As they were in a cold greenhouse will they need another cold snap?
More likely a wet spell maybe.....
You may find the link below useful.

http://webcache.googleusercontent.com/search?q=cache:http://www.ces.ncsu.edu/depts/hort/hil/hil-8704.html

I hope that it works for you, as I had a few problems with it.

Goodness, my problem is getting rid of ash seedlings! If you leave the keys on the ground over the winter you should get all the seedlings you can cope with.