Seed Starting Chart: Start Dates by Crop

How to read a seed starting chart

Every seed starting chart is two numbers hiding in one row. The first is how many weeks a seedling spends indoors. The second is the date it goes outside, counted from your last spring frost. Add them and you have the date to sow.

The first number comes from the University of Maryland Extension guide to starting vegetable seeds indoors. It lists the time to grow transplants indoors as 2 to 4 weeks for leafy greens, 4 to 6 weeks for cabbage, broccoli and cauliflower, 3 to 5 weeks for cucumber, squash and melon, 5 to 6 weeks for tomato, and 8 to 10 weeks for pepper, eggplant and onion.

The second number comes from Purdue's Indiana planting calendar. Hardy crops such as broccoli and cabbage go out 4 to 6 weeks before the average last spring frost. Semi-hardy lettuce goes out 2 to 4 weeks before it. Tomato goes out after it. Pepper, eggplant, cucumber, squash and melon go out at least 2 weeks after it.

Take a made-up last frost of April 25 and tomato as the example. Tomato spends 5 to 6 weeks indoors and goes out at the frost date (Purdue says after it, so the sums count week 0), so you sow it 5 to 6 weeks before April 25, between March 14 and March 21.

The chart: start dates counted back from your last frost

Here is the chart for five crop groups. The last column uses the same April 25 last frost, so you can check the sums.

Read the "start sowing" column as a range. The early end pairs the longest time indoors with the earliest set-out date. The late end pairs the shortest time with the latest date. With a single sowing, aim for the middle of the range.

The set-out dates are Purdue's Indiana window, so move the whole row if you plan to set plants out later. A tomato set out a week after your last frost starts 4 to 5 weeks before it, not 5 to 6.

Iowa State gives a similar count for its own state. Its crop-time list runs 6 to 8 weeks for pepper and eggplant, 5 to 7 weeks for tomato, cabbage, broccoli and cauliflower, and 3 to 4 weeks for cucumber, watermelon, muskmelon and squash. Treat the weeks as a guide and keep notes on what worked in your yard.

A seed starting schedule is only as good as the frost date you count from.

Why pepper starts before tomato: days to sprout and soil heat

Slow seed sets the order. Kansas State's planting guide lists how long each seed takes to come up and the best temperature for it, and its figures in brackets are for starting indoors.

Pepper takes up to 14 days to sprout and needs a warm mix, which is part of why it gets 8 to 10 weeks. The University of Maryland says a good mix temperature for most vegetable seeds is 65 to 75 degrees F, and that a seedling heat mat speeds germination. Once the seedlings are up, the same guide says to cool them to 55 to 60 degrees F at night and 65 to 70 degrees F by day.

Broccoli and lettuce list cooler best temperatures in the Kansas State table, 50 to 60 and 50 to 70 degrees F, so they are the trays to keep away from the heat.

Depth, light and which seeds skip the tray

Depth is in the last column above. Maryland's guide gives a rule for crops the table does not list: a suitable planting depth is typically about twice the seed's diameter, so a seed 1/8 inch wide goes 1/4 inch deep. Lettuce seed, the same guide notes, can be left on the surface, uncovered or very lightly covered.

Light is a smaller question than it sounds. A Penn State Extension page on seed biology says most seeds germinate best in the dark and may even be held back by light, and names the onion family as an example. Iowa State adds that some species need light to germinate, so those seeds get only about 1/8 inch of soil. Check the packet for lettuce and other tiny seed.

Some crops are better sown where they will grow. Iowa State's direct seeding page says the method suits crops that are not easy to transplant, like carrots and beets. Kansas State lists bush and pole beans, peas, sweet corn, radish, spinach and turnip as seeded crops, with no transplant option. The same page lists cucumber, squash and melon as crops that can be started indoors or direct-seeded in the garden.

Set up the tray so the dates hold

A chart tells you when. The tray decides whether the plants are worth planting.

Light is the usual weak spot. The University of Maryland says natural window light is seldom enough for strong seedlings, and that stems stretch and lean toward it. Keep LED or fluorescent lights 2 to 6 inches above the plants and lit 14 to 16 hours a day.

Water to keep the mix moist but not wet. Wait until the top is nearly dry before the next watering. Too much water starves the roots of oxygen, and the Maryland sheet blames water molds for damping-off, which can wipe out a tray quickly.

Almost any clean container works. Maryland says it needs drainage holes and a depth of at least 2 inches, and the guide suggests 3 to 6 square inches of cell for greens, 4 to 9 for broccoli and squash, and 9 to 16 for tomato, eggplant and pepper.

Do not sow too early. Maryland calls it a common mistake and says it gives tall, root-bound plants that do not perform well in the garden. A good tomato transplant is stocky, 8 to 10 inches tall after 5 to 6 weeks, and about as wide as it is high.

Pot up seedlings when the first true leaves appear if they share a flat. Maryland says leaving seedlings in the seed flat too long is one of the most common mistakes.

Fit the chart to your own yard

Your last frost date sets every row, so get it right. Use the average last frost for the weather station nearest you, then move your dates later if your yard runs cold and earlier if it is sheltered. Your own notes from past years beat any table.

Moving the date moves every row by the same number of weeks. That is why one chart works in any US zone, as long as you count from your own frost date and not from a printed month.

If you grow only tomatoes, peppers and a few brassicas, and your state extension office publishes a seed-starting calendar for your county, use that calendar. A local sheet fits your climate better than a general count, and it is free.

The Seed-Starting Countdown is for the grower who wants the whole indoor season in one file. It does not look up your frost date, but it holds a table of last and first freeze dates (10, 50 and 90 percent) for 108 NOAA stations, from the 1991 to 2020 normals, to help you pick one. The countdown page shows what to sow this week, one log page for each week from 14 weeks before frost takes the sowing date, and the transplant day log takes what happens later. To sow by hand from the table above, you do not need any of it.