How to Grow Tomatoes from Seed: A Week-by-Week Guide

Growing tomatoes from seed: the short version

Growing tomatoes from seed takes 6 to 8 weeks indoors before the plants go into the garden. Sow the seed 1/8 inch deep in sterile seed starting mix, keep the mix at 75 F or warmer until the seedlings emerge in about a week, then grow them on at about 70 F under lights set 2 to 6 inches above the leaves for 14 to 16 hours a day. Harden the plants off for about ten days and transplant after the last frost, once the soil has warmed to about 60 F.

Those numbers come from Cornell, the University of Maryland and the University of New Hampshire extension services, and the rest of this page walks through them in order. The work is light. Most failures trace back to three causes: sowing too early, germinating too cool, and running too little light.

When to start tomato seeds

Count back from your transplant date, which sits a little after your last spring frost. Cornell and New Hampshire both give 6 to 8 weeks from sowing to setting out. Maryland uses a shorter 5 to 6 weeks, which suits growers with strong lights and warm rooms. Starting earlier than 8 weeks rarely helps, because a seedling that outgrows its pot and its light turns tall and weak before the weather is ready for it.

Cornell’s advice is to wait at least a week or two after the last frost before transplanting. A gardener with a last frost around May 10 would therefore plan to transplant near May 20 and sow in the last week of March or the first week of April. The seed starting calendar covers how to find your frost date and count back for other crops.

Variety choice affects the far end of the schedule. Cornell groups tomatoes as early (65 days or less from transplanting to first ripe fruit), mid-season (65 to 80 days) and late (80 days or more). Whether a variety is determinate or indeterminate changes spacing and support later, though seed starting is identical for both.

Tomato seedlings week by week

The schedule below assumes an 8-week run. For a 6-week run, the growing-on stretch in weeks 5 and 6 is the part that shrinks.

Week	What the seedling is doing	Your job
1	Germinating; emerges in about a week at 75 F	Sow 1/8 inch deep, keep the mix warm and moist
2	Seed leaves open	Lights on, 2 to 6 inches above; thin to the strongest seedling per cell
3 to 4	First true leaves appear	Pot up shortly after true leaves show
5 to 6	Putting on leaves and stem	Grow at about 70 F; raise lights as plants grow; water when the surface is nearly dry
7 to 8	Ready for outdoor conditions	Harden off for about ten days
End of 8	Stocky plant with several sets of leaves	Transplant after frost into soil near 60 F

Sowing: mix, depth and containers

Use a sterile seed starting mix. Garden soil is too dense in a small cell for air and water to move around the roots. A light mix also helps against damping-off, a disease that can quickly kill a tray of seedlings when the mix is kept too wet. Moisten the mix before filling, then sow tomato seed 1/8 inch deep, the depth Cornell gives. Maryland’s general rule of planting at about twice the seed’s diameter lands in the same place for a seed this size.

Cells, plug trays or small pots all work, provided they drain. Sowing two or three seeds per cell gives a spare, and after germination you keep the strongest plant in each cell and remove the others. Label every variety on sowing day, since tomato seedlings look alike for weeks. If you are still choosing containers, the seed starting tray guide compares cell sizes.

Tomato seed germination: temperature and days

Temperature decides how long germination takes. Cornell says tomato seed germinates best at 75 to 90 F, emerges in about a week at 75 F, and germinates very slowly when cooler. University of California, Davis research, republished by Oregon State University Extension, puts numbers on that slowdown:

Soil temperature	Days to emergence
50 F	43
59 F	14
68 F	8
77 F	6
86 F	6
95 F	9

The same data set lists 50 F as the minimum and 95 F as the maximum for tomato germination, with little or no emergence outside that span. A tray on a 60 F basement floor may take two weeks to show, and a tray on a heat mat holding 77 F takes six days. If seeds seem slow, measure the mix temperature before resowing. Bottom heat is only needed until the seedlings are up. After that, Cornell recommends growing transplants at about 70 F, which keeps growth compact.

Light, water and thinning after germination

The day seedlings emerge, they need strong light. Maryland’s guidance for fluorescent and LED fixtures is to hang them 2 to 6 inches above the plants and run them 14 to 16 hours a day. Raise the fixture as the plants grow to hold that gap. Stems that stretch and lean mean the light is too weak or too far away, and most windowsills fall short. The grow light comparison explains what output a seedling shelf needs.

Water when the top of the mix is nearly dry. Constantly wet mix invites the root rot and damping-off fungi mentioned above, so a short dry interval at the surface protects the seedlings. Remove any cover or humidity dome as soon as sprouts appear, which lets the surface dry between waterings.

Maryland also notes that gently brushing the tops of tomato transplants with your hand or a wooden stick helps control their height and makes them stockier.

Potting up and feeding

Move seedlings to larger containers shortly after the first true leaves appear above the seed leaves. True leaves have the toothed outline of a tomato leaf; the first pair of smooth, narrow leaves are the seed leaves. Handle seedlings by a leaf and not by the stem, which bruises easily. Tomatoes root along any buried stem, so the seedling can sit deeper in its new pot than it did in the cell.

Feeding needs less attention than many guides suggest. According to Maryland, most potting mixes contain enough fertilizer to grow a seedling for at least 6 weeks. Check the bag: if your mix lists no added fertilizer, feed lightly following the product label once true leaves are growing.

Hardening off

Indoor seedlings have never met direct sun, wind or a cool night, and they scorch if moved out all at once. New Hampshire’s schedule starts about ten days before transplanting: set the plants outdoors for a few hours each day, and bring them in at night and on any day when temperatures fall below 60 F.

Illinois Extension describes a longer two-week version of the same idea. Begin with 2 to 3 hours outside during the warmer part of the afternoon, lengthen the stay each day until the plants are out for 10 to 12 hours, and then leave them out for a full 24 hours for a couple of days before planting. Reduce watering frequency gradually over the same period without letting the plants wilt. The day-by-day routine is laid out in how to harden off seedlings.

Transplanting tomato seedlings outside

Wait for two conditions: the danger of frost has passed, and the soil has warmed to about 60 F. Cornell adds that nighttime temperatures should be consistently above 45 F. A soil thermometer settles the question faster than the calendar, and when to plant tomatoes outside goes deeper on reading it.

Plant deep. Cornell says to set transplants so the soil level is just below the lowest leaves, and Maryland describes covering the main stem until only two or three sets of true leaves are exposed. The buried stem roots and anchors the plant.

Spacing depends on growth habit and support. Cornell’s figures are:

Type	Spacing between plants
Determinate	12 to 24 inches
Indeterminate, staked	14 to 20 inches
Indeterminate, unstaked	24 to 36 inches

Set stakes or cages at planting time or soon after so you are not driving them through established roots in June. Tomatoes produce best at a soil pH of 6.0 to 6.8.

How long from seed to ripe tomato

Add the indoor weeks to the variety’s days to maturity, which seed packets count from transplanting. A mid-season tomato needs 6 to 8 weeks indoors and then 65 to 80 days in the garden, so the first ripe fruit arrives roughly 15 to 19 weeks after sowing. An early variety rated at 65 days or less shortens that, and a late variety at 80 days or more lengthens it. Seed sown in early April and transplanted in late May should be ripening a mid-season crop from late July into August.