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)
Soaking.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)