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.
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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 a space of 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.
Organic 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.