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Providing the proper treatment for optimum germination
A lot of seed, especially cold germinators, need a cooling-period after sowing. It is beneficial to cover the sowing with snow during that period.
The temperature below snow usually keeps the optimum range of -4 to 0°C (25 to 32°F). The sowing is kept moist, and the melting snow helps to soften the shell, make it porous which is advantageous for the germinating seedling
Putting dry seed in the freezer, or in the refrigerator, does not break dormancy! Freezing seed can damage it. Seed needs to be soaked fully with water in order to break dormancy but do not put seed trays in a freezer!
For hard seed like members of the Peafamily scarify seed with sand or rub seed with sandpaper. See Big Seeds. Often the seed coat is so hard that neither gas nor water can be exchanged. Scarifying breaks this condition
Seed needs to be fully soaked with water before the germination process can start. Soak big seeds in lukewarm water for one night prior to sowing
Control the main germination conditions very carefully: light, water, temperature! Avoid stressing the seed, while germinating. Maybe you do not see the seed, but it is there!
A number of species need a very warm treatment to germinate best (over 24°C/75°F) like Scabiosa caucasica, Bergenia, Delphinium, Rudbeckia 'Goldsturm' and many of the Prairie plants (interesting enough a cold treatment has the same effect on these species!)
Avoid temperatures below -6°C (20°F). If this happens during winter it will not harm the seed but the germinating process (or breaking dormancy) can come to a standstill. Therefore the cooling process has to be extended (in nature the freezing process happens slowly and the seed can regulate/compensate the pressure differences!) Do not put seed trays in a freezer!
A number of perennials like Helleborus, Corydalis and Asarum need a warm treatment of 6 to 8 weeks (Summer) to fully develop the embryo to a size before it is capable of germination - followed by a cold treatment (winter) to break dormancy. If either of these have not been long enough, the same treatment has to be given again until it is able to germinate
Basically you can not do any harm by treating every perennial species as a cold germinator. The other way around - by only keeping them at warm temperatures - you can fail with a large number of perennials
After the cold treatment (0 - 5°C/32 - 40°F) it is important to raise temperature slowly up to 10 - 12°C (50 - 54°F) until germination occurs. If raised directly to 20°C (68°F) right away a second dormancy can occur and this is even harder to overcome. Alpines, especially, germinate better at cool temperatures like Gentiana and show little or no germination over 18°C (65°F)!
A lot of perennials benefit from day and night temperature differences - warm during the day (21°C/70°F) and cool during the night (13°C/55°F) like Aquilegia
Gibberellic acid (GA 3) can help stimulate germination on some species, approx. 4 gr. per gallon. But be careful - it can also do as much harm as it does good!