How to Start Seeds Indoors: Timing, Light and Hardening Off

Count each crop's weeks indoors back from your plant-out day, which your last spring frost decides: roughly 5 to 8 weeks for tomatoes, 6 to 10 for peppers and 2 to 5 for squash, cucumbers and melons. Sow in a sterile soilless mix kept warm and moist until seedlings show, give them 12 to 18 hours of strong light, and harden them off over one to three weeks.

For that light, see the best grow lights for seedlings; for cells, domes and bigger pots to move seedlings into, see the best seed starting trays.

When to start seeds indoors: count back from your plant-out date

Pick the day you expect to plant each crop outside, then count back by its weeks indoors, printed on the seed packet. Warm crops such as tomatoes, peppers, melons and cucumbers go out after your average last frost date; broccoli, cabbage and some other crops that shrug off a light freeze can go 2 to 4 weeks before it. Look up your average last frost date by ZIP code, then choose a plant-out day that leaves room for a late frost.

Each range spans the guides' figures, some given as approximate and some written for South Carolina. The MN figures come from a sowing calendar for Minnesota gardens, so its cool-season figures run long.

A worked example. For a May 15 plant-out day, sow tomatoes (5 to 8 weeks) between March 20 and April 10, peppers (6 to 10) between March 6 and April 3, and squash (2 to 5) from April 10 to May 1. Redo the count with your own date.

What you'll need for starting seeds indoors

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• 

  Old Potters Germination Mix

  Sow in a sterile soilless mix; the basic kit starts with a soilless seed starting mix.

  Check price on Amazon
• 

  Bootstrap Farmer 32 Cell Seed Starter Kit

  A clear humidity dome holds moisture until seedlings are up.

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• 

  Barrina TX-S50 4FT LED Grow Light

  Seed starting calls for lights on a timer.

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• 

  GE 24-Hour Plug-In Mechanical Timer, 2 Pack

  Lights on a timer are in the basic seed starting kit.

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• 

  VIVOSUN 10 x 20.75 Seedling Heat Mat and Digital Thermostat

  A seedling heat mat helps in a room colder than the 65 to 75 F that suits most seed.

  Check price on Amazon

Which vegetables to start indoors and which to sow outside

Start slow, long-season crops indoors and sow fast or touchy ones outside. Tomatoes, peppers, eggplant, broccoli, cabbage, kale, lettuce, onions and sweet potatoes take well to transplanting, while beans, corn and carrots transplant poorly and belong in the garden from seed, though corn and beans can be raised as transplants for just 2 to 4 weeks.

• Warm crops with a long season. Tomatoes, peppers and eggplant may not have time to mature if sown outdoors once it is warm enough. In Minnesota, okra can start with them, about 9 weeks ahead; see our okra guide.
• Cool crops that must beat the heat. Cabbage, broccoli, cauliflower and lettuce get an early start indoors so they mature before summer heat.
• Either way. Squash, cucumbers and melons can be started inside or sown in the bed; indoors, raise them in fiber or paper pots planted whole, since they transplant poorly.
• Outside from seed. A direct-sown list includes beans, beets, carrots, sweet corn, kohlrabi, lettuce, peas and radishes, so lettuce can go either way; see our beans guide for sowing dates.

Seed starting mix and containers

Use a sterile, soilless seed starting mix in clean containers that drain. Soilless mixes cut down on soil-borne disease such as damping off, so skip bagged topsoil and garden mixes. Commercial mixes with no true soil, usually peat and vermiculite, are sterile, light and weed-free.

Why not garden soil. True soil weighs roughly 75 pounds per cubic foot, too dense for air and water to move in a small pot, while a good soilless mix weighs about 10; soil can also carry weed seed and disease.

Seed starter or potting mix. A fine-textured "seed starter" mix helps very small seeds but is not needed for most vegetable seed, though the stricter advice is to skip ordinary potting mixes for seed. Either way, dampen the mix before you fill, since dry peat sheds water.

Containers. Almost any clean container works if it drains and holds at least 2 inches of mix. Cell trays with one seedling per cell keep roots apart, since tangled roots are likely to tear at transplanting. Onions and leeks can grow together in one larger flat. Cell sizes by crop:

• Leafy greens: 3 to 6 square inches per cell.
• Squash, cucumbers, broccoli and cabbage: 4 to 9.
• Tomatoes, peppers and eggplant: 9 to 16.

Reusing trays. Wash and disinfect old trays, either in 1 part chlorine bleach to 9 parts water or with a 30-minute soak in disinfectant at label strength, then a rinse.

