Late winter has its own particular restlessness for gardeners. Seed catalogs pile up, the days start getting longer, and there is a strong urge to plant something, anything. Starting seeds indoors scratches that itch, and it does something useful too: it gives warm-season crops the head start they need to produce well in a limited outdoor season.
The trick is timing. Start too late, and your tomatoes will still be tiny when the weather warms. Start too early, and you will have tall, floppy seedlings with cramped roots, waiting by a window for weeks with nowhere to go. The key to getting it right is a single date: your average last spring frost. This guide explains how to find it, how to count backward from it for each crop, and how to grow strong seedlings once they are up.
Why start seeds indoors?
Some vegetables need a long, warm season to produce. Tomatoes, peppers and eggplant can take two to three months or more from transplanting to their main harvest, and much longer from seed. In much of the US, there simply are not enough frost-free days to sow them outdoors and still get a good crop. Starting them indoors six to ten weeks early makes up the difference.
Indoor starting also offers far more variety than the garden center. You can grow unusual tomatoes, specialty peppers and heirloom varieties that are rarely sold as plants. And it is much cheaper per plant than buying transplants, especially if you grow a lot.
Not everything benefits, though. Some crops are better sown straight into the garden, as we will see below.
Step one: find your last frost date
Your average last spring frost date is the starting point for almost all seed-starting timing. It is the average date of the last freezing temperature in spring at your location, based on decades of weather records.
You can usually find it through your state Cooperative Extension service, many seed company websites, or local gardening groups. Some sites look it up by ZIP code from weather station records. Use the nearest station that is similar to your garden in elevation and setting, because frost dates can vary a lot over short distances.
An average is not a guarantee. The average last frost date is the point where frost later in spring is roughly as likely as not. Many gardeners plan seed starting around the average date, but wait a week or two past it before setting out frost-tender plants, and keep row cover handy just in case.
Do not confuse your frost date with your USDA hardiness zone. Your zone describes how cold your winters get, which matters for perennials. Your frost date describes when your growing season starts, which is what matters for vegetables. Two towns in the same zone can have last frost dates weeks apart.
Step two: count back for each crop
Each crop has a typical indoor growing period before it goes into the garden. Count back that many weeks from your last frost date to find your sowing date.
| Crop | Start indoors | Move outdoors |
|---|---|---|
| Onions (from seed) | 8–10 weeks before last frost | A few weeks before last frost |
| Peppers, eggplant | 8–10 weeks before | 1–2 weeks after last frost, in warm soil |
| Parsley | 8–10 weeks before | Around last frost |
| Tomatoes | 6–8 weeks before | 1–2 weeks after last frost |
| Broccoli, cabbage, cauliflower | 6–8 weeks before | About 2 weeks before last frost |
| Lettuce | 4–6 weeks before | 2–4 weeks before last frost |
| Basil | About 6 weeks before | After last frost, in warm weather |
| Cucumbers, squash, melons | 3–4 weeks before | After last frost, in warm soil |
These ranges match the starting guidance in our plant database. Seed packets sometimes give slightly different figures for specific varieties. When they do, follow the packet.
A worked example
Suppose your average last frost date is April 20. Counting backward gives these sowing dates:
- 10 weeks before (February 9): onions, the early pepper and eggplant sowings, parsley.
- 8 weeks before (February 23): the rest of the peppers and eggplant, the first tomatoes, broccoli and cabbage.
- 6 weeks before (March 9): the main tomato sowing, basil, lettuce.
- 4 weeks before (March 23): squash and melons for an early start.
- 3 weeks before (March 30): cucumbers.
Then, outdoors: broccoli and cabbage go out around April 6, and tomatoes, peppers, basil and cucurbits go out from late April into May, once nights stay warm.
Writing these dates on a calendar in January, while the seed packets are fresh in your mind, is one of the most useful things you can do for your garden.
Crops you should not start indoors
Some crops do better sown directly in the garden.
- Root crops: carrots, radishes, beets, turnips and parsnips. Transplanting disturbs the taproot and can leave you with forked or stunted roots.
- Beans and peas: they germinate quickly outdoors in the right conditions and gain little from an indoor start.
- Corn: it grows quickly and dislikes root disturbance.
- Spinach and many quick greens: usually easier sown in place, in cool spring or fall weather.
Cucumbers, squash and melons are borderline. They can be started indoors, but only a few weeks ahead, in individual pots, and transplanted gently. In regions with long, warm summers, sowing them directly is often simpler.
What you need to start seeds
Containers
Almost anything with drainage holes works: plastic cell trays, small pots, soil blocks, or recycled containers cleaned well. Cell trays are space-efficient for many seedlings. Larger crops such as tomatoes and peppers will later need potting up into bigger containers.
Seed-starting mix
Use a fine, light seed-starting mix, not garden soil. Garden soil is too dense in small containers, drains poorly, and can carry disease organisms that cause seedlings to collapse. Bagged seed-starting mixes are usually based on peat or coir with perlite or vermiculite. Moisten the mix before filling containers.
Light
Light is the single most common limiting factor indoors. A sunny windowsill is rarely enough in late winter. Days are short, the light comes from one side, and seedlings stretch toward it, becoming tall and weak. A simple LED shop light or grow light, hung close above the seedlings and run for about 14 to 16 hours a day on a timer, makes a dramatic difference. Raise the light as the plants grow, keeping it close enough to prevent stretching, following the manufacturer’s guidance for distance.
Warmth
Many seeds germinate fastest in warm soil. Tomatoes, peppers and eggplant in particular germinate much better with a seedling heat mat under the tray. Once seedlings are up, remove the heat and keep them in a comfortable room. Slightly cooler temperatures produce sturdier plants.
