Seed Starting Indoors: Mix, Trays, Lights and Timing

Which seeds to start indoors

Starting indoors pays off for crops that need a long season or a head start on the weather. Clemson lists broccoli, cabbage, kale, tomatoes, peppers, eggplant, lettuce and onions as crops that transplant easily. Beans, corn and carrots do poorly as transplants and should be sown straight into the garden, and UGA adds large-seeded crops such as pumpkins and root crops such as beets. Squash, cucumbers and melons can be started indoors, but their roots resent disturbance, so they go in individual pots for a short 3-5 weeks.

Timing is counted back from the date you plan to plant out. Clemson’s rough guide: squash, cucumbers and melons 3-5 weeks; broccoli, cabbage, kale and lettuce 5-7 weeks; tomatoes and eggplant 6-8 weeks; peppers 8-10 weeks. The calculator above turns those weeks into dates for your ZIP code. Starting earlier is not better: Colorado State warns that plants started too soon get crowded and spindly before it is safe to set them out.

Seed starting mix

Seed starting mix is a soilless growing medium: it contains no garden soil. Most blends are mainly sphagnum peat moss or coconut coir, with vermiculite and/or perlite for air space and lime to adjust the pH (Utah State, Maryland). Peat holds a great deal of water, while the coarse particles leave large pores so excess water drains away and roots get oxygen. Bagged mixes are also generally free of weed seeds, insects and disease organisms.

Why not garden soil. Garden soil is too heavy for a small cell. Maryland puts it at about 75 lbs per cubic foot, against about 10 lbs for soilless mix. In a container it stays wet, starves roots and seeds of oxygen, and brings weed seeds and the fungi that cause damping-off. Wisconsin’s plant disease clinic is blunt: do not use garden soil, because it often contains damping-off pathogens. Illinois adds compost and used potting mix to that list for seed starting.

Seed starting mix or potting mix. Extension advice differs here. Clemson recommends a product labeled as seed-starting mix and advises against bagged topsoil, vegetable garden mixes and potting mixes. Maryland says a standard all-purpose transplant mix is fine and a finer seedling mix is only needed for very small seeds. Both agree on avoiding heavy mixes: Maryland warns against products containing “forest products” and bags that feel heavy, and Utah State against mixes with large wood chips.

Nutrients. Many seed-starting mixes contain little or no fertilizer. UGA prefers starting seed in a mix without fertilizer; some mixes include a starter charge that Utah State says lasts a few weeks at most. Check the bag.

Making your own. Utah State suggests equal parts peat moss, perlite and/or vermiculite, though it warns you will not save money. UGA’s recipe is 4 quarts shredded sphagnum peat, 4 quarts fine vermiculite, 1 tablespoon superphosphate and 2 tablespoons ground limestone, wetted and left 5-6 days before sowing so the lime can react with the peat.

Moisten the mix before filling containers. Fill firmly but do not pack it down (Colorado State).

Seed starting trays, cells and containers

Any container works if it drains and is deep enough for roots. Maryland’s minimum is about 2 inches deep. The common choices:

• Flats (trays). The standard nursery flat is about 10.5 by 21 inches and 2 inches deep, sold with or without drainage holes (Maryland). A tray without holes goes underneath a holed tray or cell insert to catch water and makes bottom watering easy.
• Cell inserts. Plastic inserts divide a flat into individual cells. Home growers usually use 36, 48 or 72 cells per flat; Utah State notes that very high counts (up to 288 cells) dry out fast and need more care. One seedling per cell means no root tearing at transplant time.
• Open flats. Sowing a whole flat of mix saves space for small seeds, but roots tangle and must be teased apart when you pot up.
• Peat pots, fiber pots and peat pellets. These go into the ground with the plant, which suits cucumbers, melons and squash. Slit the sides and bury the rim completely: an exposed peat rim wicks water out of the root ball (Clemson, Maryland). Utah State notes they can be slow to break down in dry climates.
• Reused containers. Yogurt cups, salad clamshells and milk cartons work if you punch drainage holes.

