From: Jane McGary Roots that come out the drain holes of pots may be of this type [contractile]. In fact, one danger of growing bulbs in pots is that, if not lifted often enough, they will be pulled down into the drain hole and plug it up, resulting in no drainage, rot, or having to break the pot to free the bulb. This last is an argument for using plastic mesh pots instead of solid pots in a plunge situation -- though I still rely mostly on terracotta. That would only be a problem if the pot is not deep enough. Clearly, contractile roots do not pull the bulb down forever, else .... I use plastic pots almost exclusively because they are cheap and light, and I find they keep the soil more uniformly moist than clay. But you must use a porous mix to give good drainage and aeration. Plastic pots come in a variety of sizes and depths (although especially deep or very shallow pots can be hard to find). I used to use terracotta , but the pots got salty, and burned roots which were invariably attracted to the sides of the pot. David Ehrlich