When to Start Seeds Indoors

Seed-starting dates count backward from your typical last spring frost. Start too early and seedlings get leggy and root-bound; too late and they're still small at planting time. Here are the weeks for each crop and how to grow sturdy transplants.

Count backward from your last frost

Every seed-starting schedule hangs on one date: your typical last spring frost. Each crop needs a certain number of weeks indoors to reach transplant size, so you count that many weeks back from the frost date.

For example, Chicago's typical last frost is April 10, which puts indoor seed starting at: onions Jan 30 – Feb 13; peppers Jan 30 – Feb 13; tomatoes Feb 13 – Feb 27; basil Feb 27 – Mar 6. See the full Chicago calendar.

Weeks to start each crop

These windows follow common university extension guidance. "Plant outdoors" is when the transplant typically goes into the garden, relative to the same frost date. Cold-hardy crops like broccoli and onions go out before the last frost; warm-season crops like tomatoes wait until after it.

Your city page applies these windows to your own frost dates, so you get actual calendar dates rather than week counts. Look up your ZIP code.

Crops to skip starting indoors

These crops are usually planted straight into the garden: pumpkins, corn, green beans, peas, lettuce, spinach, kale, arugula, Swiss chard, carrots, beets, radishes, turnips, potatoes, cilantro, dill, sunflowers, nasturtiums. Root crops like carrots, beets, and radishes form poor roots when transplanted, and beans, peas, and corn grow fast enough from seed that a head start gains little. Leafy greens can be started indoors if you want an early harvest, but sowing them directly is simpler.

Squash, cucumbers, and melons sit in between. They can be started indoors three or four weeks ahead in short-season climates, but they resent root disturbance, so use pots you can plant whole (or handle the root ball gently), and don't start them early.

A setup that grows sturdy seedlings

• Light is the big one. A sunny windowsill is rarely enough in late winter; seedlings stretch toward the glass and grow weak and leggy. Shop lights or LED grow lights kept a few inches above the seedlings, on a timer for about 14–16 hours a day, make the biggest single difference.
• Warmth for germination. Peppers, tomatoes, and eggplant germinate fastest in warm soil, roughly 75–85°F. A seedling heat mat speeds them up considerably. Once seedlings are up, normal room temperature is fine.
• Fresh seed-starting mix. Use a sterile, fine-textured mix rather than garden soil, which compacts in containers and can carry disease.
• Containers with drainage. Cell trays, small pots, or recycled containers all work as long as water drains out the bottom.
• Even moisture. Keep the mix damp, not soggy. Bottom-watering (setting trays in water for a few minutes) avoids washing out tiny seeds.
• Air movement. A small fan on low strengthens stems and helps prevent damping-off, a fungal disease that topples seedlings at the soil line.
• Pot up if needed. If seedlings outgrow their cells before planting day, move them to larger pots so roots don't circle.

Common timing mistakes

• Starting too early. The most common mistake. Tomatoes started 12 weeks ahead are usually tall, pale, and root-bound by planting day, and often do worse than ones started at 6–8 weeks.
• Using the wrong frost date. A frost date for a city 50 miles away, or a different elevation, can be off by a week or more. Use the date for your own location.
• Forgetting to harden off. Seedlings raised indoors need a week or more of gradual outdoor exposure before planting. See how to harden off seedlings.

Planning tip

Work out every crop's start date in one sitting and write them on a calendar. Onions, peppers, and slow herbs come first, often in late winter; tomatoes follow a few weeks later; squash and melons last. A single planning session in January prevents the scramble of discovering in April that the peppers should have been started in February.