Most seeds need only to be placed in warm, moist soil to leap into growth. Others, due to hard seed coats or other factors, are slower to break dormancy. Improve the germination rates of such seeds, sown indoors or out, with the following treatments.
Some seeds come clad in a protective coating that is particularly hard and water-resistant. In nature, weather, gritty soil, and even acid in the digestive tracts of animals abrade the seed, letting moisture in to spark germination. Gardeners can mimic this process at home by scratching the seed coat, which allows water to permeate the seed and prompts germination.
Method: Scuff individual seeds by dragging them across a sheet of medium-grit sandpaper. Avoid making deep scratches that expose the underlying, lighter-colored embryo, and prevent damaging the seeds' growth points by sanding only their sides. Scarify large amounts of seed by placing them in a jar with coarse sand and shaking vigorously until the seed coats are dull and scratched. Sow scarified seed immediately.
Best for: Morning glory and moonflower (Ipomoea), castor bean (Ricinus communis), and New Zealand spinach (Tetragonia expansa)
The seeds of many perennials, trees, and shrubs need periods of chilling and warming to turn off the chemical inhibitors that prevent germination. Sowing these seeds outdoors in fall exposes them to variable temperatures, but refrigerating the seeds prior to starting them indoors is a more reliable way to condition them.
Method: Soak the seeds in water for 24 hours. Skim off any floating seeds, as they are most likely not viable, and drain the remaining ones. Fold large seeds into a piece of damp, long-fibered sphagnum peat moss and mix small ones with barely moistened vermiculite; place them in a plastic bag. Seal and label the bag. Refrigerate the seeds at 40° to 45°F. Refrigeration times vary from 1 to 4 months, depending on the seed. Look up exact refrigeration times in a propagation reference, such as Making More Plants, by Ken Druse (Clarkson Potter, 2000). Sow the seeds indoors under lights following the refrigeration period. You can sow the small seeds with the vermiculite, but pick out the large seeds from the peat moss before sowing.
Best for: Perennial phlox (Phlox paniculata), ornamental onions (Allium), columbine (Aquilegia), violas and pansies (Viola)
Presoaking softens the coats of seeds that are slow to sprout and primes them for germination.
Method: Soak the seeds in a jar filled with tepid water for at least 4 hours and up to one day. Skim off any floating seeds. Pour the remaining seeds into a wire mesh strainer and rinse well with fresh, cool water. Sow immediately.
Best for: Edible peas (Pisum sativum), sweet peas (Lathyrus odoratus), parsley (Petroselinum crispum), beets and chard (Beta vulgaris)