properly developed. For tomatoes that is around two weeks after sowing. Later and the roots tangle; earlier and the seedling is too fragile to handle.
How
- Bottom water an hour before so the mix holds together.
- Fill new pots with slightly richer compost and make a hole with a pencil.
- Lift the seedling by a cotyledon, never by the stem. A snapped stem is the end; a torn seed leaf is not.
- Lever from underneath with a dibber so the roots come with it.
- Set the plant deeper than it stood. Tomatoes, tomatillos and marigolds can go up to the cotyledons and root along the buried stem. Brassicas, lettuce and cucurbits stay at the same depth.
- Firm gently and bottom water.
- Keep them cool and lightly shaded for a day.
Cucumbers, squash, pumpkins and melons are not pricked out at all – their roots will not take it. Sow one seed straight into an 8–10 cm (3–4 in) pot and leave it there.
Hardening off without losing the plants
An indoor-raised plant has thin leaves, a thin wax layer and stomata that have never met dry wind. Put it straight into May sun and the leaves bleach within hours. Hardening off builds that protection over one to two weeks.
It takes seven to fourteen days and starts one to two weeks before planting out, as the frost risk fades.
- Days 1–2: two to three hours in full shade, out of the wind, in the warm part of the afternoon. Then back in.
- Days 3–4: four to five hours in shade with a spell of morning sun.
- Days 5–7: all day outside in part shade, in at night.
- Days 8–10: all day in sun, and outside overnight if it stays above 10 °C (50 °F).
- Days 11–14: outside around the clock. Now they can be planted.
Two things to watch. Wind is more dangerous than sun at first – dry air scorches unhardened leaf edges faster than light does, so set the trays in the lee of a wall. And water use climbs sharply outdoors; a pot that lasted two days indoors can dry out in one windy morning.
If time is short, a cold frame or a bench under fleece does much of the work. Lift the cover a little more each day. Plant out on an overcast afternoon and water in immediately. A raised bed can be planted a few days earlier than open ground because the soil is warmer.
Common problems and how to fix them
Damping off – seedlings collapse at the base
Damping off is caused by soil-borne fungi and fungus-like organisms, chiefly Pythium, Rhizoctonia, Fusarium and Phytophthora. The symptoms are distinctive: round bare patches where seeds died before emerging, or seedlings with a pinched brown stem base that flop flat within a day.
The organisms are almost always present. Conditions decide the outcome, and there are three: constantly wet mix, still air and dense sowing. Extension services also single out dirty containers and old compost as a common route of infection.
How to avoid it:
- Use fresh seed compost with perlite, and washed containers.
- Water from below and let the surface dry out between times.
- Take the lid off as soon as seedlings emerge.
- Sow thinly. Five tomato seeds in one small pot is five problems.
- Give air movement – a fan on its lowest setting daily.
- Avoid feeding during germination.
Once it starts there is no cure for affected seedlings. Remove them and the compost around them, stop overhead watering and increase air movement. Healthy seedlings in the same tray can be pricked out into fresh compost.
Leggy seedlings – tall, pale and floppy
The most common problem of all, and the cause is nearly always the same: too little light for the warmth. The plant builds stem instead of leaf.
Fixes, in order of effectiveness: drop to 16–18 °C (61–65 °F), move the light closer or get one, extend the day to 14–16 hours, thin the seedlings, and touch them daily.
A tomato that has already gone leggy can be saved: prick it out and bury it up to the cotyledons, and it roots along the over-long stem and comes back compact. The same works on tomatillo and marigold. It does not work on brassicas, lettuce, cucurbits or herbs – there you sow again.
White or green growth on the surface
White fluffy growth on the compost is usually a harmless saprophytic mould living on organic matter, not on the plant. It is still a warning: the surface is too wet and the air too still, precisely what damping off waits for. Scrape off the top layer and switch to bottom watering.
Green film is algae – harmless, same message about moisture. A hard white crust is different: salt from fertiliser or hard tap water. Flush with tepid water and cut the feed rate.
Nothing germinates at all
Work through it in order: compost too cold, seed too deep, a light-requiring seed that got covered, a surface that dried out during germination – or seed that is simply too old.
Seed longevity varies. Onion, leek, parsley and parsnip last one to two years. Tomato, carrot, bean, brassicas and squash three to four. Cucumber, lettuce and radish can still germinate after five or six. Test old seed: ten seeds on damp kitchen paper in a bag somewhere warm, counted after the days given in the chart. Seven to nine, sow thicker. Six or fewer, buy fresh.
How to start seeds indoors step by step
- Fix the planting-out date first. Look up your zone above and add seven to ten days of insurance for frost-tender crops.
- Count backwards. Planting-out date minus the weeks in the chart, plus a few days for germination. Write the date on the packet.
