Seed Stratification Guide
What is Seed Stratification?
Stratification is a process of pre-treating seeds in order to simulate natural conditions that seeds would experience in the soil over-winter. Pre-treating seeds helps the seed "break dormancy" and initiate the germination process. Stratification pre-treatments are applied when the seed is being propagated in a greenhouse setting or in a way/schedule that is not aligned with the natural germination cycle. Seeds and seed mixes which are sown outdoors in the fall do not require stratification pre-treatment.
Stratification information for individual seed species that we sell is noted in the "Cultural Details" section on each species product page. The information is also available in the Seed Stratification & Propagation Guide PDF.
Dry stratification is a seed pre-treatment in which the seed is subjected to cold temperatures of 32˚F or lower, for a period of a month or longer. This simple treatment of keeping the seeds in cold, dry storage helps increase germination rates by immitating a natural winter dormant period. Many native prairie species require only Dry Stratification. This can be accomplished in temperate climates by storing the seed in Ziploc bags in a refrigerator, or in a rodent-proof container in an unheated building over winter for 30-120 days (depending upon the species). Most warm season prairie grasses and wildflowers require only 30 to 60 days of dry stratification to break seed dormancy.
Prairie Nursery seed is “dry stratified” prior to shipping to help break seed dormancy.
Many prairie seeds germinate best when seeded in fall, in a dormant seeding. The exposure to cold, damp conditions over the winter is a nature's "moist stratification," and signals to the seed that it is time to germinate when the soil warms up in spring. Some wildflowers need this process, more than others. A moist stratification seed treatment is used to mimic the effect that winter bestows upon the seed.
- Moist stratification is a process wherein the seed is mixed with moistened inert material (fresh sawdust, builder’s sand, vermiculite, peat moss or similar material) and stored cold for ten days to three months.
- Planting the seed outside in the fall (direct seeding) achieves the same results as Moist Stratification. It is not necessary to use Moist Stratification pre-treatment for seed that is to be planted in the fall, as dormancy will be broken naturally in the soil over winter.
- To Moist Stratify seed: In a plastic bag or re-sealable container, mix the seed with an equal volume of moist (not wet) sawdust or clean builder’s sand (if moisture can be squeezed out of the sawdust or sand, it is too wet). Refrigerate at 34-38 degrees F (do not freeze!). Most flower seeds require three to four weeks of treatment. Legume seeds generally only require ten to fifteen days. Some flowers require two to three months.
- After removing the moist stratified seed from the refrigerator, it can be sown in pots or flats, or diluted with a much larger volume of inert carrier material and direct-seeded onto the soil.
- When planting moist stratified seed, it is best to water the area for one to two months after seeding. Seed may revert to dormancy if it does not receive rain or moisture within a certain number of days after planting. All direct seedings in spring or early summer, pre-treated or not, will benefit from irrigation during the first two months after seeding.
Members of the legume or bean family can benefit from an inoculation with Rhizobium bacterium prior to planting. All of our legume seeds have been pre-inoculated with this bacterium. The bacterium works with the plant to form nodules, on the roots of legume plants, which are capable of taking nitrogen from the atmosphere and incorporating it into the plant.
Scarification of seed is the process of scratching the seed coat to allow moisture to enter the seed itself. If a seed requires scarification, gently scratch the seed with a sandpaper block prior to planting outdoors, or prior to moist stratification. Many members of the pea family (legumes) benefit from scarification.
The seeds of 'Double dormant' species must spend two consecutive winters in the ground in order for the seed to break dormancy. These seeds are best planted in the fall, although they typically will germinate two years after being seeded in the spring.
Hot water is used for species that require heat to stimulate germination. Scarify the seed, then place it in a bowl. Pour boiling water over the seed and allow to cool. This mimics the effects of a prairie fire. Pour off the water, collect the seed, and Moist Stratify for 30 days as per directions above.