Seed-starting Basics

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Fill your planting containers with whatever germinating medium you’ve chosen, gently firm the surface and water it so the medium is thoroughly moistened but not soggy. Use warm water for quick absorption. If you water after planting your seeds, you will wash them into corners or, in the case of tiny seeds, bury them too deeply.

Next, plant your seeds. Place them on the damp soil surface, no closer than one-fourth inch for tiny seeds and half to two-thirds inch for larger ones. Scatter a thin covering of soil over the seeds or just press in those that need sunlight to germinate (see the “Spring Indoor Seed-starting Guide”). Gently firm the seeds and soil in place. Then, before you do anything else, label the flat or row in a 10-row flat with the variety name and planting date.

You’ve given your seeds two of the conditions they need to germinate: moisture and supportive surroundings rich in air spaces. They need no nourishment until they sprout; in fact, soil with too much organic matter can produce an overabundance of carbon dioxide that can deter germination of some seeds.

Now you need to provide the warmth that most seeds need to encourage sprouting. Most homes have warm spots — the top of the water heater, near a radiator or heat vent, close to a woodstove — that will help nurse planted seeds to germination. If your house is cool or those sites are impractical, you can put a commercially made soil-heating cable under your flats.

Water the containers as needed to keep them evenly moist but not sopping wet. Bottom watering, by setting the containers in trays of warm water, is best for two reasons: The seeds are less likely to be flooded, and you’ll avoid surface puddling of water, a sure invitation to soilborne diseases.

Most of the seeds you’re likely to plant indoors will germinate best at temperatures between 75 and 90 degrees. I often cover flats of planted seeds with damp newspapers, but I have learned not to enclose them in plastic bags, which encourage mold. Flats of quick-sprouting plants, such as lettuce, should be checked daily or at least every other day. Once a sprout nudges above the soil surface, even if it is just the “elbow” of a stem — not yet a leaf — expose the seedling to light. Those that lack sufficient light or that grow too closely together develop long, spindly, weak stems.

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You’ll notice that the first leaves — the “seed leaves” — of the new, little sprout are less notched and differentiated than the leaves that will appear later. Let the plant subsist on its seed leaves for a few days before you consider transplanting it to a larger container so the roots can develop more fully. You can wait to transplant until the seedling develops its first true leaves, but get the job done before the second set of true leaves appears. If the seedlings are growing too thickly in their germinating flat, then you can thin them by snipping off the extras with scissors, but don’t pull them. Pulling out the excess can disturb the roots of adjacent seedlings if the plants are crowded and developing well.

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