Determining the Last Spring Frost Date.

Poverty, Frost, Famine, Rain, Disease, are the beadles and guardsmen that hold us to Common Sense.
~Ralph Waldo Emerson

I have started seeds for all my lettuces and leafy greens, radishes, onions, leeks, herbs (chives, dill, oregano, basils, thyme, cilantro, lavender), sweet and hot peppers, peas, broccoli, celery and some of my tomatoes indoors under a grow light. Normally some of these plants I'd seed right outdoors like the radishes, onions, and peas but I am waiting for masons to finish up work in my garden area, so I am conducting an experiment to see if I can have any success very gently transplanting these plants once I can get into my garden. Fingers crossed.

Starting seeds indoors is a great way to get a head start on the growing season, shortening the wait to harvest, and also lengthening the growing season. But when you start seeds indoors, you add the additional step of transplanting. When to transplant often coincides with the last Spring frost date, which is an indicator of when you can directly sow seed outdoors for plants like, beans, corn, peppers, tomatoes and members of the curcubitaceae family (melons, gourds and curcubits—cucumbers, pumpkins and squashes).

So how do you determine the last Spring frost date for your area? Last Spring, Cheryl Long of one of my favorite magazine's Mother Earth News, wrote an excellent article on the subject called, Know When to Plant What: Find Your Average Last Spring Frost Date. As she suggested in the article, you can reference the U.S. Department of Commerce's National Climatic Data Center's Freeze/Frost Data, and enter your State to download a PDF file that breakdowns the threshold temperatures and probability of frost for your local area. Although the article suggests otherwise, the 90% probability column is actually the earliest and riskiest dates to plant, as there is a 90% probability that there will be a frost after those dates. If you need to get your plants in the ground at these early dates, you can minimize the likelihood of frost damage by using cold frames, cloches and Reemay cloth. The 50% probability columns mean that there is 50/50 chance of frost after those dates. And the 10% probability column means that there is only a 10% chance of frost after those dates, and therefore on or after those dates is the safest time to seed or plant outdoors.