The length of the growing season is measured as the time between the last frost in the spring and the first frost in the fall. The depth of winter cold limits the perennial plants that will grow in an area, but for annual plants, the length of the growing season is the most important climate measure.
While vegetable and flower seeds usually are planted in the spring based on the date of the last spring frost (See chart below; or, for a more accurate local estimate, contact your local Cooperative Extension Service.), you can get a jump on spring by starting seeds well before the weather is warm enough for planting directly in the garden. You can plant seeds indoors or outdoors in a cold frame, hotbed, or individual covers to get an early start. These simple structures provide warmth and protection for the developing seedlings.
Seeds can be started on a sun porch or a windowsill 6 or 8 weeks before the weather is mild enough to plant them outside. In short-summer growing regions, this is the only way you can get some tender vegetables to bear at all. In all regions, this method allows earlier harvests and greater yield from vegetables and earlier blooms from annual flowers.
The last frost in the spring and the first frost in the fall are likely to be radiation frosts. Radiation frosts occur on clear, still nights as the heat absorbed by the soil during the warm day is radiated to the dark sky of a cold night. Either wind or a cloud layer will usually prevent a radiation frost. The easiest way to protect young plants from a radiation frost is by covering them. An old bed sheet or a piece of cardboard is often all that is needed to keep vulnerable young shoots or spouts from being damaged. Plants in containers can be moved under the eaves, which will offer them enough protection.
If you just can’t wait to work on your landscaping or in your garden, you can start plants outside or set them outside early with a little protection.
Average Last Frost Dates Across the Country
The dates supplied are averages. There is a 50% chance that frost will still occur after these dates.