Growing Tomato Seedlings from Seed to Garden

Tomato seed goes indoors 5 to 6 weeks before the plants are to go outside, according to University of Maryland Extension; Clemson Cooperative Extension’s table puts the same crop at 6 to 8 weeks from seeding to transplanting. Either way, the target is a stocky plant about as wide as it is tall, with a stem that has not been stretched by weak light. Sow too early and you get a tall, root-bound plant that sulks in the garden. Sow at the right time, give the seedlings strong light and cool nights, and the transplant goes into warm soil with a good root system and starts growing almost at once.

When to start tomato seeds

Count backward from your average last frost date, not from a calendar date that worked somewhere else. Maryland Extension’s figure for tomato is 5 to 6 weeks indoors; Clemson’s is 6 to 8 weeks. The difference reflects how fast plants grow under different indoor conditions, so treat the range as a window and aim for the middle. A tomato sown 5 to 6 weeks before planting out makes a stocky 8- to 10-inch plant, Maryland Extension notes.

Tomatoes are tender. They go outside after all danger of frost, when the soil has warmed. That is later than the last frost date in most years, so the sowing date should be set against the planting date you actually expect, not the earliest possible one. If you are working out dates for several crops at once, the method is the same: when to start seeds indoors explains counting back from your last frost.

Peppers and eggplants, which are usually started at the same time on the same shelf, need a longer run-up; see pepper and eggplant seedlings for their timing.

One seed per cell, or a flat and a potting-up

Tomato seed can be sown one per cell in a container at least 2 inches deep, or broadcast in a flat and moved up later. Maryland Extension suggests cell or pot sizes of 9 to 16 square inches for tomato, eggplant and pepper. A standard plastic flat is 10 by 20 inches and holds anywhere from 10 to 120 plants depending on cell size.

Sow about twice the seed’s diameter deep — a seed 1/8 inch wide goes about 1/4 inch deep. Use a soilless seed-starting mix, not garden soil; garden soil weighs about 75 pounds per cubic foot and may carry weed seeds and pathogens, while a good soilless mix weighs about 10 pounds per cubic foot. Moisten the mix to the feel of a wrung-out sponge before filling, and fill containers to within one inch of the top.

Keep the mix at 65° to 75°F for germination. A clear cover holds moisture until sprouts appear, then comes off. Once seedlings are up, Maryland Extension gives 55 to 60°F at night and 65 to 70°F by day.

If you started in a flat, move seedlings to their own containers shortly after the first true leaves appear. Tomatoes started one per cell are potted up 2 to 3 weeks after germination. Hold a seedling by a leaf or a seed leaf, never by the stem.

Light and temperature for stocky plants

A windowsill is seldom enough. Stems stretch and lean toward the glass, and a stretched tomato seedling is hard to correct later. 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 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 the plants 1 to 2 inches from the lights.

Cool nights and bright light together produce the compact growth you want. Warmth without enough light does the opposite.

Brushing and stem strength

Tomato seedlings respond to touch. Brushing the tops by hand makes them stockier. A small fan near the seedlings from the two-true-leaf stage does something similar: it makes them sway and begins toughening them, Clemson notes, though it also dries the mix faster, so watch moisture. Water when the top of the mix is nearly dry, from the top until it drains or from below in a shallow tray.

Watering and feeding

Most potting mixes carry enough fertilizer for at least 6 weeks, and some labels say 6 to 8 weeks after germination. Clemson’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 burn easily.

Hardening off before planting out

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.

Start in a shaded, wind-protected spot for part of the day and add an hour or two of sun each day. Water less often but never let plants wilt. Do not put tender seedlings out on windy days or below 45°F, and bring tomatoes in if the night will drop below 50°F. A cold frame is an excellent place to do this. After proper hardening, Maryland Extension notes, tomato plants can usually tolerate a light, unexpected frost with little damage.

Planting tomato transplants deep

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.

Tomatoes are set deeper than they grew. Maryland Extension’s crop profile says to plant so only two or three sets of true leaves show, and warns against setting the root ball deep into cold soil. A tall transplant can go in a trench with only 2 or 3 sets of leaves above ground; the buried stem roots along its length. Black plastic laid 2 to 3 weeks before planting warms the soil.

Soil temperature matters more than the calendar. Wait until the soil has warmed; a tomato set into cold ground sits and stalls. The general steps for any transplant — hole size, firming, watering in, temporary shade — are covered in transplanting seedlings into the garden.

Spacing and support

Maryland Extension gives tomato spacing as 18 to 36 inches in the row with 48 to 60 inches between rows, depending on whether plants are staked or caged. Follow the seed packet’s spacing where it differs; squeezing plants closer causes disease and lowers yield. Full sun means at least 6 hours of direct light a day, with 8 to 10 hours preferred. Set stakes or cages at planting time, before the roots spread.

What goes wrong with tomato seedlings

Starting too early. Maryland Extension names this as a common mistake. Early sowing gives tall, root-bound plants that do poorly in the garden. If seedlings are ready weeks before the soil is, the plants lose more than they gain.

Leggy, stretched seedlings. Too little light — fixtures too far away or a window — makes tomato seedlings tall and weak, and warmth and crowding make it worse. Lower the fixture so the light sits closer, cool the nights and give each plant room. The causes and fixes are set out in leggy seedlings.

Crowding in the flat. Leaving seedlings crowded too long is one of the most common mistakes. Move them to their own containers shortly after the first true leaves appear.

Damping-off. The water molds Pythium and Phytophthora and the soil fungus Rhizoctonia need free water and kill seeds before they emerge or collapse seedlings at the soil line. Use a 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 them with their mix, wash the 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.

Cold soil at planting. A tomato set out before the soil warms stalls, and the root ball should not go deep into cold ground. Wait for warm soil, then plant deep.

Buying or keeping the wrong transplant. A good tomato transplant is stocky with deep green leaves and white, fibrous roots. Avoid overgrown, woody plants with a thick mat of circling roots, and plants with pests. A plant about 6 to 10 inches tall is the size to look for.