The map above is based on average dates when you can begin setting out plants without fear of frost in the U.S and Canada.

Many old garden hands in Mid-Atlantic states, for example, will tell you that the first day for outdoor spring planting is actually April 20, when they will set out frost-resitant varieties of vegetables (peas, radishes) before planting warm weather crops and flowers nearer the end of the month.

In short, use the map as a guideline only, and if you are still unsure of the safe frost-free date in your area, take adventage of local garden wisdom — by asking knowledgeable neighbors when they begin planting their spring gardens.

“Climate is what we expect, weather is what we get.”
Mark Twain

More Frost Free Dates:

U.S. Spring Freeze Occurence

The Atlas of Canada - Last Frost in Spring

UK First & Last Frost Dates