Pepper and Eggplant Seedlings: Slow Starters That Need Heat

Peppers and eggplant are the slowest of the common vegetable seedlings, and they are often started too late, kept too cool, or set out into soil that has not warmed. Both need a long indoor run-up — Maryland Extension puts pepper, eggplant and onion at 8 to 10 weeks before the plants go outside, and notes that some pepper types need 12 weeks from seed. Clemson Extension’s table gives pepper 8 to 10 weeks and tomato and eggplant 6 to 8 weeks. The practical reading: count back from your average frost-free date, sow pepper seed indoors 8 to 10 weeks before it, and give both crops warmth from the first day.

Why these two crops get the longest head start

A tomato sown 5 to 6 weeks before planting out makes a stocky 8- to 10-inch plant. A pepper given the same window arrives at planting time as a small seedling that then sits in the garden doing very little. Peppers and eggplant grow slowly at every stage, and they grow slowest when the mix and the air are cool. The indoor weeks are not padding; they are the time the plant needs to build a stem, a root system and a few sets of true leaves before it faces the garden.

Sowing too early is still a mistake. Maryland Extension names it as a common error: seed started too soon gives tall, root-bound plants that do poorly after transplanting. The window is long but it is a window. Work from your frost date rather than from the first warm day in February — when to start seeds indoors explains how to count back.

Getting pepper seed to germinate

Pepper seeds not germinating is a common early complaint, and the usual cause is temperature. A potting-mix temperature of 65° to 75°F suits most vegetable seeds, Maryland Extension reports. Pepper and eggplant sit at the upper end of that range and below it they simply take longer, or rot.

• Use a soilless mix. Seed-starting or potting mix, not garden soil, bagged topsoil or garden mix. Garden soil is too dense in a container — about 75 pounds per cubic foot against about 10 pounds per cubic foot for a good soilless medium — and may carry weed seeds and pathogens. Moisten it before filling; it should feel like a wrung-out sponge.
• Sow at the right depth. About twice the seed’s diameter: a seed 1/8 inch wide goes about 1/4 inch deep. Maryland Extension’s germination page states the rule as 2 to 3 times the seed’s diameter.
• Hold the warmth. A seedling heat mat speeds germination, and a plastic tent over a grow light can hold 70° to 75°F. The top of a refrigerator is a warm spot. A clear cover or dome keeps moisture in until sprouts appear and comes off as soon as they do. Seed germination temperature, domes and heat mats covers the setup in detail.
• Be patient before you resow. Eggplant and pepper are not fast, and a cool flat slows them further. Check the mix moisture rather than adding water on a schedule; saturated media starve roots of oxygen.

Once seedlings are up, Maryland Extension gives a night temperature of 55 to 60°F and a day temperature of 65 to 70°F. That is a drop from germination warmth, and it is deliberate: warm, dim conditions produce stretched stems.

Light, potting up and the two-true-leaf stage

A windowsill is seldom enough. Stems stretch and lean toward the glass, and any seedling stretches in weak light. Keep fluorescent or LED fixtures 2 to 6 inches above the plants, on for 14 to 16 hours a day with an outlet timer, Maryland Extension advises. Clemson Extension gives 16 to 18 hours of bright light with tubes 2 to 4 inches above the seedlings, raised as the plants grow. After potting up, keep the tops of plants 1 to 2 inches from the lights. Broad-spectrum LED grow lights and cool-white fluorescent tubes both work.

Pepper and eggplant started one seed per cell are moved up 2 to 3 weeks after germination, Maryland Extension says. Handle seedlings by a leaf or the cotyledons — the seed leaves — never by the stem. Leaving seedlings crowded in the flat too long is one of the most common mistakes, and with these slow crops the crowding lasts longer than the grower expects.

A small fan near the seedlings from the two-true-leaf stage makes them sway and begins toughening them. It also dries them faster, so watch moisture. Brushing the tops by hand is the method Maryland Extension describes for making tomato seedlings stockier.

Feeding: most potting mixes carry enough fertilizer for at least 6 weeks, some labels say 6 to 8 weeks after germination. Clemson Extension’s practice is to feed at every other watering once the first true leaves appear, with plain water between feedings. Rinse liquid fertilizer off the leaves; young seedlings are easily burned.

When to plant peppers and eggplant outside

Both are very tender. The pepper figure to remember is 65°F: Maryland Extension’s pepper profile says plants set out before the soil reaches 65°F “just sit there.” Eggplant is just as tender. For both, wait until the soil has thoroughly warmed, not merely until the last frost has passed.

A cloudy, calm day is planting 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. Peppers and eggplant benefit from that shelter more than most.

Harden them off first. Maryland Extension describes a 5- to 14-day period depending on the crop, beginning 1 to 2 weeks before planting out; Clemson Extension starts once plants are about three inches tall and continues for two to three weeks. Start in a shaded, wind-protected spot for part of the day, add an hour or two of sun each day, and water less often but never let plants wilt. Bring warm-season crops in if the night will drop below 50°F. Do not put tender seedlings out on windy days or below 45°F. How to harden off seedlings walks through the schedule.

Hot nights, cold soil and fruit set

Pepper is sensitive to temperature in both directions. Very hot nights — above 75°F — slow or stop fruit set, Maryland Extension notes. A stretch of weather that keeps nights that warm will drop flowers, and the plant usually resumes when nights cool. Slow early growth in pepper usually means cold conditions or an overgrown, stressed transplant.

Eggplant has the same need for warmth and no tolerance for a cold check. Setting either crop into soil below the temperature it wants costs weeks that the short season may not return.

Spacing matters for airflow as much as for room. Follow seed-packet spacing; squeezing plants closer causes disease and lowers yield. Full sun for fruiting vegetables means at least 6 hours of direct light a day, and tomatoes — the closest comparison in the garden — want 8 to 10 hours where it is available. Growing tomato seedlings from seed to garden is a useful contrast: the tomato is the faster, more forgiving transplant, and the pepper is the one that punishes an early start in cold ground.

What goes wrong

• Seed sown in a cool room. Pepper seed below the 65° to 75°F range germinates slowly or not at all. Warm the flat before blaming the seed.
• Sown too early. Tall, root-bound plants that have run out of room do poorly in the garden. A 12-week start is for the pepper types that need it, counted back from the frost date.
• Stretched seedlings. Too little light, fixtures too far away, warmth and crowding all push stems up. Lower the lights and move plants into their own containers on time.
• Damping-off. The water molds Pythium and Phytophthora and the soil fungus Rhizoctonia need free water. Use soilless mix and clean containers, do not overwater, and let the top of the mix dry slightly between waterings. 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.
• 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; read and follow the label.
• Planting into cold soil. A costly mistake with both crops. Wait for the soil, not the calendar.
• Overgrown, stressed transplants. Good transplants are stocky with deep green leaves and white, fibrous roots. Avoid woody plants with a thick mat of circling roots.

A last note on what not to start indoors: asparagus, horseradish, rhubarb, potato, garlic and sweet potato are grown from plant parts — crowns, tubers, cloves, slips — not from seed in a flat. Peppers and eggplant are the opposite case: they need one of the longest indoor stays in the vegetable garden, and they repay it only if the flat is warm and the garden soil is warmer still.