Seed starting for beginners: the essentials

Starting plants from seed needs five steps, and beginners who keep them in order usually get sturdier seedlings. Use clean containers with drainage, a fine seed-starting mix, even moisture, strong light once seeds germinate, and a gradual move outdoors before planting. Skipping one step often shows up later as weak or leggy seedlings, or as a tray that fails all at once.

Seed packets are the best source for details that vary by variety, such as sowing depth, the number of days to germination, and whether the seed needs a cold period or light to sprout. When a packet's instructions differ from the general guidance here, follow the packet.

Starting with clean containers and drainage

Trays, cell packs and small pots all work for seed starting, as long as they drain freely. Clean used containers with hot, soapy water and rinse them well. Many gardeners also soak them in a dilute bleach solution, about one part bleach to nine parts water, to lower the chance that disease carries over from earlier crops. Let the containers dry before filling them.

Label every row or pot with the plant name and the sowing date from the start. Seedlings that look alike at a few weeks old are easy to mix up, and a written label prevents that. A clear dome or lid can hold moisture while seeds germinate, but remove it as soon as the first seedlings appear.

Start with fresh seed where you can. Seed that is old or stored poorly often germinates unevenly or not at all. Keep leftover seed in a sealed container in a cool, dry, dark place, and note the sowing year on the packet so the next season starts from good stock.

Choosing a seed-starting mix over garden soil

Garden soil is usually too heavy for seed trays. It compacts, drains poorly in containers and can carry weed seeds and soil pathogens. A seed-starting mix is lighter and finer, holds moisture while still letting air reach the roots, and suits young seedlings better. Moisten the mix before filling the containers, because very dry mix can shed water and leave patches that never wet evenly.

Sow seeds at a depth of about two to three times their own width, unless the packet gives different instructions. Very fine seeds that need light to germinate are usually pressed onto the surface and covered lightly or not at all. Firm the surface gently so each seed makes good contact with the mix. Fill the containers loosely and tap them on the bench to settle the mix, leaving a small gap below the rim so watering does not spill over the edge. Packing the mix hard makes it dense and slow to drain, which works against the light structure that young roots need.

Giving seedlings enough light

Once seedlings emerge, they need strong light for many hours each day. A windowsill often gives less light than seedlings need, and they respond by stretching into thin, pale stems that flop over. A grow light positioned a few inches above the seedlings, and raised as they grow, is the usual remedy.

Turn trays regularly if they sit near a window, so the seedlings do not lean toward the glass. If seedlings still stretch under good light, the lamp is probably too far away. Adjust its height rather than hoping the plants will recover on their own.

Thin crowded seedlings once they have their first true leaves. Crowded seedlings compete for light and water and grow weak together. Keep the strongest plant in each cell or spot, and snip the others off at the soil line instead of pulling them, which avoids disturbing the roots of the seedling you keep.

Keeping moisture even without drowning seedlings

Seedlings need steady moisture, and both drying out and waterlogging can kill them. Water from below by setting the tray in a shallow amount of water until the surface feels damp, then drain away the excess. Keeping the surface drier this way helps prevent damping off, a fungal problem that makes young stems collapse at the soil line.

Airflow matters too. A small fan on low, placed so it moves air around the trays without blasting them, reduces the damp conditions that favor disease. Check the mix each day by touching the surface, and water only when it begins to feel dry.

Hardening off before planting outdoors

Seedlings raised indoors need a gradual move to outdoor conditions, a process called hardening off. Sun, wind and temperature changes can scorch or stunt tender growth when plants go out all at once. Start about one to two weeks before the planting date. On the first day, set the trays outside in a sheltered, shaded spot for a few hours. Each day, increase the time outside and gradually expose the seedlings to more sun and wind.

Bring the trays indoors if frost or strong wind is forecast, and avoid moving seedlings out during cold snaps. Plant them after the last frost date for your area, when the soil has warmed and the forecast is steady. Seedlings that hardened off well usually settle quickly into the garden or their containers. Expect a brief pause in growth during the first days outside. Some wilting or a slight pale flush on the leaves is normal as the plants adjust to brighter light and wind, and it usually fades as the stems and leaves toughen.

Some seeds need a cold period or a light nick in the seed coat before they sprout, and the packet should say so. When seedlings look weak, check light and moisture first. Those two factors are the easiest to correct, which makes them the best place to start.