Guide to Woody Seed Stratification

Scarification

• A survival adaptation of many woody plant species is a hard outer seed coat. Freezing temperatures, microbial activities and passage through the digestive tract of an animal naturally alter the seed coat during the winter, allowing moisture to reach the seed embryo. Gardeners can artificially reproduce this process using a metal file, sandpaper, knife or hammer, or by soaking the seed in hot water. During the scarification process it is important to alter only the shell. If the seed is damaged it will not germinate.

Stratification Medium

• Plastic food storage bags and other sealable containers are ideal for seed stratification at home. Commercial nurseries commonly use vermiculite, a mixture of vermiculite and perlite, sterile sand or sterile milled sphagnum peat moss for the stratification medium. Use 10 to 20 times the amount of medium to seed volume. The chosen medium must be moist, but not so much that it is dripping wet. Moisten and mix the medium thoroughly and allow the excess water to drain through a strainer before placing the seeds in it.

Cold

• The seeds of many plants naturally ripen in late fall or early winter. These seeds spend the winter months among the natural ground cover, which keeps them just above freezing through the winter. However, the seed only knows what temperature it is, not where it is. The crisper drawer in the refrigerator provides the temperature range -- between 32 and 40 degrees Fahrenheit -- needed in cold stratification without the temperature fluctuations found in nature.

Warm

• Seeds that naturally mature in late summer or early fall require a warm stratification period, between 62 and 75 degrees Fahrenheit. The species of seed determines the length of the stratification period. The seeds require warm, moist conditions to break dormancy, however these same conditions encourage fungal growth. Milled sphagnum moss has fungicidal properties, and nurseries often use it instead of adding fungicide to the stratification mix.

Variable

• Some seeds, regardless of what season they ripen, require a combination of stratification methods. The seeds undergo warm stratification, followed by a cold period and then natural warm, moist germinating conditions. Soak the seed for 24 hours before placing it in the stratification medium. The presoak and warm stratification period soften the seed coat and allow moisture to reach the embryo during the cold stratification period. Only a mature seed embryo responds to cold stratification.

A Good Look

• Remember to label the seeds with the date stratification started, the date the seeds should be ready and the plant name. Some seeds will germinate during the stratification process; sow these in pots indoors until proper planting time. Check for signs of mold or inadequate moisture every two weeks. Throw out any seeds that show signs of mold. If the medium itself molds, replace it.