You can start seeds in almost any kind of container that will hold 1 to 2 inches of starting medium without becoming waterlogged. After seedlings form more roots and develop leaves, though, they grow best in larger individual containers that provide more space for root growth and have holes for drainage.
Flats are large, rectangular containers that hold many seedlings. Many gardeners start their seeds in them, then transplant the seedlings to individual containers after the first true leaves unfold. If you raise lots of seedlings, it’s useful to have interchangeable standard-size flats and inserts. You can buy flats at a garden center, or make your own by constructing a rectangular wooden frame, 3 to 4 inches deep. Nail slats across the bottom, leaving â…› to ¼ inch between them for drainage.
Although individual containers dry out faster than flats, they are better for starting seeds because you don’t have to repot as often, so the seedlings’ tender roots are less likely to be damaged by constant handling. Some containers, such as peat pots, paper pots, and soil blocks, go right into the garden with the plant during transplanting so the plants’ roots are never disturbed.
If you choose to use homemade containers, such as old milk cartons, yogurt cups, or egg cartons, keep in mind that square or rectangular containers make better use of space and provide more area for roots than round ones do. Also, be sure to poke a drainage hole in the bottom of each container. Also keep in mind the harmful chemicals like BPA that can leech into your soil from plastic.
Or you could splurge: Spend a couple of bucks on containers that are designed for starting seeds. Most garden centers and many home and hardware stores carry cell packs, plastic trays that have individual 2-or 263-inch-deep pockets with drainage holes. (Leftover “six-pack” containers from the garden center will work fine, too.) These special-purpose packs range from 6 cells to 40 or more, and many include a clear plastic dome that helps maintain humidity until the seeds have sprouted.
Peat pots, made entirely of peat moss, are popular because you can plant them “pot” and all—you don’t have to worry about extracting the seedlings from the containers before you set them in the garden. Also, the peat absorbs excess moisture naturally, so seedlings are less susceptible to damping-off, a fungal disease that often occurs when soil is too soggy. But because peat pots do dry out faster than plastic containers, you’ll need to check their moisture level daily. Also, peat is not a sustainable resource, so the better option is paper pots.Like peat, paper pots also break down in the soil, allowing you to place them right in the garden, pot and all. They also draw excess water away from the seed-starting medium, although not to the degree that peat does. You can buy pots made from recycled paper or make your own pots from newspaper strips.
Another option is to skip the pots completely and start your seeds in soil blocks. If you go this route, you can do without containers, but you will need a soil-block maker—a device that compresses the seed-starting medium into cubes in which you plant your seeds. You can choose between block makers that produce four 1¾-inch cubes (just right for big seeds, like those of tomatoes, peppers, and cucumbers) or twenty ¾-inch cubes (for lettuce, alyssum, and other tiny seeds). Like peat and 27paper pots, soil blocks go right into the garden without disturbing delicate seedling roots. But, also like peat, the cubes dry out quickly, so you’ll need to water them often to keep the seedlings from drying out. See page 29 for details on making soil blocks.
No matter what kind of container you start your seeds in, you’ll also need a tray to put beneath the containers. This allows you to water your seedlings by filling the tray rather than dumping water on them from the top. A plastic lid or wrap for covering the containers after you seed them will help keep the seed-starting mix moist and encourage germination. Also be sure to have some simple markers or labels on hand so that you can note the variety and sowing date.
Seed-Starting and Potting Mixes
Seeds contain enough nutrients to nourish themselves, so a seed-starting mix doesn’t have to contain nutrients. But it should provide plenty of air spaces, hold moisture well, and be free of weed seeds and toxic substances. Peat moss, compost, perlite, and milled sphagnum moss—either alone or in combination—are all good materials for starting seeds. Don’t use plain garden soil, though; it hardens into a dense mass that seedling roots can’t penetrate.
