Starting Lettuce and Leafy Green Seeds Indoors

Leafy greens are the quickest transplants you can raise indoors. Maryland Extension puts the indoor stretch at only 2 to 4 weeks for lettuce, spinach, kale and similar greens before they go outside, and Clemson Extension’s table gives lettuce 5 to 7 weeks from seeding to transplanting. Either way, the plants spend less time on the shelf than tomatoes or peppers, which means less light to supply and less chance to stall. The two things that decide success are sowing the seed at the right depth and keeping the seedlings cool. Lettuce seed germinates best in light, so it is left uncovered or barely covered, and the mix should stay on the cool side rather than being pushed along with heat.

When to sow lettuce and kale indoors

Count backward from the date you plan to set the plants out, which for cool-season greens comes before the last spring frost. Clemson’s table gives lettuce, kale and collards 5 to 7 weeks from seeding to transplanting. Maryland Extension’s shorter figure of 2 to 4 weeks for leafy greens reflects how fast they grow once they are up; if you sow too early, the plants sit in their cells and stretch.

Greens are cool-season crops, so they can go outside earlier than tomatoes or peppers. Maryland Extension sets broccoli, a close relative of kale, out four weeks before the last spring frost; that figure is given for broccoli only, so check the seed packet or the local Cooperative Extension office for the planting date of each green. The soil does not need to be as warm as it does for fruiting crops.

For a fall crop, sow indoors in summer and set plants out in time for them to size up before hard frost. Maryland Extension notes that fall transplants of broccoli take about 4 to 6 weeks from seeding, and that fall broccoli often yields better than spring in the mid-Atlantic; that figure is for broccoli, but greens grow quickly in summer warmth too. Plan the fall sowing date from your first fall frost rather than your last spring one; see fall vegetable transplants for how to work backward from that date.

A short indoor schedule for greens

Sowing lettuce seed: shallow, uncovered, cool

Lettuce and celery seed germinate best in light and are left uncovered or barely covered. Maryland Extension’s rule for sowing depth is about twice the seed’s diameter, with a seed 1/8 inch wide going about 1/4 inch deep; for lettuce, treat that as a maximum and leave the seed on the surface, pressed into the mix. Burying lettuce seed is a common reason a flat comes up bare.

Use a soilless seed-starting mix, not garden soil. Moisten it before filling the containers; it should feel like a wrung-out sponge. Fill cells to within one inch of the top. For greens, Maryland Extension suggests cells or pots of 3 to 6 square inches, which is small; a standard 10-by-20-inch flat holds 10 to 120 plants depending on cell size.

Temperature matters more than most beginners expect. A potting-mix temperature of 65° to 75°F suits most vegetable seeds, but lettuce germinates best when the mix is not pushed to the top of that range. Soil over 85°F is usually a problem for spinach and lettuce seed. Keep the flat in a cool room rather than on a heat mat; if you use a dome to hold moisture, take it off as soon as sprouts appear. For more on domes and bottom heat, see germination temperature and heat mats.

After germination, seedlings do well with a 55 to 60°F night temperature and a 65 to 70°F day temperature. A cool windowsill or a basement shelf under lights suits greens better than a warm living room.

Light, water and the short time on the shelf

A window is seldom enough for seedlings; stems stretch and lean toward the glass. Keep fluorescent or LED fixtures 2 to 6 inches above the plants, on for 14 to 16 hours a day with an outlet timer, as Maryland Extension describes. Clemson gives 16 to 18 hours of bright light and keeps tubes 2 to 4 inches above the seedlings, raising them as the plants grow. After potting up, keep the tops of plants 1 to 2 inches from the lights.

Water when the top of the mix is nearly dry. Water from the top until it drains, or set the containers in a shallow tray of water and let them absorb it; Clemson suggests filling a tray with 1 to 2 inches of water and letting the cells absorb it for 3 to 10 minutes. Saturated mix starves roots of oxygen and invites damping-off.

