Artificial Vernalization of Potted Cypripediums

A few days ago I took my Cyp. reginae f. albolabium seedling out of vernalization. This is the only Cyp that I have growing in a container currently, and I'm doing this because it is a fragile young plant - I never plant seedlings out until they are 2-3 years old. Anyway, I wanted to discuss the issue of dormancy in Cypripediums when you are container growing. This applies to seedlings and mature plants alike. Because Cyps are temperate plants, they require a cold winter dormancy in order to survive. During this period the plants die to the ground and rest for 3-4 months. Obviously in the garden nature takes care of this, but if you are container growing, you have to simulate it. The best option with mature plants, if they are hardy in your region, is to sink the pots into the ground for the winter. But if you are growing seedlings, this is undesirable as conditions outside are quite harsh for such tiny plants (although some people simply plant them in protected sites). As well, some people like to have mature Cyps growing at specific times of the year, ie for shows and such. Finally many people grow the more tender species in northern areas and can't overwinter them outside. In these cases, we use a method called artificial vernalization, in which basically the plants are refrigerated for a few months.

Cyps do not need freezing temperatures during their dormancy; in fact this might kill them in pots. Outside, the plants are insulated from the cold by the ground, mulch, and snowcover. In pots they don't get this insulation. So, temperatures of around 3-5 C are adequate for dormancy. Any standard refrigerator works for this. The next thing is soil moisture. Although the plants should have significantly less moisture during dormancy, they should still not be bone dry. As dormancy approaches (signalled by the yellowing of the leaves), gradually reduce watering so that by the time the plant is brown it is just being kept slightly damp. Now, place the pot in a Ziploc bag and seal it, and place it in the crisper drawer of the refrigerator. This will create a humid environment so that the soil doesn't dry out, and the bag should help keep fungi out. However, as there is moisture and there are cool temperatures, you could get mold growth on the soil so watch for this throughout dormancy and pick it out as it appears. I like to prick holes in the Ziploc bag to vent it, which I find helps reduce mold growth. This doesn't work as well for everyone though, I've heard. Also throughout the dormancy, check the soil moisture every once in a while and dampen it a little if it gets dry.

With mature plants, it is usually not feasible to refrigerate them because the pots are too big. For them, you probably have to store the pots in the garage (unheated) or a really cool room in the basement over the winter. In the case of an unheated garage, simply cover the pots really well with blankets, etc. to keep it from freezing.

And that's basically it! All you have to remember besides that stuff is that the plants need 3-4 months dormancy depending on the species (and formosanum appears to only need 2 months). When the time is up, just remove the pot from the fridge and start watering it (put it in a bright location). If the plant starts emerging from the soil while it is in the cold treatment, remove it immediately. I'll be posting pictures of my seedling in the coming weeks.