When your seedlings have their first set of true leaves, you’ll need to transplant them to a nutrient-rich potting mix. You can either use a commercial mix (check the label to make sure it doesn’t contain a synthetic chemical fertilizer) or make your own. To make a basic mix, try this popular organic potting soil recipe:
- 1 to 2 parts good-quality garden soil
- 1 part builder’s sand or perlite
- 1 part compost
Each component provides specific benefits to plants. Soil contains essential minerals. Sand and perlite assure good drainage. (Perlite, an expanded volcanic rock with many air spaces, will make the mix lighter than if you use sand.) And compost releases nutrients slowly, helps maintain proper soil pH, improves drainage, and holds moisture.
Seedlings need more intense light than full-grown plants. If they don’t get 14 to 16 hours of strong light a day, most become spindly and weak. Although many gardeners start their seeds on windowsills, the light from a window during the short days of winter often isn’t enough to grow strong, sturdy seedlings.
A grow-light system will provide seedlings started indoors with enough light to produce healthy, compact transplants for the garden. Here’s how to get the most from your lights:
- Growing seedlings need lots of bright light. Keep the lights on for at least 14 hours a day, and suspend the lights close to seedling leaves.
- Because tubes produce less light at the ends, choose the longest tubes you have room for and rotate seedlings at the ends into the middle every few days.
- Keep the tubes clean. Dust can decrease the amount of light available.
- To increase light from a fluorescent fixture, position a mirror or aluminum foil alongside it to reflect light back onto seedlings.
Shopping for a Seed-Starting Mix
When shopping for a seed-starting mix, be sure the product is specifically labeled for seed starting and not as “potting soil.” Good-quality mixes contain some vermiculite, a form of the mineral mica, to promote drainage. Recently, it has been discovered that vermiculite can contain trace amounts of asbestos, a mineral that can cause lung disease. The EPA recommends working with this material in a well-ventilated area (preferably outdoors) and dampening the mix as soon as you open it to keep any particles from becoming airborne.
When you’re ready to transfer seedlings to larger pots, you can use potting soil. Look for products that contain composted fir or pine bark (don’t settle for plain “bark” or “processed bark”). Research has shown that properly made bark compost helps prevent diseases, such as damping-off, a fungal disease that can quickly kill newly sprouted seedlings.
Most seedlings will do better if you grow them beneath fluorescent lights. You can buy expensive grow lights, but the 4-foot-long shop lights sold at hardware stores work just as well and cost much less. Start with new tubes—fluorescent tubes become dimmer over time. Don’t bother with incandescent bulbs. Their light does not stimulate growth well.
Suspend your light fixture from the ceiling over a table or bench. To protect the table or bench from water, cover it with a plastic sheet.
Heating Mats and Cables
Most seeds—including tomatoes and peppers—germinate much faster in warm soil (about 70° to 75°F). To provide those toasty temps, heating mats and cables come in handy. You just plug them in and set your containers on top of them.
Some of these devices connect to a control unit that allows you to set the temperature at the exact level needed for germination. Others have thermostats, switching on and off automatically to keep the soil temperature in the mid 70s or so.
Keep in mind that most sprouted seedlings grow better in slightly cooler temps (upper 50s to lower 60s), so remove the heating mat or cables after the seeds have sprouted.
For peat pots, paper pots, and soil blocks, a capillary mat can be a big help. Capillary mats allow the seed-starting medium to draw water from a reservoir as needed, so you don’t have to monitor moisture levels as often. Simply set your containers on top of the mat and keep the reservoir filled (usually one filling every 4 or 5 days is all that’s needed). The mats are made of a fiber that “wicks” water, spreading the moisture evenly from the reservoir to all corners of the mat. You can even put liquid fertilizer in the reservoir to feed larger seedlings. One drawback is that, as the seedlings grow, their roots may attach to the mat; if that happens, just peel off the mat at transplant time.
Seedlings growing in a soil-free or lean potting mix will need small doses of plant food, starting at the time the first true leaves develop. For the first 3 weeks, water them once a week with a half-strength solution of fish or seaweed fertilizer, compost tea, or one of the liquid organic fertilizers specially formulated for seedlings. After that, feed the seedlings with a normal-strength solution every 10 to 14 days. If you’re growing your seedlings in a potting mix that contains compost or other nutrients, you may not need to feed them as often.