Most potting mixes carry enough fertilizer to grow a seedling for at least 6 weeks, and some labels say 6 to 8 weeks after germination. Greens are in their cells for only a few weeks, so they usually need nothing more. If you do feed, Clemson’s practice is to feed at every other watering once the first true leaves appear, with plain water between feedings, and to rinse liquid fertilizer off the leaves because young seedlings burn easily.

A small fan near the seedlings from the two-true-leaf stage makes them sway and begins toughening them, but it dries them faster, so watch moisture.

Hardening off cool-season greens

Hardening off gradually adapts indoor-grown plants to sun, wind and cold. Maryland Extension describes a 5- to 14-day period depending on the crop, beginning 1 to 2 weeks before planting out; Clemson says to start once plants are about three inches tall and continue for two to three weeks. Greens are cool-season crops, so they tolerate more cold than tomatoes, but they still need the gradual introduction.

Start in a shaded, wind-protected spot for part of the day, increase sun by an hour or two each day, and water less often but never let plants wilt. Do not put tender seedlings out on windy days or below 45°F. Bring cool-season crops such as lettuce and cabbage in if a freeze is forecast. A cold frame is an excellent place to harden off greens. The full method is in how to harden off seedlings.

Overdoing hardening harms some crops: cauliflower can form thumb-sized heads and stop. Hardening matters most for cabbage, broccoli and cauliflower; it matters less for warm-season transplants set out after all danger of frost into soil above 60°F.

Setting greens out and spacing them

Plant on a cloudy, calm day. Water the plants several hours before, dig a hole large enough for the root mass, firm the soil, and water in with a weak liquid fertilizer at label dilution. Shade and shelter new transplants for a few days with baskets, pots or floating row cover.

Good transplants are stocky with deep green leaves and white, fibrous roots. Avoid overgrown, woody plants with a thick mat of circling roots, and check for pests before planting.

Follow the seed-packet spacing for each crop; squeezing plants closer causes disease and lowers yield. Full sun for fruiting vegetables means at least 6 hours of direct light a day, and greens generally want similar light in spring, though they appreciate some afternoon shade as summer heat builds. Biodegradable pots can go in the ground, but remove the base and any rim above the soil line, because an exposed rim wicks water away from the roots.

What goes wrong with lettuce and kale seedlings

Burying the seed. Lettuce seed germinates best in light. If it is buried in mix, germination drops or fails. Sow on the surface and press it in.

Too much heat. A heat mat or a warm room speeds other crops but can keep lettuce and spinach from germinating well. Keep the mix cool.

Holding plants too long. Greens grow fast, and a flat left too long becomes crowded and leggy. Maryland Extension calls leaving seedlings crowded in the flat one of the most common mistakes. Pot up or plant out on schedule.

Leggy seedlings. Tall, stretched, weak stems come from too little light, fixtures too far away, or a windowsill. Warmth and crowding make it worse. Move the lights closer and keep the room cool.

Damping-off. The water molds Pythium and Phytophthora and the soil fungus Rhizoctonia need free water; seedlings under lights die when the mix is poorly drained or kept too wet. It can kill seeds before they emerge and collapse young seedlings. Infected seedlings cannot be saved: remove and discard them with their mix, wash containers, soak them for 1 to 2 minutes in 1 part chlorine bleach to 10 parts water, and rinse. Prevent it with soilless mix, clean containers and letting the top of the mix dry slightly between waterings.

Fungus gnats. Tiny dark flies around pots are mainly a sign of overwatering. Let the mix dry and use yellow sticky traps to monitor. Products based on Bacillus thuringiensis var. israelensis are labeled for the larvae; follow the label.

Summer heat. Lettuce bolts and turns bitter in heat. For a summer succession, start seeds indoors in a cool spot and set them out where they get afternoon shade, or plan the main crop for spring and fall.

A note on other transplants

Tomatoes and peppers need a longer indoor run than greens, and their seedlings are handled differently. If you are raising both, keep the schedules separate; see growing tomato seedlings for the warm-season timing.