Learn how to start seeds indoors and reap the rewards. You’ll save money compared with buying starter plants—and you’ll be able to grow the exact varieties of flowers or herbs you want, not just what your local nursery carries.
Starting seeds indoors also gives you a head start on the growing season. Otherwise, you’ll be waiting for the soil outdoors to warm up enough for seeds to germinate. And if you live in a colder region with a short growing season, it may be the only way you can grow seed-to-harvest tomatoes, peppers, and other heat-loving plants that need time to mature. Once the danger of frost has passed, you can plant those seedlings outside and stay on pace with gardeners in warmer climates.
Here’s what else you need to know about how to start seeds indoors, including when to start, watering and light needs, and more.
When to Start Seeds Indoors Before the Last Frost
It can take from 3 to 15 weeks to grow a seedling big enough to move into your garden, says the University of Missouri Extension. First, get a sense of when your last frost date might be. Next, read the seed package for how long it takes for a crop to mature, and count backward from the estimated frost date.
How to Prepare Containers and Seed-Starting Mix
Seeds can be planted in any shallow container (about 2 to 3 ½ inches deep) that has drainage holes. Plastic milk jugs with the tops cut off, paper cups, peat pots, or shallow compartmented trays with plastic covers (sold in garden supply stores) all work well. If you’ve used a container before, the University of Maine Cooperative Extension suggests cleaning it with a solution of one part chlorine bleach to nine parts water to prevent potential disease contamination from previous plantings.
Fill your containers ¾ full with a fine-textured soilless starter mix (use fresh every year), water well, then gently press seeds into the surface. Read the label to see how deep to plant. Mist lightly, then cover with plastic wrap or the plastic dome lid that came with your tray.
How Much Light Seedlings Need to Grow Strong
Place the containers in a warm location, about 65 to 75ºF, for germination; avoid drafty windowsills. Once the seedlings emerge, remove the plastic or cover and move them to a location with bright light. They need 12 to 14 hours of light per day so they don’t become leggy and spindly. For best results, use a grow light set about 3 inches above the trays, and adjust the height as the seedlings grow.
When and How to Fertilize Seedlings
Once the second set of leaves appears, water with a half-strength solution of liquid fertilizer about once a week (starter mixes contain no fertilizer). Let the plants dry out a little before watering again to prevent damping-off, a common disease that causes the plants to shrivel at the base and fall over. Transplant the seedlings into larger pots in a soilless growing mix to give them space to grow, but handle them by the leaves, not the stem, to avoid bruising them, says the University of New Hampshire Extension.
How to Harden Off Seedlings Before Planting Outside
To avoid transplant shock, acclimatize your seedlings by a process called “hardening off.” To do this, slowly expose them to outdoor conditions to toughen them up. First, move them to a shady spot for a few days, then into sunlight for a short amount of time each day. Don’t put them out when it’s windy or colder than 45 degrees. After a week or two, they’re fine to plant if the risk of frost is past. One note of caution: Every spring is different, so it’s not a bad idea to wait a week past your area’s last estimated frost date.
