When to Start Seeds Indoors in Washington: A Crop-by-Crop Calendar

Near Seattle, onions and leeks go into flats first, from November 14 to December 10, and sweet potatoes go in last, from January 30 to February 25. Peppers fall between December 26 and January 14, and tomatoes between January 16 and February 11. Each window is counted back from a last spring frost that arrives between February 27 and March 18 on the mild, maritime side of the state. East of the Cascades the frost comes later, so every date moves later with it.

The date everything counts back from

NOAA’s NCEI 1991-2020 Climate Normals, taken at the Seattle-area station (USW00024233), put the 32°F freeze probability in the 30 to 70 percent band between February 27 and March 18. That span is the last spring frost for the Puget Sound lowlands. Every sowing date in this article is counted back from it. Write down February 27 to March 18, then subtract each crop’s weeks indoors from both ends.

The last frost arrives as a range because a late-winter freeze is a matter of odds. Some years the final one comes in the last days of February. In others it lands in mid-March. If you count from the early end and spring runs cold, seedlings wait in their pots with nowhere to go. They stretch toward the window, run out of root room, and arrive in the garden already stressed. If you count from the late end and spring runs mild, you have given away weeks of growth that tomatoes and peppers, with their long season, can ill afford to lose.

How do weeks indoors turn into sowing dates?

The weeks come from the University of Minnesota Extension table “When to plant seeds and how long to keep them indoors.” It lists how long each crop usually spends indoors before it goes out: leeks and onions 14 to 15 weeks, celery 12 to 13, the cabbage family and head lettuce 10 to 11, eggplant, okra and peppers 9, tomatoes 5 to 6, and sweet potatoes 3 to 4. Count that many weeks back from each end of the frost range and you get a window. Onions at 14 to 15 weeks, for example, land between November 14 and December 10. Only the weeks carry over from the Minnesota table. Its calendar months belong to Minnesota’s climate, and none of them apply here. Extension guidance like this is the usual practice, and a cold spring can still change the plan.

A mild frost date does not mean an early planting date. Cold, wet soil and low light hold spring crops back well past the last freeze, so seedlings started on schedule often wait indoors longer than the arithmetic suggests. That is one reason many western Washington gardeners sow in the later half of a window.

These dates are for the western Washington lowlands. The USDA Plant Hardiness Zone Map 2023 spreads the state across many zones, from the high Cascades to the mild coast, but that system rests on one variable, the average annual extreme minimum temperature. It says little about when spring settles. Eastern Washington beyond the Cascades is drier, with colder winters and a later last frost, and the colder parts of western Washington, farther north or higher up, also have a later last frost. In those places, sow later than the dates here. Local ranges are laid out in First and Last Frost Dates in Washington.

The calendar, crop by crop

The table below gives the usual extension weeks for each crop and the resulting indoor sowing window for the Seattle area. Both ends of every window are the computed dates, so there is no need to recount them.

The order of the table is the order of the work. Onions and leeks go in first because the seedlings grow slowly and spend the longest indoors. The broccoli family and head lettuce follow in the dead of winter. They are cool-season crops, so a cool room suits them better than it would a pepper. Light is their limit. If you only have a bright windowsill, start in the second half of the window rather than the first.

Peppers and eggplant need the most steady warmth of the group. Nine weeks is a long time to hold a seedling in a cold house, so a heat source under the tray earns its keep. Tomatoes go in much later, and you can find the full routine in Growing Tomato Seedlings Indoors. Sweet potatoes round out the list. They are grown from slips rather than true seed, and many gardeners buy slips instead of starting their own. The 3 to 4 weeks is the figure in the Minnesota table.

Every date in the table shifts later in a colder spot. Gardeners on higher ground or north of Puget Sound should push each window back and let the local last frost, not the table, set the final week.

Which crops should not be started indoors?

Carrots, beets, radishes and parsnips form a taproot or a delicate root that resents being moved, so they grow best where they were sown. Beans germinate fast in warm soil and sulk after transplanting. Peas and sweet corn follow the same rule. Squash, cucumbers and melons are usually sown in place as well. A few gardeners give them a short head start in individual pots, but the gain is small and the roots are easy to damage. Sow roots, beans and most squash straight in the garden, and save your indoor space and light for the crops in the table. The Washington Planting Calendar shows when those direct sowings go out.