How to sow: depth, moisture and domes

Sow most seeds 1 to 2 times as deep as they are wide, typically about twice, then firm them in and keep the mix moist under a cover until sprouts appear. A seed 1/8 inch wide can go 1/4 inch deep, so one more seed would fit on top.

1. Fill and firm. Fill trays with the dampened mix to about an inch below the rim and press it level.
2. Sow thinly. Rows spaced 1 to 2 inches, with seed 1/8 to 1/4 inch down, are easier to label and lift later. In cells, sow two or three seeds of cucumber, squash, melon or older seed, and later snip all but the strongest at soil level instead of pulling them.
3. Mind seeds that need light. Lettuce and some others sprout only in light, so press them onto the surface without covering.
4. Cover for moisture. A clear dome or plastic bag keeps humidity in. Keep the cover off the surface and take it off once sprouts appear, or at the latest when the seedlings touch it; moving air helps keep damping off away.
5. Water gently from below, setting the flat in water until the surface turns damp, so the seeds stay put.

Bottom heat: how warm seeds need to be

Most vegetable seed sprouts well in a mix held at 65 to 75 F, and a heat mat under the tray can hold that in a cool room. Other figures for most seeds are a steady 70 to 75 F, or anywhere from 68 to 86 F. For a mat, see our picks for the best seedling heat mats.

By crop. Onions, lettuce and spinach can sprout in soil as cold as 32 to 40 F, while peppers, eggplant, melons, squash and okra need at least 60 F. The chart gives each crop's figures, lowest to highest.

The same figures as a table

Germination figures by crop

Crop	Lowest soil temperature (F)	Ideal range (F)	Days to sprout
Arugula	Not listed	40 to 55	5 to 7
Cilantro	Not listed	55 to 70	about 7 to 21
Peas	40	50 to 75	6 to 17
Spinach	32 to 40	55 to 75	6 to 14
Swiss chard	40	45 to 86	7 to 14
Carrots	40	50 to 85	about 6 to 21
Lettuce	32 to 35	55 to 80	about 3 to 14
Kale	Not listed	65 to over 75	4 to 7
Radishes	40	55 to 85	3 to 10
Broccoli	40	60 to 85	about 3 to 14
Onions	32 to 35	about 60 to 85	4 to 20
Basil	Not listed	65 to 85	5 to 14
Cauliflower	40	65 to 85	about 3 to 14
Leeks	32 to 40	65 to 85	7 to 12
Sweet corn	50	55 to 95	about 3 to 10
Tomatoes	50	65 to 85	5 to 14
Turnips	40	60 to 95	about 1 to 10
Cucumbers	50 to 60	65 to 95	3 to 10
Eggplant	60	70 to 90	5 to 14
Melons	60	70 to 90	3 to 12
Peppers	60	65 to 95	5 to 20
Snap beans	55 to 60	70 to 90	6 to 14
Okra	60	70 to 95	about 6 to 7
Squash and pumpkins	60 to 65	75 to 95	3 to 12
Watermelon	60	75 to 95	about 2 to 12

Each range runs from the lowest figure in the guides to the highest. Ideal ranges are soil or mix temperatures, except one table that gives the air around the tray. Seed takes longer to sprout outside its ideal range, and may not sprout at all when it is far too cold or hot.

Do you need a heat mat? Not always: a warm spot in the house usually does the job, and mats or heating cables suit cool rooms. Bottom heat can help most seeds sprout sooner and resist damping off, and the mix in a tray can run as much as 5 F colder than the room. Plug the mat in on its own, not into the timer that runs your lights.

Cooler after sprouting. Once seedlings are up, they can go cooler: 60 to 70 F by day and 55 to 60 F at night, which heads off soft, leggy growth. Another rule is about 10 F below the seed's optimum germination temperature.

How much light do seedlings need?

Seedlings need strong light for 12 to 18 hours a day from fluorescent or LED lamps hung 2 to 6 inches above them; a window alone is seldom enough.

Raise the lamp as they grow. Hang the fixture on chains, start it just above the mix and raise it as seedlings grow, since a small gap makes a stocky plant. With 48-inch fluorescent tubes, that gap is 2 to 4 inches.

In the drawing: a lamp on chains and its timer, a cell tray over a drip tray, a clear dome, a heat mat and a fan. For the trays and the light, see our picks of the best seed starting trays and the best grow lights for seedlings; What you'll need lists the rest.

Windowsills. A sill can be the coldest spot in the house at night and the hottest by day, and seedlings there often grow tall with thin, bent stems. Growth is usually acceptable only in a sunny, south-facing bay or sunroom, though a south-facing window can do for a few plants on wide sills.

