Down to Earth: It’s time to start vegetable seedlings indoors

Although most of the vegetable garden is still months away from planting time, gardeners can begin enjoying the season now by starting their own seeds.

Plants that require a long growing season, like tomatoes, peppers, eggplants, melons and winter squash, can get a jump start indoors on a sunny windowsill.

Plants like lettuce, cabbage, broccoli and basil don’t need a long growing season, but these, too, can be started indoors to make harvest time earlier.

If you’ve never started your own seeds, give it a try. This is one of my favorite gardening activities, and is a good one to do with kids.

What you’ll need

In order to become vigorous, productive plants, seeds have the same requirements, whether they are started indoors in a container or sown straight into garden soil: a loose, disease-free planting mixture; even moisture; fertilizer; good drainage; the right amount of light; and the right temperature for germination.

The plants that you are most likely to start indoors are most susceptible to disease between the period when germination occurs and when the first few true leaves emerge. Most of these diseases are soil-borne, so the best way to avoid disease is to use a sterilized soilless seed-starting mixture, rather than standard garden soil.

Soilless mixtures are typically a combination of sphagnum peat moss, vermiculite and perlite, which produces a mix that is light and holds moisture well. Some soilless mixtures also contain fertilizer. If your mixture doesn’t contain fertilizer, wait to fertilize the seedlings until after their first true leaves emerge.

There are many different kinds of seed-starting containers available, and you’ll likely find your personal favorites for particular plants over time.

Some containers have trays segmented into cells, and some are made of peat or soil blocks. Some have transparent plastic lids that turn them into miniature greenhouses.

The most important thing to remember when choosing a container is drainage. Drainage holes in the bottom of your container ensure that seeds don’t sit in water. Seeds that sit in standing water rot rather than sprout , so make sure your container has good drainage.

There’s also no reason to buy a special seed-starting container. Although the store-bought ones work well, you can also recycle plastic butter tubs or yogurt containers, or cutoff milk cartons and start seeds in those. One of my favorite containers is a pot I make myself out of newspaper.

Seedling requirements

In order to get your seeds to germinate, the most important things to pay attention to are providing an even moisture level and the correct soil temperature.

After germination occurs, light will become extremely important. Germinated seeds require 12 to 16 hours of light a day to grow. You can provide this light by moving your seed-starting containers into a sunny windowsill, preferably one that faces south. On cold nights, be sure that seedlings are protected from drafts.

If you don’t have a sunny window, you can rig fluorescent lights over your seedling trays instead. Keep the lights about 3 inches above the tops of the plants and move them up as the seedlings grow.

Moisture and temperature will still be important to your germinated seeds. Try to keep the temperature near what’s required for optimum growth, and make sure that your soilless mix never dries out. Use bottom watering, or a watering can with a fine stream to avoid damaging tender seedlings.

After the first true leaves appear, fertilize seedlings once a week with a half-strength mixture of fish fertilizer when you water.

True leaves versus seed leaves

Seedlings usually have two types of leaves. The first to appear are called cotyledons , or seed leaves. After these, the first set of true leaves develops, typically growing above the seed leaves.

When planting a winter squash seed, the arched stem of the seedling will emerge first from the soil, followed by a pair of thick seed leaves, often with the seed coat still attached. After that, the first pair of true leaves grows above the seed leaves.

Peas are an exception to this rule, with the seed leaves emerging underground, so the first leaves you see above ground are true leaves.

Corn and onions produce only one seed leaf instead of two, and the seed leaf itself is very similar to the true leaf — the true leaf also forms below the seed leaf. For narrow-leaved vegetables such as these, the best way to tell if true leaves are emerging is to count the leaves. If there are two or more leaves on these seedlings, true leaves have emerged.

Once true leaves have appeared on a seedling, it’s time to transplant. Although onions, celery and lettuce can stay in your seed-starting flat, tomato, eggplant and pepper seedlings should be transplanted into 4-inch pots to give them room to grow. These plants have stems, leaves and roots that will grow very quickly, so the larger growing space will keep roots from becoming cramped. Larger pots will also encourage the plant to produce stronger stems.

When you are transplanting a seedling, it’s a good idea to start introducing the plant to garden soil. The recommended mixture is usually 1⁄3 soilless seed-starting mix, 1⁄3 garden soil, and 1⁄3 compost.

Also, never handle a seedling by its stem. If it’s damaged, a stem will usually not recover, and the plant may die. If a leaf is broken or bruised, the plant will likely survive. Cradle the root ball in your hand and keep the plant steady by gently holding a leaf between your fingers as you transplant.

Come join the conversation on my blog at blogs.roanoke.com/downtoearth/ for more tips on starting seeds indoors.

Karen Hager’s column runs every other Saturday in Extra.

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