Many seeds require cold weather to break dormancy and germinate into new plants, a process known as stratification.
They are stratifying seeds to speed up their germination rate! Discover how stratifying can speed the germination process!
How to Stratify Seeds
At first, many seeds from trees, shrubs, and perennial flowers (such as Agastache, Delphinium, Foxglove, Hollyhocks, Liatris and Rudbeckias) require pretreatment to break dormancy and allow germination. This stratification process involves exposing them to cold and warm, moist environments that simulate nature to help break out their natural dormancy and allow germination. Each seed packet for these species will usually indicate how long cold/warm stratification should last for successful germination.
Stratification can be completed indoors or outdoors, using your refrigerator, or cold frame, where temperature fluctuation mimics natural seed growth conditions. Seeds should be placed in a container with equal parts sand, perlite and vermiculite, along with some water to keep their surface damp; then put into your fridge until indicated on their seed packet by either letter WS (warm stratification) or CS (cold stratification).
Many seeds that require stratification also need to be sacrificed – this involves altering their outer seed coat by either nicking or rubbing it to promote imbibition and absorb moisture, which aids germination. Be sure to follow any instructions on your seed packet regarding scarification, as some seeds are pretty fragile and must be handled carefully.
Seed packets generally list the time necessary for cold and warm stratification and a code to denote which method was chosen (e.g. CS 13 = Cold stratify for 13 weeks). You can find this information on our website as well. Additionally, many seed companies include information regarding minimum/maximum growing zones, which is especially helpful when living in different climates. You want to ensure you buy appropriate seeds for where you live!
Refrigerator Stratification
When your seeds don’t sprout after sowing them, you may think something is amiss with the seeds, your planting technique or your soil. But what your seeds need help getting out of dormancy: stratification offers this easy solution.
Many tree, flower, herb and vegetable seed varieties require cold weather stratification to overcome dormancy and begin their germination processes. Dormancy is when all development stops temporarily to protect it from freezing or being consumed by birds or animals until conditions are better for growth.
One way to help nature do its work is to scatter seeds over raked soil in the fall. This allows natural freezing/thawing cycles, combined with moisture from rain and snowfall, to break down seed coats and prepare them for spring germination. This method is often recommended on seed packets; it may work for certain seed types but can take time and be less controllable than using different indoor techniques; leaving outdoor seeds exposed could even get eaten by animals! It’s wiser to only employ this approach with hardy seed varieties or varieties where temperatures stay below freezing in winter.
Alternately, you could try spreading the seeds on a sanitized paper towel or in a container filled with moistened soil, peat, vermiculite or sand and wrapping them with damp paper towels before sealing them in plastic bags or containers for one to three months, depending on the seeds’ requirements. Be sure to label each container so you know exactly what’s inside and the required storage period.
If you don’t have access to a refrigerator, a cooler can still serve the purpose of stratification; ensure it does not get too warm and there are enough ice packs inside for cooling during its duration. Otherwise, seeds could become exposed to too much heat before cooling enough for dormancy breakage.
Freezer Stratification
Seed stratification may seem complicated, but it’s one of the easiest and quickest ways to increase germination rates. All it requires is a refrigerator and the time your seeds need to break their dormancy, and sowing can begin! Cold stratification works for perennial flowers, shrubs and trees that require frigid temperatures to germinate, such as Larkspur (Consolida), Love-in-a-mist (Nigella) and annual poppies (Papaver spp). Iowa State University Extension recommends cold stratification when sowing tree seeds like Redbud Maple Oak species.
Refrigerating large seeds such as nuts or fruit trees in the freezer is an invaluable way to maximize limited storage space. It protects them from being devoured by rodents who might otherwise get into your refrigerator and destroy them. Similarly, freezing seeds helps you protect them against rodents who could otherwise get access to them and nibble at them while they sit unprotected in storage containers or fridges.
Freezer stratification emulates natural conditions by exposing seeds to cold and dry temperatures found when they land in soil, similar to what they encounter when planted outdoors. Gardeners frequently utilize this approach when growing fruit trees as it yields better results than indoor, refrigerated stratification.
When deciding whether your seeds need stratification, consult the seed packet or species-specific requirements in a reference book. Plenty of gardening websites and books offer step-by-step instructions on how to stratify seeds so you can get going immediately.
Certain seeds, such as those belonging to black-eyed susan and purple coneflower plants, do not require cold stratification to grow. Most warm-weather flowers and vegetables also do not require stratification for germination, although in some instances, the thickness of the seed coat can indicate need – thicker coatings indicate a greater likelihood that cold temperatures must be used to break dormancy and germinate the seedlings.
If cold stratification is needed, scatter your seeds onto raked or tilled soil in the fall and allow nature to take its course: freezing-thawing cycles combined with moisture from rain or snowfall will gradually break down their seed coat and allow the seeds to germinate more readily throughout winter.
Soil Stratification
Some seeds require cold stratification before germinating can begin, rather than waiting for nature to kick in in spring. You can accelerate their success by using a few materials and your refrigerator to speed up this process and be patient.
Woody species and perennial wildflower seeds like cleome and foxglove benefit from being stratified, as do annual varieties such as sunflowers and zinnias. While some seeds can sprout quickly when sprinkled onto bare soil in early spring, others contain hard shells which need time to break open under ideal growing conditions before sprouting occurs – cold stratification ensures they don’t germinate too early, where they could perish from an unexpected winter cold snap, or too late where competing plants outshout compete against them for survival.
Although the best stratification method varies by species, stratification generally involves mixing dried seeds with prechilling material such as sand or peat layers and then placing the whole batch in the refrigerator for one to four months before checking to ensure it remains moist and mould-free.
Your second option for stratifying seeds is using the dry stratification method, which involves placing them in a plastic bag or jar and placing them in the fridge for one to four months. This approach works particularly well when cultivating perennial wildflowers such as Larkspur, Love-in-a-mist and annual Poppies, and hardy grasses like Penstemon that thrive in dry soil or have adapted well to winter climate conditions.
Patience is at the core of stratification; otherwise, it can take days, weeks, or even months for your seeds to emerge from dormancy and germinate. If this becomes impossible for you to endure until the planting date arrives, alternative germination methods, such as hot water or chemical treatments, might need to be utilized instead. While the process might seem overwhelming at times, remember that you provide a gentle push in their direction so they emerge when and if needed.