Seed Germination Time Chart

How to read this chart

Three numbers decide when a seed sprouts and when you sow it. The first is the days to germination, the window between sowing and the first green loop pushing out of the soil. The second is the germination soil temperature, which is warmer than the temperature the plant wants once it is growing. The third is the light requirement: some seeds need light to sprout, some need darkness, and some want a light covering.

That last one trips people up. A seed that requires light is pressed into the surface and left uncovered, because burying it stops it from sprouting. A seed that requires darkness is covered with a quarter to half inch of mix. Get this backwards and the seed either never sprouts or takes twice as long as the packet promised.

The data here comes from the germination tables published by Iowa State University Extension, which list the ideal temperature, light requirement and days to germination for common vegetables, herbs and annuals. Use the chart as a planning tool, then check the seed packet, because varieties within a crop can differ.

Days to germination for vegetables and herbs

D = needs darkness and a light covering of mix. L = needs light, so press the seed in and leave it uncovered. L-D = barely cover the seed. Weeks to planting is the time from sowing to a transplant ready for the garden.

Notice how wide some windows are. Carrots take 7 to 21 days, and onion seed can sit for 4 to 20 days depending on freshness and soil temperature. That spread is why sowing too early is a common mistake: the seed is not late, the soil is cold. Most seeds sprout fastest in the middle of their temperature range.

Worked example: starting tomatoes indoors

Tomatoes need 6 to 12 days to sprout at 70 to 75°F, and 5 to 7 weeks of indoor growth before they go outside. Say your last frost date is May 15.

• Count back 5 weeks from May 15: April 10
• Count back 7 weeks from May 15: March 27
• So sow between March 27 and April 10
• Sprouts appear 6 to 12 days later, between April 2 and April 22

Sowing at the early end of the window gives bigger transplants, but plants started too early get tall and leggy in a windowsill and stall when they go outside. If you do not have a grow light, sow closer to the 5 week mark.

Keep the flat at 70 to 75°F until the sprouts appear, then drop the temperature to about 65°F and put the seedlings under bright light. A warm room with weak light is the recipe for pale, stretched stems.

Worked example: peppers and parsley

The slow crops are the ones worth planning around. Peppers take 7 to 10 days to germinate and 6 to 8 weeks to grow on, so with a May 15 last frost you sow between March 20 and April 3. That is nearly two months before the plants go in the ground.

Parsley is slower still: 20 to 25 days to germinate, plus 6 to 8 weeks of growth. Sow it between March 20 and April 3 and expect sprouts between April 9 and April 28. Some gardeners soak parsley seed overnight or pour hot water over the flat to speed it up, because the seed coat is tough.

Compare that with radish, which germinates in 3 to 4 days and is sown straight into the garden. The gap between radish and parsley is a factor of about six, which is why a single "days to sprout" number for all seeds would be useless.

Germination temperature and light

Soil temperature drives germination speed more than air temperature. Cool season crops such as lettuce, peas, spinach and radishes germinate at 50 to 65°F, and they sprout faster in cool soil than in warm. Warm season crops such as tomatoes, peppers, melons and squash want 70 to 85°F, which is why a heat mat under the flat is worth the $25.

Temperature also sets the upper limit. Lettuce seed goes dormant above about 80°F, which is why summer lettuce sowings fail. If you are starting lettuce in a warm room, move the flat to a cool basement corner or the bottom of the fridge for a few days.

On light: seeds that need darkness get covered with a quarter to half inch of mix, and seeds that need light are pressed into the surface and left bare. Water from the bottom or with a gentle mist so you do not wash the uncovered seeds into a corner. A clear lid or plastic wrap holds moisture, but take it off as soon as the first sprouts show.

Why seeds do not sprout

When a flat sits for two weeks with nothing happening, the cause is usually one of five things.

• Old seed. Germination rates fall with age, and onion, parsley and parsnip seed lose viability fastest, often within a year or two. A seed storage and germination guide explains how to test a pinch of seed on a damp paper towel before you commit a whole flat.
• Cold soil. Warm season crops planted in 60°F soil can take twice as long as the packet says, or rot before they sprout.
• Too wet or too dry. Soggy mix drowns the seed and grows mold. Bone dry mix stops the process entirely. The mix should feel like a wrung out sponge.
• Buried too deep. A seed planted deeper than the packet says runs out of stored energy before the shoot reaches the surface. The general rule is to cover a seed about two to three times its own thickness.
• Wrong light. Light requiring seeds that got buried will not sprout, and darkness requiring seeds left on the surface dry out.

Give it the full window before you give up. Parsley at 25 days is normal, not a failure.

Hardening off and planting out

Seedlings grown indoors are soft. They have never felt wind, direct sun or a cold night, and moving them straight into the garden shocks them. Hardening off is the week long process of getting them used to the outside.

Start about 7 to 10 days before your planting date. Put the plants outside in a sheltered spot for an hour on day one, then two hours the next day, and keep adding an hour or two. By day five they can take half a day of filtered sun, and by day seven they are ready for a full day. Bring them in at night for the first few days, and water them before they go out.

Plant on a cloudy day or in the late afternoon so the roots settle before the sun hits them, and water them in well. The University of Maryland Extension seed starting guide covers the same sequence, along with which crops transplant well and which prefer to be direct sown.