How to Start Seeds Indoors

You’ve browsed the catalogs and now have a stack of seed packets. In the weeks while you’re waiting for the weather to warm, you can get a jumpstart on your garden by starting seeds indoors. Use this step-by-step guide to set up a seed-starting space and successfully grow transplants that will outshine the local nursery.

1. Sort Fresh Seeds

Group your new seeds by how long they will need to start: 12 weeks before the last-frost date, six weeks before, et cetera. If you don’t know the age of your seeds, do a quick germination test. Soak a piece of paper towel or coffee filter in water. Place 10 to 15 seeds in a line on the paper. Spray them with water and then fold the paper over and place in a loosely closed plastic baggie. Check on the seeds every few days until no new seeds are germinating. The percentage of seeds that germinated from your original count is your estimated germination rate.

2. Mix Healthy Potting Soil

For best germination and early root growth, use a soil rich in nutrients and well-balanced in texture to hold just the right amount of moisture. Many growers choose an organic bagged blend or mix their own potting soil from materials like vermiculite, coir and compost.

“In a pinch, garden soil can be used as an ingredient for homemade potting mix,” says Milan Karcic, operator of Peace, Love, and Freedom Gardens in Columbus, Ohio. “It should be sterilized by putting no more than a 1 inch layer on a baking tray in the oven for 25 minutes at 200 degrees.”

Sterilized potting soil ensures that mold and bacteria will not infect your seedlings.

3. Choose Your Seed Tray

Growers can select from a variety seed trays suited to variety of purposes. If space is no issue, you can use 3- to 4-inch pots, but to maximize limited space, use plastic plug trays instead and later transplant the seedlings into pots. Other gardeners make blocks of compressed soil with no pot for easy transplanting. Still another alternative are compostable peat pots or newspaper-wrapped soil blocks for a bit more stability.

Whichever soil container you choose, you’ll want a sturdy bottom tray to hold the seedlings. Plastic trays can be reused year to year but must be washed thoroughly to prevent cross contamination.

4. Select Your Light Source

Seedlings grow best with consistent exposure to sunlight. For urban farmers living in the northern U.S. or those without sufficient indoor lighting, natural light during typical seed starting time is not predictable. In that case, seedlings will grow strongest under artificial light placed 4 to 6 inches above the top of the soil. Many growers prefer the efficiency and lumen output of a four-bulb t-5 lighting unit.

5. Set Up Your Space

Whether or not you use grow lights, set your seedlings in an undisturbed space that maintains an even temperature. The ideal space will include a table or rack, access to water and an adjustable hanging system for lights. Some gardeners prefer a basement or closet to keep pets and young children away from the trays.

6. Seed Your Trays

Once your setup is ready, it’s time for the action to begin. Lay out your washed trays or pots and fill them with soil, tapping the trays on a table so the soil settles. Avoid compressing too much or delicate roots might not be able to break through. Dampen the soil gently with a sprayer nozzle on a hose, then seed according to your tested germination rate: If you’re using new seed or your tested germination rate was near 100 percent, plant one seed per plug or pot. If not, use two or three seeds. Water the trays thoroughly and place them in your growing space.

7. Water Judiciously

Leave your trays alone until you see the tips of the germinating seeds, then only water when the soil is dry to the touch.

“Little plants are tough—or at least you want them to be tough!” says Erin Harvey of The Kale Yard, based in Ohio. “Don’t water too often. It’s important that seedlings get a good ‘wet/dry swing’ to prevent greenhouse diseases.”

She warns to be especially cautious of watering at night. Many growers prefer bottom watering, i.e., filling the bottom tray with water so that the soil and roots percolate the water up to the surface to avoid wet leaves or damaging tiny stems.

8. Take Notes

The best farmers keep detailed notes about every part of the growing process so they can make slight improvements year to year. Karcic recommends keeping track of your seeds traits, the conditions they grew in, the germination rate, and how many generations you’ve planted the same seed. You should also record the size plug trays you used, days to germination, and notes about anything unusual, like inconsistent germination.

9. Thin Seedlings

As your seedlings grow, remove all but the strongest seedlings. “I know it’s hard to do, but keep just one plant per cell or soil block or pot,” Harvey says. “Snip rather than pull so you don’t disturb the tiny roots. Starting out life with less competition will give you bigger, stronger plants sooner that will thrive later on.”

10. Transplant

If you’re using small peat pots or cell trays, you’ll need to transplant when the seedlings have fully rooted out, meaning you can see roots coming out of the bottom. This usually happens after four to six weeks. Farmers often time their seed starting so that transplanting can happen in later spring, when sun hours are longer and transplants can be moved outside to a hoop house or cold frame.

The transplant process is similar to seed-starting. Fill a larger clean pot with an enriched potting soil. Gently make a hole in the soil and transplant the cell or soil block into the center of the pot equal to the height of the soil surface. Gently tamp the soil. Water thoroughly, place in a bottom tray, and position under lights, in a warm windowsill, or in a hoop house or cold frame.

Bare in mind, seedlings can undergo transplant shock if they aren’t gently exposed to the weather before being planted outside, so they’ll need to be hardened off. When your seedlings are fully rooted in their transplanted pots and the weather is warm, carry your trays outside during the warmest time of day for a few hours at a time. Choose a slightly shaded and protected spot so that strong sun, rain or wind doesn’t shock the plants. Gently increase the outdoor exposure over a week or two. At this point, your seedlings are finally ready to be transplanted into their final homes in the ground.

Shepherding new plants from tiny seeds to full grown seedlings is one of the greatest joys a gardener can experience. The steps may seem numerous, but by following this guide, you’ll be rewarded with strong healthy plants that will produce exceptionally well because they’ll be under your care from start to finish.