Factors that Affect Seed Germination
Why didn’t my seeds germinate? On occasion, we are asked this question, and the following article addresses several important factors, both general and specific, of prairie seed germination:
- Timing of planting
- Soil temperature
- Pretreatment of the seed, or lack thereof
- Growing conditions
Timing & Soil Temperature
Most native wildflowers germinate best when seeded in fall as a “dormant” seeding. The exposure to cold, damp conditions in the soil over winter, followed by warming of the soil in spring, breaks seed dormancy and stimulates germination. This is believed to be an adaptation that ensures that seeds do not germinate when they ripen and fall to the ground in autumn, which would result in the loss of small seedlings with the onset of winter.
Most warm season prairie grasses, including Big Bluestem, Little Bluestem, Side Oats Grama, Switchgrass, and Indiangrass germinate best after the soil warms up in May and June. These grasses exhibit significant reductions in germination when seeded in fall as a dormant seeding.
Prairie Dropseed does not germinate well when seeded into warm soils, and is best seeded in April, in fall, or as a late winter “frost seeding” by scattering seed on frozen ground on a light cover of soft snow in February or March. The seed will work its way down into a previously prepared seedbed as the soil freezes at night and thaws during the day, opening up micro-fissures in the ground. Warm season prairie grasses generally perform better with frost seedings than with fall dormant seedings. Prairie wildflowers also exhibit high rates of germination with late winter frost seedings compared to spring and early summer seedlings.
Never seed onto a crusted layer of snow or ice, as the seed can blow away.
Prairie Cordgrass and all sedges are best seeded in fall as a dormant seeding, since they require extended exposure to cold, damp conditions to break seed dormancy. Spring seedlings of these species result in little or no germination.
When seeded in spring, many flowers, sedges, and grasses that require exposure to cold, moist conditions to break seed dormancy may not germinate the year they are planted, but often will come up in the following spring. The seeds of some flowers are known to remain dormant in the soil for a number of years before finally germinating 3-5 years after seeding. Patience is a virtue with native seedlings.
Pre-Treatment for Double Dormant Species
Many members of the Rose and Lily families exhibit a phenomenon known as “double dormancy,” in which the seeds require exposure to cold conditions over winter, warm over summer, and then cold again over a second winter. The seeds then germinate in the second spring, some 20 months after they matured and fell to the ground. This process can be hastened by planting double dormant seeds in pots or flats when ripe in summer, and then covering them with plastic wrap to retain moisture and placing them in a refrigerator for two months (October-November). Remove the pots or flats from the fridge and place in a warm environment for two months (December-January). Place them back in the fridge for two months (February-March) and bring them out to germinate around April 1st. This will usually yield reasonably good germination. If germination is spotty, the pots or flats can be placed back in the fridge in fall (November) and brought out in early spring (March-April) for a second round of germination for those seeds that did not come up the first year.
Daylength Factor and Greenhouse Propagation
When germinating seeds indoors or in a greenhouse, daylength can be a factor. Many seeds germinate best when the days are longer, as in spring. When days are getting shorter in fall, or are still short in winter, germination of some species can be reduced or non-existent until the days begin to get longer. This can be overcome by installing plant growing lights and maintaining 12-18 hours of light per day by setting them to come on for 4-10 hours just before the sun sets at day’s end.
Growing conditions can exert a significant influence in germination and seedling survival. First and foremost, each species must be planted in the appropriate soil type, moisture regime, and sun conditions to which it is adapted. If planted in the wrong soil or light conditions, the chances of success are greatly reduced.
Good soil structure is essential fo good germination and plant growth. Soils that are low in organic matter typically have poor soil structure. For instance, heavy clay soils without much organic matter are hard and impermeable, preventing the absorption of moisture and free flow of air to plants’ roots. This leads to rapid soil drying, and limited development of a plant’s root system because of the difficulty in penetrating the hard clods of clay. Stunted plant growth or death are common results. Dry sandy or rocky soils with limited organic matter have limited nutrient and water-holding capacity, so seeds and young seedlings can dry out and die during germination. Organic matter holds far more plant-available water and nutrients than the mineral portion of the soil, making it indispensable in promoting good soil structure and healthy plant growth.
Soil issues can be overcome using three different strategies, which can also be applied with good soils to enhance results of native prairie seedings:
- SEED IN FALL. Fall dormant seeding will “pre-position” the seeds so they are in place to germinate at the first opportunity in spring. Soil moisture is typically high and daytime temperatures are cool, reducing heat and drought stress on small seedlings. The young plants have an opportunity to develop more extensive root systems prior to the onset of hot summer weather, compared to later spring seedings.
- APPLY MULCH OR AN EROSION BLANKET. Mulch the seeding with clean, weed-free winter wheat straw at the rate of 3000 lbs. (1.5 tons) per acre = 70 lbs. per 1000 square feet (two 35 lb. bales), or install a light duty straw erosion blanket and stake it into place. The straw will help retain soil moisture during the critical germination process in spring and early summer. Erosion blankets are essential for preventing soil and seed loss from washing on hillsides and in swales. The openings in the top side plastic netting should be at least 3/4” by 3/4” and no smaller, to allow for proper emergence of the native wildflower seedlings through the net. The filler material in the blanket should be either straw of excelsior wood shavings, such as the ‘Curlex 1’ blanket made by the American Excelsior Company. These allow for the proper emergence of small prairie seedlings through the blanket. Dense blankets consisting of coir (coconut husks) or other thick materials are not recommended, as they do not allow for proper seedling emergence. Woven coir or jute (plant fiber) mats are good for holding the soil and seed in place, and the openings between the weave allow for seedling emergence. However, they do not provide the same moisture retention as straw or excelsior blankets.
- WATERING. Water the seeding early every morning after seeding in spring or early summer, or for fall dormant and frost seedlings during dry periods in the first spring. This can make a huge difference in seed germination and seedling survival, especially in a dry spring. Water early in the morning around dawn until the surface soil is just moist, usually 10-20 minutes depending sprinkler output. Never water in late afternoon or at night, as this can create perfect conditions for “damping off” of young seedlings by plant-attacking fungi.
Water un-mulched seedings daily (unless the soil is damp from a previous rain) for the first 3-4 weeks after seeding. Once seedlings begin to appear after a few weeks, watering frequency can be reduced to once every 2-3 days. Most prairie seeds germinate between mid-April and mid to late July. Watering is most effective in stimulating germination in the first 4-6 weeks after seeding. Seedings that have been mulched or covered with an erosion blanket will benefit from watering during dry periods at intervals of every 2-3 days. Fall dormant seedings do not benefit from watering, since the seeds will not germinate until the following spring.