I am in the process of building a greenhouse and need a cheap retaining wall. I thought about using railroad ties, but am a little concerned about creosote since I will be growing vegetables (and possibly fish) in this greenhouse.

Any ideas on a low cost retaining wall? I need three walls built, the highest is around 3 feet and about 17ft long. The other 2 walls are around 12 feet long and 2 feet high.
Any time you can get out to hunt, is a good day to hunt.