- Check the light. A sowing date before mid-March needs a lamp. No lamp means moving the date, not hoping.
- Wet the mix before filling. Blend seed compost with a fifth perlite, water through and drain.
- Sow thinly and at the right depth. Two to three times the seed's diameter; light-requiring species onto the surface. Two seeds per cell, not five.
- Label straight away. Variety and date. You will not remember.
- Hold 20–25 °C (68–77 °F) until emergence. Lid or clear bag on, lifted daily, off when the first cotyledons show.
- Drop to 16–18 °C (61–65 °F) and switch the lamp on. 12–16 hours, 5–15 cm above the leaves, on a timer.
- Water from below, letting the surface dry between times. Rotate the tray every other day.
- Prick out at the first true leaves, lifting by a cotyledon. Tomatoes go in deep; cucurbits are not pricked out.
- Feed weakly. A quarter-strength liquid feed weekly from the third week after pricking out.
- Harden off over 7–14 days. Shade and shelter first, sun and nights last.
- Plant out on an overcast afternoon and water in well. Use fleece the first week if nights are cold.
Common mistakes
- Sowing too early. The most enjoyable mistake and the most expensive. A tomato sown in January without a lamp is worse in May than one sown in March. Count from the planting-out date, not from impatience.
- Leaving the heat on after germination. 22–24 °C (72–75 °F) is right for the seed and wrong for the seedling. Drop to 16–18 °C as soon as the cotyledons are up, or you get leggy plants however good the lamp is.
- Hanging the lamp too high. Intensity falls off fast with distance. A lamp 50 cm above does almost nothing; the same lamp at 10 cm does everything.
- Watering from above every day. A wet surface plus still air is the recipe for damping off. Water from below and let the top dry.
- Sowing too densely. Five seeds in a small pot gives five seedlings that shade each other and trap moisture between them. Sow two and thin to one.
- Starting carrots, dill and radishes indoors. Taproot crops fork from root disturbance, and fast germinators have no time to save.
- Skipping the hardening off. A plant taken straight from windowsill to May sun scorches in a day and then sulks for three weeks.
- Reusing old compost and unwashed trays. Damping-off fungi overwinter in residue. Five minutes at the sink saves a whole tray.
Frequently asked questions
What does starting seeds indoors mean?
It means sowing in pots or cells under cover and planting out a finished transplant once the weather allows. The point is to move the plant's first six to twelve weeks into a space where you control the temperature, so long-season crops can finish inside a short cold-climate summer.
When should I start seeds indoors?
Count backwards from your planting-out date. Chillies and peppers 10–12 weeks before, tomatoes 6–8, brassicas and lettuce 5–6, cucumbers and squash only 3–4. In zone 5 that means chillies in late February, tomatoes in late March, cucumbers mid-April. Without grow lights, sow nothing before mid-March.
What temperature do seeds need to germinate?
Most vegetables germinate best at 20–25 °C (68–77 °F) in the compost. Chillies, peppers and aubergine want 24–28 °C (75–82 °F). Lettuce is the exception at 15–20 °C (59–68 °F); above 25 °C it enters thermal dormancy. After germination, drop to 16–18 °C (61–65 °F) for everything but chillies and peppers.
How deep should I sow seeds?
Two to three times the seed's own diameter. A tomato seed ends up just under a centimetre down, a bean three to four centimetres. Light-requiring species such as celeriac, many lettuces and several bedding annuals are not covered at all, only pressed onto the damp surface. Sowing too deep is a common reason nothing appears.
Why are my seedlings tall and thin?
Too little light for the temperature, so the plant stretches instead of building leaf. Drop to 16–18 °C (61–65 °F), move the lamp to 5–15 cm above the leaves, extend the day to 14–16 hours and thin the tray. A leggy tomato can be pricked out and buried up to the cotyledons, where it roots along the stem.
What is damping off and how do I stop it?
Damping off is caused by soil fungi such as Pythium and Rhizoctonia. The seedling develops a brown, pinched stem base at soil level and collapses, and affected plants cannot be saved. Prevent it with fresh compost, washed containers, thin sowing, bottom watering and air movement, and remove the lid as soon as seedlings emerge.
Do I have to prick out my seedlings?
Only if they are crowded or have outgrown the container. Prick out once the first pair of true leaves has developed, lifting by a cotyledon and never by the stem. Cucumbers, squash, pumpkins and melons should not be pricked out at all – sow those straight into an 8–10 cm pot.
How long does hardening off take?
Seven to fourteen days. Start with two to three hours in shade and shelter, increase time and sun daily, and leave them out overnight at the end if it stays above 10 °C (50 °F). Wind scorches leaves faster than sun early on. Plant out on an overcast afternoon and water in straight away.
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