Starting vegetables from seed is cheaper than buying plants, gives you far more varieties to choose from, and is deeply satisfying — but it is also where many beginners go wrong, usually by starting too early, sowing too deep, or keeping seedlings too wet. This guide covers vegetables seed starting properly: which vegetables to start indoors and which to sow direct, when to start each one, what compost and containers to use, how much light seedlings need, how to water them, and how to harden them off before planting out. The timings below use published ranges for common vegetables, which vary by variety, climate and soil. As with everything in this guide, check your local extension service for advice suited to your area.
The first decision is indoor or direct. Some vegetables hate transplanting and must be sown where they are to grow: carrots, parsnips, radishes, beans, peas and sweet corn are all direct-sown, because their roots dislike disturbance and they grow fast enough not to need a head start. Others benefit greatly from an indoor start: tomatoes, peppers, eggplant, cucumbers, zucchini and brassicas all gain weeks from being started indoors and planted out after the last frost. Lettuce, spinach, kale and beets can go either way — start them indoors for an early crop, or sow direct for the main crop.
Start indoors or sow direct?
| Vegetable | Method | Days to maturity | Notes |
|---|---|---|---|
| Tomatoes | Start indoors | 60–85 | Start 6–8 weeks before last frost; needs warmth |
| Bell peppers | Start indoors | 60–90 | Start 8–10 weeks before last frost; slow to germinate |
| Eggplant | Start indoors | 70–85 | Start 8–10 weeks before last frost; needs warmth |
| Cucumbers | Start indoors or direct | 50–70 | Start 3–4 weeks before last frost, or direct sow after frost |
| Zucchini | Start indoors or direct | 45–55 | Start 3–4 weeks before last frost, or direct sow after frost |
| Broccoli | Start indoors or direct | 60–85 | Start 6–8 weeks before last frost for an early crop |
| Cabbage | Start indoors or direct | 60–90 | Start 6–8 weeks before last frost for an early crop |
| Lettuce | Either | 45–60 | Start indoors for early crop; direct sow little and often |
| Kale | Either | 50–65 | Hardy; direct sow or start indoors |
| Beets | Direct (or start indoors) | 50–70 | Direct sow preferred; transplant carefully if started indoors |
| Carrots | Direct sow only | 60–80 | Never transplant; roots fork if disturbed |
| Radishes | Direct sow only | 25–40 | Too fast to need an indoor start |
| Green beans | Direct sow only | 50–65 | Sow after last frost; roots dislike disturbance |
| Peas | Direct sow only | 60–70 | Sow early; hardy; roots dislike disturbance |
| Sweet corn | Direct sow only | 60–90 | Sow after last frost in blocks, not rows |
| Onions | Start indoors or from sets | 90–120 | Start seed indoors 8–10 weeks before last frost, or plant sets |
| Leeks | Start indoors | 90–120 | Start 8–10 weeks before last frost; transplant young |
Tomatoes are the classic indoor-start vegetable: sow them indoors 6–8 weeks before the last frost, keep them warm, and plant them out after frost when they are sturdy young plants. Peppers and eggplant need even longer indoors — 8–10 weeks — because they germinate slowly and grow slowly at first. Carrots are the opposite: sow them direct, thin them to 2–3 inches apart, and never transplant them, since a disturbed carrot root forks. Beans, peas, radishes and sweet corn are all direct-sown for the same reason. The seed starting calculator works out sowing dates for your area, and the seed starting mix guide covers the right compost for starting seed.
Containers, compost and sowing depth
Use clean pots, trays or modules, and fill them with fresh seed compost — not garden soil, which can carry damping-off fungi, and not ordinary potting mix, which is often too coarse for tiny seeds. Seed compost is fine, light and low in nutrients, which is exactly what a germinating seed needs. Sow seeds at a depth of roughly twice their width: tiny seeds such as lettuce sit on the surface and need light to germinate, while large seeds such as beans go an inch deep. Covering a tiny seed too deep is one of the most common reasons for failure — if in doubt, sow shallow.
Water from below if you can: stand the tray in a shallow dish of water and let the compost soak it up, then drain it. Watering from above with a can can wash tiny seeds away or bury them too deep. Cover the tray with a clear lid or plastic wrap until seeds germinate, to hold in moisture, then remove the cover as soon as seedlings appear — leaving it on invites damping-off. Label everything with the vegetable, variety and date, because seedlings all look alike at first and you will forget what you sowed within a week.
Light, warmth and watering
Light is the most common reason indoor seedlings fail. A windowsill is rarely enough in early spring: seedlings stretch towards the light, grow tall, pale and leggy, and flop over. A grow light, hung a few inches above the seedlings and run for 14–16 hours a day, makes a dramatic difference — keep the light close and raise it as the plants grow. If you have no grow light, use the brightest windowsill you have, turn the trays every day, and accept that seedlings will be leggier than ideal.
