How to start seeds indoors

Learning how to start seeds indoors gives warm-season crops a head start the outdoor season cannot. A tomato sown indoors in March can be a sturdy transplant by the time the soil is warm enough to plant out. Getting there takes the right sowing time, enough warmth to germinate, and enough light to grow stocky.

When to start

University of Maryland Extension publishes the approximate time needed to grow transplants before outdoor planting: leafy greens 2 to 4 weeks; cabbage, broccoli, and cauliflower 4 to 6 weeks; cucumber, squash, and melons 3 to 5 weeks; tomatoes 5 to 6 weeks; peppers, eggplant, and onions 8 to 10 weeks (source).

These are transplant-growing durations, not weeks-before-last-frost figures. The right way to use them: set a transplant target date from frost risk and the crop's temperature needs (peppers want nights reliably above 50°F, not merely frost-free), then count backward by the crop's duration. Our planting date calculator computes sowing and transplant dates from your average last frost date using per-crop week offsets — for tomatoes, six weeks before to sow and one week after to transplant. The calculator works from frost dates, not soil temperatures; check soil warmth with a thermometer separately. Sowing too early is the classic mistake — Maryland notes it produces tall, root-bound plants that perform poorly after transplanting.

Containers

Almost any clean container works if it allows good drainage and is at least 2 inches deep — reused yogurt cups, cell packs, and standard flats all qualify (source). What matters is cleanliness: reused containers can carry damping-off pathogens.

If you disinfect, use a dilute bleach soak. Maryland Extension specifies soaking washed containers and tools for 1 to 2 minutes in 1 part household chlorine bleach to 10 parts water, then rinsing with clean water. Household bleach is typically around 5–6% sodium hypochlorite (check the label — concentrations vary), so this 1:10 dilution is roughly a half-percent solution. That is very different from a 10% active-ingredient concentration, which would be dangerously strong; the supported procedure is the dilute soak plus rinse.

Growing media

Do not use garden soil. Maryland Extension is explicit: at roughly 75 pounds per cubic foot, soil compacts in small containers, drains poorly, and may contain weed seeds and pathogens (source). Use a soilless seed-starting mix — typically peat or coir with perlite or vermiculite, around 10 pounds per cubic foot, fluffy and free-draining. Fresh bagged mix is generally free of weeds and pathogens; that is cleanliness, not a guarantee, and damping-off prevention still depends on drainage, air movement, and not overwatering.

Pre-moisten the mix before filling containers. Peat-based mixes are hydrophobic when dry and hard to wet evenly once seeds are in. Our garden soil vs potting soil vs topsoil article explains the differences in more depth.

Sowing

Maryland Extension gives a suitable planting depth of about twice the seed's diameter — a seed an eighth of an inch wide goes a quarter inch deep (source). Label everything at sowing; seedlings of different varieties look identical for weeks.

Lettuce is the notable exception: Maryland notes its seeds can be left on the surface, uncovered or very lightly covered, since they need light to germinate. Check the packet for each crop rather than assuming.

Sow tomatoes, peppers, and eggplant singly (one seed per cell); they need the space and are typically moved to larger pots 2 to 3 weeks after germination. For cucumbers, squash, melons, corn, and beans, Maryland suggests 2 to 3 seeds per cell, keeping the strongest after germination.

Temperature

Soil temperature, not air temperature, governs germination speed. The UC Master Gardener Program publishes optimum germination ranges from the California Master Gardener Handbook (Table 5.2) (source):

As a general rule, Maryland Extension states 65° to 75°F potting-mix temperature suits most vegetable seeds.

Speed follows temperature. UC's emergence data shows tomato seed germinating in about 6 to 8 days within the 65–85°F optimum range, but taking about 43 days at 50°F — and effectively none at 104°F. A heat mat under the tray is one way to reach the optimum range for warm-season crops; a warm spot such as the top of a refrigerator is another. Remove bottom heat once seedlings emerge, and give them cooler air after germination — Maryland suggests 55–60°F nights and 65–70°F days.

Cover the tray with a humidity dome or plastic (not touching the mix) until sprouting, then remove it promptly. Trapped humidity after germination invites damping-off.

Light

Maryland Extension states that natural light from a window is "seldom enough for strong seedling growth" — stems stretch and lean, producing weak plants. The exception it allows: acceptable growth "usually only occurs in south-facing bay windows or sun rooms that receive ample direct sunlight" (source).