Light, warmth and hardening off

Winter days in western Washington are short, and many of them are overcast. A window delivers weak light from one side only. Seedlings respond by stretching toward it: the stem lengthens, turns pale and thin, and eventually flops over. Once a seedling is leggy it rarely recovers fully, because the extra length is weak tissue. A fixture of grow lights hung close above the seedling tops and raised as they grow solves the problem. Put it on a timer so the hours stay steady. A small fan nearby, running gently, strengthens the stems and keeps air moving across the soil.

Warmth matters just as much at the start. Seeds germinate fastest in steady warmth, and a cold windowsill, garage or back bedroom gives patchy, slow germination. A heat mat under the tray, ideally with a thermostat, keeps the mix evenly warm. Peppers, eggplant and tomatoes are the slowest in a cold room, while onions and the cabbage family are more forgiving. Keep the mix evenly moist and cover the tray with a clear dome until the first sprouts show. Then remove the dome and the extra heat. Seedlings grown a little cooler, under bright light, come out stockier than ones kept hot.

How do you harden off seedlings?

University of Minnesota Extension advises that all plants be hardened off over the last 1 to 2 weeks before planting out. In practice that means moving the trays outdoors to a sheltered, shady spot out of the wind for a short stretch on the first day. Each day after that they stay out a little longer and take in a little more sun. Water slightly less as the week goes on, and carry the trays back inside whenever a frost is forecast. By the final days, a seedling in mild weather can stay out day and night. Skipping this step is a common reason transplants burn, wilt or stall. Harden off over the last 1 to 2 weeks, and count that time into your schedule so the planting-out date is not a surprise.

What goes wrong

Starting too early is a common mistake. The seedlings look fine for a month, then turn pale, tall and root-bound while the weather outside stays cold. Tomatoes may even flower in their cells. Count from the window in the table, favor the later half when light is limited, and move seedlings into larger containers if they outgrow their first ones before the garden is ready.

Damping off is a fungal problem that rots seedlings at the soil line. The stem pinches thin, and the seedling topples over, sometimes with a gray fuzz at the base. Once it starts, affected seedlings rarely recover. Prevention is simple. Use clean containers, fresh seed-starting mix instead of garden soil, water from the bottom, keep air moving, and avoid soaking the mix. Thin crowded seedlings early so leaves dry between waterings.

Planting out into cold soil is the third. The last frost range marks where a 32°F freeze becomes much less likely, not impossible, and at the late end of the range, around March 18, a freeze is still possible. Tomatoes, peppers and eggplant planted the week after that date often sit in cold, wet soil. Growth stops, and leaves can turn purplish. Grab a handful of soil: if it squeezes into a muddy ball, it is too wet and too cold. Wait until the soil has drained and warmed, even if March 18 has passed, and use a row cover or cloche on any transplant that goes out early. Cool-season crops such as cabbage, broccoli and onions handle the cool better than the tender ones.

Common questions

When should I start tomato seeds indoors in western Washington?

Sow between January 16 and February 11. That counts 5 to 6 weeks back from a last frost between February 27 and March 18, using University of Minnesota Extension weeks. Tomatoes then go out after the last frost, once the soil has warmed.

Do the same dates work east of the Cascades?

No. Eastern Washington generally has a later last frost, so every sowing date moves later. Ask your county extension office for the local last frost, then count the same weeks back from it.

Should I start head lettuce indoors?

The extension weeks for head lettuce are 10 to 11, which gives a window from December 12 to January 7. Lettuce is a cool-season crop that handles cool weather, so many gardeners also sow it outdoors. Details on both routes are in Starting Lettuce.

Is the last frost date the same as the planting-out date?

No. The last frost date marks when a 32°F freeze becomes much less likely, though not impossible. Tomatoes, peppers and eggplant need warm soil as well, and cool-season crops go out earlier. Check the soil, and for regional differences use the Washington guides.