Sowing and growing seedlings
- Sow at the right depth. A good rule is about two to three times the seed’s diameter. Tiny seeds such as lettuce are barely covered.
- Label everything. Seedlings of different tomato varieties look identical. Write the variety and sowing date on a label in each container.
- Cover until germination. A clear dome or plastic wrap keeps humidity in while seeds sprout. Remove it as soon as most have emerged.
- Water from below. Set trays in a shallow tray of water for a few minutes, then let them drain. Keep the mix moist but not soggy.
- Thin to one per cell. If several seeds sprout, snip the extras at soil level with scissors rather than pulling them.
- Provide air movement. A small fan on a low setting nearby strengthens stems and reduces fungal problems.
- Pot up. When tomatoes, peppers and other larger crops have a couple of sets of true leaves, move them into bigger pots. Tomatoes can be buried deeper than before, because they grow roots along the buried stem.
- Feed lightly. Seed-starting mixes contain few nutrients. Once seedlings have true leaves, feed with a diluted liquid fertilizer every week or two.
The seed quantity calculator helps you decide how many seeds to sow. Many gardeners start about 20 to 30 percent more plants than they need, then keep the strongest.
Hardening off: the step people skip
Seedlings grown indoors are soft. They have never felt wind, direct sun or cool nights. Moving them straight outdoors can scorch leaves, stunt growth or kill them outright. Hardening off is the process of gradually toughening them up, and it usually takes about a week to ten days.
- Day 1–2: put seedlings outside for an hour or two in a sheltered, shady spot.
- Day 3–4: leave them out for half a day, with some gentle morning sun.
- Day 5–7: extend to most of the day, with more direct sun.
- Day 8–10: leave them out all day and, if nights are mild, overnight.
Bring them in if frost or strong winds are forecast. Then transplant on a cloudy day or in the evening, and water them in well.
Troubleshooting common seedling problems
| Problem | Likely cause | Fix |
|---|---|---|
| Tall, thin, floppy seedlings | Too little light, or too much warmth | Lower the light, run it longer, keep the room a little cooler |
| Seedlings collapse at the soil line | Damping off, a fungal disease | Use fresh mix and clean containers, avoid overwatering, add air movement |
| Seeds do not sprout | Too cold, too deep, too dry, or old seed | Use a heat mat, check depth, keep moist, test seed viability |
| Pale or yellowing leaves | Lack of nutrients, or overwatering | Start a dilute feed, let the surface dry slightly between waterings |
| White fuzz on the soil | Usually harmless mold from constant moisture | Water from below less often and improve air flow |
| Purple undersides on tomato leaves | Often cool temperatures or low phosphorus uptake | Keep them warmer and feed lightly; usually fades outdoors |
Starting seeds for fall
Seed starting is not just a spring activity. Broccoli, cabbage, cauliflower, kale and lettuce for fall harvests are often started in mid to late summer, indoors or in a shaded spot outdoors, and transplanted as the heat breaks. For fall crops, count back from your first fall frost date instead, using the crop’s days to maturity plus a couple of extra weeks to allow for slower growth as days shorten.
Is it worth it?
For crops like tomatoes, peppers and onions, almost always yes. A few seed packets and a light can produce dozens of plants for the price of a handful of nursery transplants, in varieties you will never find at a garden center. There is also real pleasure in watching seedlings appear in February, months before the garden wakes up.
If this is your first year, keep it simple: start just tomatoes, peppers and basil, and sow everything else directly or buy transplants. Our list of the 10 easiest vegetables for first-time gardeners notes which are best from seed and which from transplants.
A simple, low-cost seed-starting setup
You do not need a greenhouse or expensive equipment to raise healthy seedlings. A basic setup for a typical home garden can include:
- A shelf or table in a room that stays roughly at normal indoor temperature.
- One or two LED shop lights hung on chains or adjustable cords so you can raise them as seedlings grow.
- An outlet timer set to run the lights for 14 to 16 hours a day.
- A seedling heat mat for tomatoes, peppers and eggplant, used only until they germinate.
- Cell trays or small pots, plus solid trays underneath for bottom watering.
- A bag of seed-starting mix, moistened before use.
- A small fan for gentle air movement.
- Labels and a waterproof marker.
Most of this lasts for many years. Compared with buying transplants every spring, it often pays for itself within a season or two, especially if you grow several varieties of tomatoes and peppers.
Window light versus grow lights
A south-facing window can work for a few seedlings in places with bright late-winter sun, especially for crops started later in spring when days are longer. Turn the trays every day so seedlings do not lean toward the glass, and expect them to be somewhat leggier than plants grown under lights. For anything started in January or February, or for more than a tray or two, supplemental lights give far more reliable results.
Frequently asked questions
When should I start tomato seeds indoors?
About six to eight weeks before your average last spring frost date. Transplant them outdoors one to two weeks after that date, once nights stay reliably warm.
When should I start pepper seeds?
About eight to ten weeks before your last frost date. Peppers germinate and grow slowly, and they benefit from a heat mat for germination.
Can I start seeds on a windowsill?
You can, but seedlings often grow tall and weak because winter window light is too dim and one-sided. A simple LED light hung close above the seedlings for 14 to 16 hours a day gives much sturdier plants.
What if I started my seeds too early?
Pot them up into larger containers so roots have room, keep them cooler, and give them plenty of light. Tomatoes that have grown leggy can be planted deeper when transplanted.
Do I really need to harden off seedlings?
Yes. Seedlings moved straight from indoors to full sun and wind are often scorched or stunted. A week to ten days of gradual exposure makes a big difference.
How do I find my last frost date?
Your state Cooperative Extension service, many seed company websites and local gardening groups publish average frost dates. Some sites look them up by ZIP code from nearby weather stations.