Clean containers before reuse. Wash off old soil, then soak in a 10% bleach solution (1 part bleach to 9 parts water): Wisconsin recommends at least 20 minutes, Illinois 30, then a thorough rinse. If you have had damping-off before, both suggest new pots and trays, since used plastic is hard to fully clean.

A clear humidity dome, plastic wrap or a loose plastic bag keeps the surface moist until seeds sprout. Remove it once seedlings appear (Utah State says when about half have come up), or the humid air invites disease and stretching.

Sowing depth and germination temperature

UGA’s rule is to sow most seeds about twice as deep as the seed is wide; tiny seeds are pressed onto the surface and barely covered. Some seeds need light to germinate. Illinois names lettuce, dill, snapdragons and coleus: sow them on the surface and press them in. Sowing too deep is a common reason seeds fail to come up.

Germination is driven by the temperature of the mix, not the room. UGA’s rule of thumb is 65-75°F for most seeds, and Illinois finds most germinate best at 70-75°F. Utah State uses 70°F for both cool- and warm-season crops. Cooler mix slows germination and leaves seeds open to damping-off longer.

Heat mats. A seedling heat mat under the tray warms the mix a few degrees, which matters in a cool basement and for warm-season crops such as peppers and tomatoes. Utah State recommends a mat with a thermostat so trays do not overheat, and a cheap kitchen thermometer to check the mix. Once seedlings emerge, turn the mat off if the room is warm enough.

Grow lights: type, distance and hours

A window is rarely enough. Maryland says natural light from a window is seldom sufficient for sturdy seedlings outside a south-facing bay window or sunroom, and seedlings on a sill stretch and lean toward the glass.

Type. You do not need expensive lights sold as grow lights. Utah State finds plain fluorescent or LED shop lights affordable and sufficient. Purdue suggests a two-tube fluorescent fixture with one cool-white and one warm-white tube. Fluorescent tubes dim with age; Purdue gives about two years of useful life, and Illinois advises replacing old tubes if seedlings stretch even under lights.

Distance. Keep lights close: Clemson 2-4 inches above the seedlings, UGA and Utah State 2-3 inches, UNH and Purdue 3-4 inches, Maryland 1-2 inches for fluorescent tubes. Hang the fixture on chains or adjustable cords and raise it as the plants grow. Lights that run hot need more distance to avoid scorched leaves.

Hours. Run lights 14-16 hours a day on a timer (Maryland, UNH, Purdue); UGA uses 16 hours, Clemson 16-18, Utah State 12-14 or more. Seedlings still need a dark period each night.

Temperature after germination

Once seedlings are up, slightly cooler air keeps them compact. Purdue and Utah State recommend nights of 60-65°F with days about 10°F warmer for most vegetables and annual flowers. Cool-season crops such as broccoli, cabbage, kale and spinach prefer nights around 55°F and days near 65°F. Warmer than that, seedlings turn spindly. Maryland gives 55-60°F at night and 65-70°F by day after germination. A cool spare room or basement under lights often works well.

Watering and feeding seedlings

Keep the mix moist but never soggy. Seedlings die from both extremes: a short dry spell kills young plants, but waterlogged mix drowns roots and encourages damping-off (Utah State, Illinois). Newly emerged seedlings use little water and rarely need daily watering. Maryland waits until the surface is nearly dry. Utah State’s test: lift the tray when fully watered, learn its weight, and water when it feels noticeably lighter.

Bottom watering keeps the surface drier and does not wash seeds out. Set the tray in 1-2 inches of water for 3-10 minutes, then drain it (Clemson). Utah State suggests watering from the top now and then to flush fertilizer salts, and pouring off drainage water rather than letting the tray soak it back up.