Watering and feeding seedlings

Keep the mix moist but never soggy, and hold off on fertilizer until true leaves appear. Too much water pushes air out of the mix and the roots stop growing, so water thoroughly once the top is nearly dry or dry to the touch. Drain any water left standing in the tray.

Bottom watering. Fill a pan or lid with water 1 to 2 inches deep, sit the tray in it for 3 to 10 minutes, and put it back under the lights.

When to start feeding. Advice splits four ways:

• From the first true leaves, a water-soluble starter fertilizer at every other watering.
• From several sets of true leaves, a fertilizer at 1/4 strength once a week.
• A dilute solution every two weeks, or none if the mix has slow-release fertilizer.
• None for a while: most mixes carry enough fertilizer for at least 6 weeks, so read the bag before you add any.

Potting up seedlings

Move seedlings into their own pots at the first or second set of true leaves, while they are small and can recover fast. Leaving them too long in a crowded flat is one of the most common mistakes, since tangled roots tear more easily when separated.

• Hold a leaf, not the stem. A seedling can replace a lost leaf but may not survive losing its growing tip.
• Go up a size. Move seedlings into pots 1 to 2 inches wider than their cells, lifting each by its root ball with a spoon or plant tag.
• Tomatoes go deeper. Set seedlings with only the leaves and growing point above the mix. A tomato that gets very tall can be laid sideways in a trench at planting with 2 or 3 sets of leaves showing, and the buried stem roots.

Why are my seedlings leggy?

Too little light is the main cause of long, skinny stems, and seedlings in low light grow weak, often toppling when a few inches tall. Four other factors contribute: overwatering, high temperatures, too much fertilizer and crowding.

How to fix leggy seedlings:

• Lower the lamp and check the timer.
• Cool them down to 60 to 70 F, with nights at 55 to 60 F.
• Give them air. Set a fan near seedlings from two true leaves on, so they sway gently, and watch the watering, since moving air dries them.
• Brush the tops. Run a hand or a wooden stick over tomato transplants to help keep them short and sturdy.
• Sow later next time. Sown too early, seedlings turn tall and root-bound and do poorly once planted.

Tomatoes and the cabbage family tend to stretch toward the light, and an ideal transplant is about as wide as it is tall.

Damping off: why seedlings collapse

When seedlings collapse and die soon after sprouting, damping off is the likely cause. Several fungi and fungus-like water molds cause it, among them Pythium, Rhizoctonia and Fusarium, and a poorly drained mix, crowding and overwatering go with it. Some attack seeds before seedlings come up.

How to tell. Stems usually rot at or near the soil line, with dead seedlings scattered or in patches; in greenhouse flats, they die in a circle spreading from one point. Damage that hits nearly every plant evenly usually points elsewhere, such as too much fertilizer, drowning, drying out, heat or cold.

How to prevent damping off:

• Start with clean, disinfected containers and a sterile, well-drained mix.
• Sow thinly so air moves between seedlings.
• Keep the mix evenly moist for germination, then let it dry a little between waterings.
• Keep it warm before seedlings come up: a sheet for commercial greenhouse growers says a mix below 68 F at that stage can favor the disease.

If it strikes. It can be hard to stop once started. Drier conditions, more air movement, less frequent watering and more light may slow the spread. To start over, throw out the plants and the mix, then wash containers and tools and soak them 1 to 2 minutes in 1 part bleach to 10 parts water before a rinse.

How to harden off seedlings

Harden off seedlings by setting them outside for a little longer each day over one to three weeks, starting in shade and out of the wind. Indoor plants have never felt full sun, wind or swings in temperature, and without a gradual start their leaves can scorch, or the plants can wilt and die.

1. Start early enough. Most schedules run two weeks or more, up to three, before planting out; bedding plants should be ready after 7 to 10 days.
2. Day one. Put seedlings outside in shade for a few hours on a warm afternoon, sheltered from wind, and bring them in before it cools at night.
3. Each day after. Leave them out longer, with a little more direct sun, and check them once or twice a day.
4. Watch the weather. Keep tender seedlings in on windy days or below 45 F, and bring them in for strong wind, storms or frost.
5. Last days. On a two-week schedule, seedlings can stay out in the sun by the end of it until planting, unless a freeze is forecast.

In a cold frame, open the lid a little more each day and close it on freezing nights. For a ready-made cold frame, see the best mini greenhouses. One caution: extreme hardening can do harm. Cauliflower can form thumb-sized heads that never grow, and cucumbers and melons can stop growing entirely.

