Our agriculture extension recommends planting from February 1 to April 15 here. February 1 is the colder part of winter here. Anywhere in that time frame is still before the last frost.
Old Farmers Almanac says
"Radishes are a hardy, cool-season vegetable that can produce many crops each season due to its rapid days to maturity. Radishes can be planted in both the spring and the fall, but growing should be suspended in the warmer months. They are a very easy vegetable to grow."
http://www.almanac.com/plant/r...