Fertilizer. Start feeding when the first true leaves appear. Clemson feeds at every other watering with a water-soluble starter fertilizer. Utah State uses half-strength water-soluble fertilizer about once a week if the label gives no seedling rate. Too much feed produces soft, weak growth, so it is better to underdo it. If your mix includes fertilizer, Maryland says it usually carries seedlings for 5-6 weeks.

Air movement. A small fan near the seedlings strengthens stems and dries the air around them, which helps prevent damping-off (Clemson, Utah State). Clemson starts the fan once plants have two true leaves. Utah State notes that a gentle breeze for short periods is enough: strong wind damages seedlings and dries trays faster.

Thinning and potting up

Sow two or three seeds per cell to cover for poor germination, then thin to one once the first true leaves show (the leaves after the round seed leaves). Snip extras off at the surface with small scissors rather than pulling them, which tears the roots of the one you keep (Purdue, Colorado State, UGA). Seedlings left crowded compete for light and stretch.

Seedlings sown in an open flat need potting up into individual cells or pots when the first true leaves appear (Maryland, UGA). Utah State watches for leaves touching neighbors or roots matting at the bottom. Lift each seedling with a pencil or plant label and hold it by a seed leaf, never the stem, which is so fragile that a slight injury can do permanent damage (Clemson). Set it at the same depth or slightly deeper.

Hardening off seedlings

Seedlings raised indoors have never felt sun, wind or cold, and planting them straight out can scorch or stunt them. Hardening off acclimates them over 1-2 weeks before planting (UGA, Purdue, UNH; Clemson allows 2-3 weeks).

1. Start in a shaded, sheltered spot outdoors for a few hours a day.
2. Each day, leave them out longer and move them gradually into more sun.
3. Cut back on watering slightly and stop fertilizing (UNH), but do not let plants wilt.
4. By the end, they stay out day and night. Bring them in if frost is forecast.

Keep tender seedlings such as tomatoes, peppers, squash and melons indoors when it is below about 45-50°F (Utah State, Purdue). Utah State also warns that long exposure of cool-season seedlings such as cabbage and broccoli to temperatures below 40°F can make them flower prematurely. Some leaf yellowing during hardening off is normal. Transplant on a cloudy day or in the morning and water in well.

Common seedling problems

Damping-off. Seedlings come up healthy, then collapse at the soil line and die. It is caused by soilborne fungi and water molds, including Pythium, Rhizoctonia and Fusarium, and it is worst in cool, wet mix (Wisconsin, Penn State). Affected seedlings cannot be saved; seedlings become much less susceptible once they have true leaves. Prevention is the whole cure:

• Fresh, sterile mix and clean or new containers; no garden soil or reused mix.
• Warm mix for fast germination, and seeds no deeper than needed.
• No overwatering; let the surface dry slightly between waterings.
• Spacing and a fan for airflow, and the humidity dome off once seeds are up.

If it strikes, throw out the seedlings and the mix with them, and sanitize the container before reuse.

Leggy seedlings. Tall, thin, pale stems that flop over almost always mean too little light: a window, lights too high, or old tubes (Illinois). Warmth, crowding and too much fertilizer add to it. Lower the lights, cool the room a few degrees, thin crowded cells and run a fan. Leggy tomatoes can be planted deeper, since they root along the buried stem.

Leaning seedlings. Plants bend toward a one-sided light source. Add light or turn the trays daily (Utah State).

Poor germination. Usually old seed, mix that is too cold, too wet or too dry, or seeds sown too deep. Illinois suggests a germination test for saved seed: roll 10 seeds in a damp paper towel in a bag in a warm place and count sprouts after 7-10 days. Below about 70% germination, buy fresh seed.

Algae, fungus gnats and edema. Green film on the mix and small flies around trays both mean the mix stays too wet (Illinois). Small white or tan bumps on tomato leaves are edema, a water-balance problem that Utah State treats with more air movement, enough light and less water.