Starting seeds indoors gives you a head start on the growing season, lets you grow varieties that aren't available as transplants at your local nursery, and saves significant money compared to buying starter plants. But timing is everything. Start too early and your seedlings will be leggy, rootbound, and stressed by transplant time. Start too late and you won't gain much advantage over direct sowing.

Everything revolves around your last frost date. Seeds are started a specific number of weeks before that date, and that number varies by crop. Here's how to get it right.

Finding Your Last Frost Date

Your last frost date is the average date after which freezing temperatures are unlikely in your area. It's based on historical weather data and varies significantly even within the same state. Look up your zip code on the USDA Plant Hardiness Zone map or check with your local extension office. For zones 3-4, the last frost is typically mid to late May. Zone 5 is early to mid May. Zone 6 is mid to late April. Zone 7 is early to mid April. Zones 8-9 are March or earlier. These are averages—check your specific location for the most accurate date.

Seed Starting Timeline

Here's when to start the most common garden vegetables indoors, expressed as weeks before your last frost date. Ten to twelve weeks before last frost: Onions, leeks, and celery. These slow growers need the longest head start. Eight to ten weeks: Peppers (both sweet and hot), eggplant. These warm-season crops are slow to germinate and grow, and they need warm soil to transplant into. Six to eight weeks: Tomatoes. This is the most common timing question, and six to eight weeks is the sweet spot. At transplant time, tomato seedlings should be 6-8 inches tall with thick stems.

Four to six weeks before last frost: Broccoli, cauliflower, cabbage, and other brassicas. Also lettuce and herbs like basil. These crops can tolerate light frost at transplant time (except basil—wait until after frost for basil). Two to four weeks: Squash, cucumbers, and melons. These grow so fast that starting them more than 3-4 weeks early creates rootbound, transplant-shocked plants. Many gardeners skip indoor starting entirely for these crops and direct-sow after frost.

Crops to Direct Sow (Skip Indoor Starting)

Some vegetables actually perform better when planted directly in the garden. Beans and peas have fragile root systems that don't transplant well. Carrots, radishes, and turnips—root vegetables in general—should always be direct sown because transplanting damages the taproot. Corn needs to be planted in blocks for pollination, making transplanting impractical. These crops grow quickly from seed and don't benefit from indoor starting.

Indoor Seed Starting Setup

You don't need a fancy setup. The essentials are containers with drainage holes (cell trays, peat pots, or even recycled yogurt cups), seed starting mix (NOT regular potting soil—it's too heavy), and light. Light is the critical factor most beginners underestimate. A sunny window is NOT sufficient for most seedlings—they need 14-16 hours of direct light to grow stocky and strong. A simple shop-light fixture with LED or fluorescent tubes hung 2-3 inches above the seedlings is the most reliable and affordable solution.

Keep the lights on a timer for 14-16 hours per day. Raise the light as seedlings grow, maintaining that 2-3 inch gap. Without adequate light, seedlings stretch toward the nearest window, becoming tall, weak, and floppy—a condition called legginess that's difficult to correct.

Hardening Off: The Step Everyone Forgets

You can't move seedlings from your cozy indoor environment straight into the garden. The shock of wind, direct sun, and temperature swings can kill or severely set back even healthy seedlings. Hardening off is the process of gradually acclimating indoor seedlings to outdoor conditions over 7-10 days.

Start by placing seedlings outside in a sheltered, shady spot for 2-3 hours. Each day, increase the time and gradually introduce more direct sun. By day 7-10, seedlings should be spending full days outside and can be transplanted into the garden. This process is tedious but essential—skipping it is one of the most common reasons transplants fail.

Seed starting is one of the most satisfying parts of gardening. There's something magical about watching a tiny seed crack open on your kitchen counter in February and knowing that by July, it'll be producing tomatoes in your garden. Get the timing right, provide adequate light, and don't skip hardening off—everything else is details.