Warmth matters too, and different vegetables want different temperatures. Tomatoes, peppers and eggplant germinate best in warmth, around 70–80°F, and a heat mat or the top of a fridge helps. Lettuce, spinach, peas and brassicas germinate in cooler conditions and can sulk in too much heat. Once seedlings are up, most vegetables grow best in cooler conditions than they germinated in — around 60–65°F — which keeps them stocky rather than leggy. Water carefully: keep the compost moist but never wet, water in the morning, and let the surface dry slightly between waterings. Overwatering is the main cause of damping-off, the fungal disease that makes seedlings collapse at soil level. The damping-off seedlings guide explains it in detail.
Thinning, feeding and potting on
Thin seedlings early and ruthlessly. A tray of lettuce sown too thickly produces a mat of weak seedlings that never amount to anything; thinned to proper spacing, the same tray produces strong plants. Thin carrots to 2–3 inches apart, lettuce to 8–12 inches, beets to 3–4 inches, and do it when seedlings are an inch or two tall — the longer you wait, the more the roots tangle. Snip unwanted seedlings off at soil level with scissors rather than pulling them, which disturbs the roots of the ones you keep.
Seed compost holds little food, so start feeding once seedlings have their first true leaves — the second pair of leaves, which look like the adult plant's leaves rather than the smooth seed leaves that appear first. Feed with a half-strength liquid fertiliser once a week. Pot seedlings on into larger pots when their roots fill the module — you will see roots coming out of the drainage holes — and handle them by a leaf, never by the stem, which bruises easily. The vegetables transplanting guide covers the next step, and the fertiliser calculator helps with feeding rates.
Hardening off before planting out
Hardening off is the gradual process of getting indoor seedlings used to outdoor conditions, and skipping it is the most common reason transplants fail. A seedling raised indoors in still, warm air has soft leaves and a thin skin; put it straight outside and the wind, sun and cold will scorch it, wilt it, or kill it. Harden plants off over 7–10 days: put them outside in a sheltered, shady spot for a few hours on the first day, bring them in at night, and increase the time outside and the exposure to sun and wind a little each day. By the end of the week they should be outside all day and night, unless frost threatens.
Do not plant out tender vegetables — tomatoes, peppers, eggplant, cucumbers, zucchini, beans — until after the last frost, and wait until the soil has warmed up. Hardy vegetables — peas, lettuce, spinach, kale, broccoli, cabbage — can go out earlier, but they still benefit from hardening off. Water transplants well before planting out, plant on a cloudy day or in the evening if you can, and water them in well. The vegetables indoor growing guide covers growing indoors beyond the seedling stage.
Common seed starting mistakes
- Starting seeds too early, so plants are leggy and root-bound before the weather is ready.
- Sowing seeds too deep, especially tiny seeds such as lettuce.
- Using garden soil instead of seed compost, so damping-off kills the seedlings.
- Keeping seedlings too wet, so they rot at soil level.
- Giving seedlings too little light, so they grow tall, pale and leggy.
- Never thinning, so seedlings crowd each other and stay weak.
- Skipping hardening off, so transplants scorch or die in their first days outside.
- Planting tender vegetables out before the last frost.
- Forgetting to label trays, so you do not know what you planted.
Most seed starting mistakes are made in the first two weeks, and they are all avoidable. Sow at the right time, sow shallow, use fresh compost, give plenty of light, water carefully, thin early, and harden off before planting out. That is the whole method.
Frequently asked questions
When should I start tomato seeds indoors?
About 6–8 weeks before the last frost in your area. Starting earlier gives leggy, root-bound plants that do worse than plants started at the right time — check your local extension service for your area's last frost date.
Why are my seedlings tall and leggy?
Not enough light. Move them to a brighter spot or add a grow light hung close above them, and turn the trays every day.
Why did my seedlings collapse at soil level?
Damping-off, a fungal disease caused by wet conditions and poor airflow. Use fresh seed compost, do not overwater, and give seedlings air movement.
Can I transplant carrots?
No — carrots should always be direct sown. Transplanted carrot roots fork and grow misshapen. Sow them where they are to grow and thin to 2–3 inches apart.
How do I harden off seedlings?
Put them outside in a sheltered spot for a few hours a day, increasing the time and exposure over 7–10 days, until they are outside all day and night before planting out.
Related guides
For more vegetable help, browse the guides hub and the growing guides hub, and try the seed starting calculator and the harvest date calculator. Good next reads are the vegetables transplanting guide, the vegetables soil preparation guide, and the damping-off seedlings guide. And as always, check your local extension service — local advice beats any general guide for your soil, your weather and your varieties.