Seed Starting Tips Plus How to Care for Seedlings
Loads of helpful hints!
Seeds are programmed to grow, but sometimes they seem stubborn, or trouble ensues after germination. But don’t give up on starting seeds! Here are our best tips for getting seeds to germinate and for helping the resultant seedlings thrive.
Tips for starting seeds:
Seeds are alive, and the energy stores they contain deplete over time. Before sowing seeds leftover from prior years, roll several in a damp paper towel and set it in a warm spot. Check for germination every few days. If a good percentage sprouts, the packet is worth sowing. Storing seed in a cool, dry place—such as the refrigerator—extends its lifespan.
The seeds of most vegetables and annual flowers sprout quickly after sowing if kept moist and warm. Sow them in a potting mix formulated for seed starting, water them well and keep them at room temperature. It’s the moisture that awakens them from dormancy so they begin to swell and grow. If the pots dry out after sowing, the seeds can easily die. You can soak just about any kind of seed in water for up to 24 hours before sowing them, and this usually shortens the time it take them to sprout.
Unlike annual species, most hardy perennial, tree and shrub seed will not germinate immediately after sowing. They require a chilling period (in nature, that’s winter). When sowing these seeds for the garden, dry cold storage is not sufficient; the seed needs to absorb water to sprout. To replicate winter, mix the seed with potting soil in a self-sealing plastic bag and place it in the refrigerator. The exact length of damp chilling depends on the plant, but 90 days is a safe bet. After this time, pour the contents of the bag atop a pot of potting mix, add more mix on top and set the pot in a bright, warm spot.
Related: "Starting Seeds That Need Cold"
Lupines, roses, morning glories, many cacti and New Jersey tea are among a smaller set of plants whose seeds possess an outer layer that prevents the absorption of water from the soil. In nature, the seed sits for months or even years until the coating has weathered away. At home it’s easy to speed this process by rubbing the seed between two pieces of sandpaper for about 30 seconds prior to planting. You can check your work by soaking a seed in water for 24 hours after the sandpaper treatment. If it swells, you’ve sufficiently scratched the coating.
Seeds smaller than a grain of sand will often not germinate unless they are exposed to light after sowing. Whether you sow tiny seeds in pots or into garden soil, do not cover them. Water them carefully but frequently until seedlings pop up.
The majority of seeds will respond to one or more of the above treatments, but a few—including some woodland wildflowers—are far more challenging. Plants like trillium, Solomon’s seal and hellebores are intolerant of dry storage and even when handled fresh they may take several winters and summers to germinate. Plant these seeds directly in their preferred garden setting and forget about them. Eventually they’ll surprise you.
Tips for seedling care:
Light: Plants make their own fuel, combining carbon dioxide and water to create sugar, which they burn through respiration to power their growth. What powers the manufacture of sugar? Light, in photosynthesis. Mature plants require different amounts and intensities of light depending on species, but all seedlings should get 14 to 16 hours each day. (Think about how much fuel they need to make to support their rapid growth.)
For seedlings started indoors, providing enough light can be as difficult as it is important. Because our eyes naturally adjust to compensate for low or high light, it is hard for us to judge the true quality of light in a room. Our eyes work to make light appear even, but its intensity diminishes the farther you get from the window. Even if you place seedlings in a south-facing window, you will likely notice them stretching toward the glass rather than growing straight and stocky. Artificial light, be it from a homemade setup or a store-bought light stand, is the best bet for compact, healthy seedling growth.
After the seeds sprout, run the lights for 14 to 16 hours each day. Longer is not better; seedlings need a period of darkness during which they can rest. Keep the seedlings’ top leaves 3 to 5 inches from the bulbs.
Air: Stagnant air and wet soil contribute to two of the most common seedling problems: damping off and mold. A disease caused by certain soil fungi, damping off is indicated when young seedling stems become thin at the base and bend over. Visible mold sometimes accompanies damping off, but it can also appear after seedlings have matured a bit and are growing well. Seedlings will not recover from damping off, but they may persist through an outbreak of mold.
After germination, thin the seedlings to allow for air to circulate and light to reach the soil.
An oscillating fan set on low also helps guard against damping off and mold because it moves the air, reduces humidity and dries the top of the soil. The breeze of a fan also strengthens seedlings’ stems. It should be placed so the seedlings vibrate but don’t bend from the wind.
Moving up: If you've started seedlings indoors in a cold climate, they may need to be transplanted to a larger pot and grown indoors a while longer before they can move out into the garden.
Move seedlings out of a flat and into larger pots after they have developed one or two sets of their true leaves (different from their very first seed leaves, or cotyledons).
Plants that will go into the garden within a few weeks can move into two- or three-inch pots, while heat-loving plants that have a longer time to wait should go into 6- to 9-inch pots.
Use a lightweight potting mix. Dampen it, fill your containers and poke a hole for each transplant.
Water the seedlings a few hours before you transplant them, so that the stems and leaves are turgid and the growing mix will stick to and protect the roots.
Gently dig the seedlings up from their original container using a popsicle stick, pencil or other improvised tool. Handle the seedlings only by their leaves, never by their stems.
If you have several seedlings growing close together, select just one to grow on and snip the others off at soil level before transplanting the selection. Alternatively, gently uproot the clump and untangle them, using a nail or other sharp-tipped instrument to tease the roots apart.
Plant them into their new containers deeper than they were growing, gently firming the growing media around their roots.
Pinching back: Many plants benefit from pinching back during their early stages, because it helps them achieve better growth and shape. These include basil, coleus, cosmos, fuchsias, impatiens, marigolds, petunias, snapdragons, sweet peas and zinnias.
Wait until the seedling has developed several sets of true leaves (don't count the cotyledons, or seed leaves, that appeared when it first sprouted). Use your fingertips or a clean, small pair of sharp scissors or pruners to snip off the top of the seedling's stem just above the point where a leaf or set of leaves connect to it. Don't pinch right at the point where the leaves connect, or you may remove the bud that will become a new branch. Aim for just slightly above that point. New stems will form and grow and develop more leaves.
Later you can pinch those new stems back the same way, too, to keep creating a bushier shape. Stop pinching when you are satisfied with the shape the plant is taking, so that the stems can then continue to mature and create flower buds.
