Unlike annual seeds that sprout when weather conditions are suitable, many varieties of perennial seeds require a period of damp cool weather to germinate properly. The process of exposure to cool temperatures and high humidity is called cold stratification. Many times perennial seeds are gathered in the fall and the seeds are cold stratified to encourage them to sprout in the spring. The temperature and amount of time required for successful cold stratification varies for different perennial seeds.
Veronica (Speedwell) seed is planted in the fall or the seed is placed in cold stratification as it requires three to four months of cold stratification to germinate. It is a perennial plant that produces spikes of pink, lavender, white or purple flowers in the spring. It grows to 24 inches high and the flowers are often used as long-lasting cut flowers for floral arrangements.
Lavender (Lavandula) is a shrubby perennial that produces fragrant foliage and purple, pink or white flowers. The seed requires five to six weeks of cold stratification to germinate properly. Lavender is an herb used as a cut flower, in cooking, in sachets or harvested for its essential oil.
Phlox is a popular garden plant of which there are several types. Some types grow low to the ground and used as border plants, while other improved types grow up to 4 feet tall and produce large blooms. However, the seeds of all types need 12 weeks of cold stratification for effective germination.
Rudbeckia is a native wildflower, also known as Black-Eyed Susan, for which there are many improved varieties. In most cases, the seed falls to the ground and experiences natural cold stratification. However, for seeds purchased or gathered in the fall, the cold stratification required for germination of Rudbeckia is three months.
Delphinium is a cool weather perennial that thrives in mild climates. The large flower spikes grow to 4 feet tall and bloom in white, pink, purple or blue depending on the variety. The flower is a popular cut flower and is outstanding in the back of the garden. Delphinium blooms in early spring and goes dormant in warmer weather. The seeds need a cold stratification period of 12 weeks before planting.
Asclepias, or milkweed, is a native perennial popular for planting in butterfly and hummingbird gardens. The seed is attached to a silky substance that helps the seed become airborne in the fall. The flowers are bright orange or yellow and are important to Monarch butterflies during their fall migration. The stratification period for Asclepias seed is three months.