I have no idea what Preen is, but I might be able to help with the issue of "ferns spreading." There are only a couple of ferns that are likely to spread by root runners through the whole bed. Most ferns will stay put and just bulk up where you've planted them and won't spread whether you Preen or not. Some may spread by spores, which is the ferny equivalent of self-seeding. If Preen is some sort of a pre-emergent herbicide, then it will probably also kill fern spores, but it probably won't stop the root runners from sprouting. It all depends on what kinds of ferns you have - feel free to post the names here and someone can tell you how they grow, or look them up on the web or in a book...

karin1, I have two types of ferns. One is a Christmas fern, which has spread a bit (I started out with three crowns and now have three HUGE crowns and maybe four tiny ones very nearby.) The second is an unknown that a friend dug up for me from her wooded backyard here in SW Ohio and which I believe is probably a native. It spreads like crazy. When they come up this year, I can post some photos. Maybe someone can ID them for me.

Preen is a pre-emergent, dry granules that you sprinkle wherever you don't want weed seeds to germinate.