Vernalization
Most temperate terrestrial orchids must undergo some form of vernalization, or cold dormancy period during the winter. When seedlings have roots that are more than an inch in length and their shoot bud has elongated, they are ready for vernalization. Seedlings should be removed from their flasks and the agar thoroughly rinsed off. We often find it useful for the final rinse to consist of sterile water, processed either with an autoclave or pressure cooker for 20 minutes. Then, if any tiny fragments of agar remain, little or no bacterial growth will occur during vernalization. Seedlings are placed into well-sealed bags with only a few drops of water, or, if there are many, into sealed boxes with a small amount of water, and placed into a refrigerator.
It is extremely important that the seedlings in the refrigerator are not allowed to freeze as freeze-thaws will cause dessication and damage. Suitable storage temperatures are 33 - 39 F. We recommend that a thermometer be placed into the refrigerator for several days and checked at various times to confirm that the cooling cycle does not result in swings in temperature to below freezing or to temperatures that are too high to accomplish vernaliztion.
Seedlings must be checked every week or so to confirm that they have not broken dormancy and the shoots are greatly elongating. When this happens, the seedlings may die if not brought out and potted. Vernalization should last three months or longer for species like Cypripedium reginae and Cypripedium kentuckiense and four months or longer for other North American species. For many Asian species, four - five months may be required for the seedlings to break dormancy. A few of the more southern species, in particular Cypripedium formosanum, are notorious for breaking dormancy early so watch them carefully!