When to transplant seedlings outside

Plant them out once they are hardened off and the crop's planting date for your area has come, ideally under cloud cover or toward evening to ease the shock of the move. Warm crops wait until after the average last frost date, and some cool crops like broccoli can go 2 to 4 weeks before it.

• Water right away after planting. If plants wilt, protect them for a few days under an open milk carton or a row cover. Hardened plants that wilt in full sun usually recover within a day or so.
• Trim fiber pots. A peat or paper pot rim that sticks out of the soil wicks water away from the roots, so tear or cut it to soil level and open the bottom.

Seed starting supplies

The basic kit is short: a soilless seed starting mix, cell trays or containers, lights on a timer, a fan and a water-soluble fertilizer. A clear humidity dome holds moisture until seedlings are up, and a seedling heat mat helps in a room colder than the 65 to 75 F that suits most seed. For seed, buy fresh packets of the crops in the weeks table, sized to what you will sow in two or three years.

Sources we used

We saved a copy of each page below on October 29 or 30, 2026.

• "Clemson Cooperative Extension HGIC 1259: Starting Seeds Indoors"
• "University of Maryland Extension: Starting Vegetable Seeds Indoors"
• "University of Minnesota Extension: Starting seeds indoors"
• "Iowa State University Extension: How to Successfully Start Seed Indoors", "Germination Requirements", "Hardening Bedding Plants" and answers on "heat mats", "spindly seedlings" and "collapsing seedlings"
• "NC State Extension Gardener Handbook, Chapter 13: Propagation" and "Damping-off in Flower and Vegetable Seedlings"
• "UConn Extension IPM: Damping-off of Ornamental and Vegetable Seedlings"
• For the germination chart: "UC Cooperative Extension Sacramento County: Soil Temperature Conditions for Vegetable Seed Germination" and "Colorado State University Extension GardenNotes 720: Vegetable Planting Guide", with crop figures from Fedco Seeds (peppers), Clemson Extension (eggplant, lettuce, okra, basil), University of Minnesota Extension (broccoli, cauliflower, melons, pumpkins and winter squash), Johnny's Selected Seeds (kale, Swiss chard, bush beans, cucumbers, seedless watermelon, cilantro), Utah State Extension (lettuce, spinach, Swiss chard, basil, cilantro), Kitchen Garden Seeds (Swiss chard), Cornell Garden-Based Learning (arugula), Baker Creek (onions), Penn State Extension (peas), Iowa State Extension (sweet corn) and Oregon State Extension (cilantro)

Related guides

• How to grow tomatoes, often the first tray of the year.
• How to grow peppers, among the slowest starters in the weeks table.
• How to grow onions and leeks, which can share one flat.
• Brassicas, the cabbage family.
• Cucurbits, squash, cucumbers and melons, started either way.
• Frost dates by ZIP code, for choosing a plant-out day that leaves room for a late frost.
• Low light houseplants, with grow-light hours for foliage plants.
• Flowers from seed, on which annuals to start inside and which to sow in the bed.
• How to grow zinnias, started indoors a few weeks early or sown in place.
• Buying guides for the gear: the best seed starting trays, chosen by cell size, and the best grow lights for seedlings.
• Best seedling heat mats, for a tray in a cool room.

Frequently asked questions

When should I start seeds indoors?

Count back from your planting-out date by each crop's approximate weeks indoors: tomatoes 5 to 8 weeks, peppers 6 to 10, squash, cucumbers and melons 2 to 5. Warm crops go out after your average last frost date.

How long do seedlings need light each day?

Aim for 12 to 18 hours of lamp light daily, with fluorescent or LED fixtures on a timer. Don't leave the lights on around the clock, because many plants need some darkness every night.

How do you harden off seedlings?

Set seedlings outdoors for a few hours in shade and out of the wind, then add time and sun each day for 7 to 10 days to three weeks. Keep tender seedlings in on windy days or below 45 F.

How do you prevent damping off in seedlings?

Use clean, disinfected containers, a sterile well-drained mix and thin sowing, then let the mix dry a little between waterings after sprouting.

Do you need a heat mat to start seeds?

Not always: most seed sprouts well with the mix at 65 to 75 F, and a warm spot in the house often gets there. Bottom heat can speed sprouting and help against damping off.

What soil do you use to start seeds indoors?

Use a sterile, soilless seed starting mix, not garden soil or bagged topsoil; soilless mixes reduce damping off. Commercial mixes are usually peat and vermiculite.