For fixtures, follow type-specific guidance:

• Standard fluorescent tubes and LED shop lights: Maryland advises 2 to 6 inches above the plants, lit 14 to 16 hours a day on an outlet timer, raising the fixture as seedlings grow. The remaining 8 to 10 hours should be dark — seedlings need a nightly rest period, not 24-hour light.
• High-output LED panels: follow the manufacturer's recommended hanging distance for the seedling stage. Do not assume the shop-light spacing applies.

Begin lighting as seedlings emerge — the tray goes under the lights when sprouts appear, whenever that is, not on a preset calendar date. If stems stretch tall, thin, and pale, the light is too far away or too weak for the fixture type. Tomato and cabbage-family seedlings are particularly prone to stretching toward inadequate light.

Water and seedling health

Maryland's rule: keep the media moist but not wet, watering again when the top of the media is nearly dry. Saturated mix displaces oxygen — roots stop growing and die — and wet conditions favor the damping-off pathogens Pythium and Phytophthora (water molds) and Rhizoctonia (a soil fungus), which can kill a tray quickly.

Bottom-watering — setting containers in a shallow tray of water and letting the mix wick it up — keeps the surface drier. A small fan on low for a few hours a day thickens stems and discourages fungal problems. Thin to the strongest seedling per cell once true leaves appear.

After transplanting, establishment watering depends on weather and soil — there is no universal daily schedule. In Florida conditions, UF/IFAS Extension advises watering transplants daily for the first week as they establish, while holding fertilizer for 10–14 days until roots adjust (source). In cooler or wetter climates, check soil moisture and water as needed rather than on a fixed daily routine.

Hardening off

Seedlings raised indoors have thin cuticles and no wind or sun conditioning. Maryland Extension defines hardening-off as gradually acclimating indoor- or greenhouse-grown transplants to harsher outdoor conditions, over a 5- to 14-day period depending on the crop; the process slows growth while tissues toughen (source).

One adaptable schedule — adjust to wind, sun, and temperature; no fixed day count fits every crop and climate:

Check moisture before each outing and water as needed — small pots dry fast outdoors, but the rule is the moisture, not the calendar. If frost, storms, or high winds threaten, bring trays under cover; the University of Delaware's extension notes transplants should be moved inside when freezing temperatures or thunderstorms are expected (source). Transplant on an overcast day or late afternoon when possible, and water immediately to settle soil around the roots (Washington State University Extension, source).

Worked example: tomatoes

Treat every date below as a hypothetical local planning date for illustration — not a forecast, and not derived from any zone map.

Assume a gardener whose local records suggest frost risk typically ends around May 15. For tomatoes, the transplant target also needs soil at or above 60°F and nights reliably above 50°F.

If planting is delayed past May 15, seedlings need continued suitable outdoor exposure — keep them outside during mild days with protection as needed, rather than moving them back to indoor-only conditions. Night-temperature requirements (above 50°F for tomatoes) apply whenever seedlings stay outside overnight, whether during hardening or after a delayed transplant.

If a late frost threatens after transplanting, cover with row fabric.

How to start seeds indoors vs sowing direct

Once you know how to start seeds indoors, the remaining decision is method, not timing. Maryland Extension lists tomatoes, peppers, eggplant, cabbage, broccoli, and most herbs as indoor-start candidates, and notes cucurbits can go either way; root crops are the clearest direct-sow case since transplanting can damage their roots (source).

"Usually direct-sown" is a practical default, not a biological rule — Maryland gives container cell sizes for lettuce, beets, and spinach, so they transplant successfully; they are simply easy enough to sow where they will grow.

Sources

• UC Master Gardener Program — Seed Germination Temperature and Timing (germination ranges from California Master Gardener Handbook Table 5.2; emergence days from Harrington, UC Davis)
• University of Maryland Extension — Starting Vegetable Seeds Indoors (timing, containers, media, sowing, light, water, disinfection)
• University of Maryland Extension — Planting Vegetable Transplants (hardening-off definition and duration)
• UF/IFAS Gardening Solutions — Starting Transplants Indoors (hardening-off steps, establishment watering and fertilizer timing)
• Washington State University Extension — Home Vegetable Gardening (transplant timing and watering)
• University of Delaware Extension — Weekly Crop Update (hardening-off shelter and acclimation)