Ask Mr. Smarty Plants

Congratulations on starting a good crop of yucca seedlings. I am assuming that you have a hardy yucca (There's a list of some hardy species at the http://www.yuccaagavaceae.com/hardy.html website). The most popular are Yucca filamentosa (Adam's needle) and Yucca glauca (soapweed yucca).

Outdoors, hardy yucca need to go through a gradual progression to winter conditions so that they become acclimatized to the weather.  To do this, slowly reduce the soil moisture so they go dormant - even if they look slightly shriveled. Also, they have to have well-drained soil or else the roots will rot during the winter. Water pooling around the plant will also cause rotting issues so make sure they are not sitting in a low area. Plunge the yucca seedlings with their pots into a garden bed late in the fall before the ground freezes. Put some evergreen boughs over the yucca to help collect the snow during the winter. This will help provide insulation and reduce the chance of wind burn or freezing and thawing damage.  As a backup you can bring a few indoors and overwinter them in a warm and slightly moist environment (or keep them dormant in a cool and dry location like the back of an attached garage).