When to plant? When to plant?

Springtime is gardening time! But when can I plant my delicate vegetables without fear of losing them to the last ravages of winter? All of the seed packets and plants at the nursery say to plant only after the last frost date. Luckily meteorologists and horticulturists have been recording the last frost date in Wilmington, Delaware for more than a hundred years.

The relevant frost data for my garden is either around Wilmington, Delaware or several miles further north at Porter Reservoir. The last frost date usually published in an almanac is actually the 90th percentile date for the last frost dates in the spring complied over several years. That means that the last frost occurred before that date 90% of the years for which there are records. This makes that date a pretty safe date to plant frost sensitive plants. The chart below (click for larger version) is the histogram for the frost dates for both locations. The 90th percentile last frost date for Wilmington is April 26th and for Porter Reservoir is April 22nd.

The first frost date in the fall is also recorded by scientists. The difference between the two is the official growing season. The chart below (click for larger version) suggests, if you are suggestible, that it may have lengthened over the last decade. Is this due to global warming? The data is too noisy to tell.
The bottom line is the last frost date in the spring, the top line is the first frost date in the fall. The region between the two is the growing season, mid-spring, summer, through mid-fall.

tags: frost, gardening, meteorology, Delaware