Great article from Mother earth news on how to grow carrots in succession so you can have fresh carrots almost all year in the south east and I would think a lot of months elsewhere. They also give us a tip on planting radishes and carrots together and tell us why which I found to be very good advice and had never thought of myself.
http://www.motherearthnews.com/organic-gardening/succession-planting-lettuce-carrots-more.aspx#axzz2O2OCSutH
Succession Planting for Year-round Pleasure