The last frost date is one of the most important tools you should have in your gardening bag of tricks. That's the date in the spring when it's "safe" to plant with less worry about a late spring frost, or "light freeze". Most state Extension offices or websites can give you the date for your area. In Colorado, you can find dates at Colorado State University's online site. You can also find regional charts online that give general frost information like the one at "The Old Farmer's Almanac" or through climate statistics at NOAA.
The last frost date is an important date because most seed packets and gardening guides reference it. It also gives you a guideline for starting outdoor gardening activities.
The reason I put "safe" within the protective caveat of quotation marks above, is that the last frost date is the date of the average last spring frost; this provides a 50 percent chance that there won't be freezing temperatures past that point. If you're a "glass is half empty" kind of gardener, that also means that there is a 50 percent chance that there will be freezing temperatures past that date. Depending on how much you like to gamble with your plants, you can plant seedlings before that date or you can wait until there is no chance of a freeze.
I prefer to use the 10 percent date. That means there's only a 10 percent probability that a frost will hit after that date (or 90 percent probability that it won't). Most publications and Extension information will list both the 50 percent date and the 10 percent date. For expensive or very tender plants, I prefer a better probability of success and often wait for the zero percent date. All of these are determined by historical climate data.
According to Colorado State University Extension, in Colorado Springs the average last frost is May 6 and the 10 percent date is May 18 (May 4 and May 15, respectively, according to NOAA statistical data). That's a difference of almost two weeks by both measures. To me that two weeks is worth the wait. I've seen snow fall during the second week of May; that's not good for warm season plants. For tomatoes and peppers I, and many other local gardeners, usually wait until Memorial Day, the last Monday in May, to plant. That gives me a virtual guarantee of warm weather. There has never been a freeze past June 2nd, only a few days after Memorial Day.
Your last frost date will be different. In many parts of California it will be in February, in Louisiana it might be March, and in Illinois and most of the United States it will probably be some time in April. If you don't know your specific 50 percent and 10 percent dates, check with your Extension office or ask experienced, local gardeners when they plant and why.
On the other end of the calendar is the first frost date. That's the average date of the first frost in the fall. As in the spring, you can find the 50 percent date and the 10 percent date in publications and online. The length of your growing season is determined by knowing these two dates. Here our first frost date is October 6, the point with a 50 percent probability of frost before and after that date. September 18 is the 10 percent date, when there's only a 10 percent chance of a frost before that point. Using the average dates, we have a frost-free growing season of 152 days. Using the 10 percent dates we have a season of 126 days, a difference of nearly a month.
That's the number you need to know before you plant vegetable seeds or transplants. If the length of time on the seed packet or plant tag is less than your frost-free growing season then the plant should have enough time to fruit before a frost. The greater the difference, the longer the plants will produce for you. My 75-day tomatoes should provide ripe fruit for more than a month before the danger of frost, even using the 10 percent dates. If I plant 90-day tomatoes, I may only have a few weeks of fruit. For that reason, I try to choose plants with small numbers on the packet. I won't even try beans or melons that have 120 days to harvest.
The last frost date comes into play when you grow from seeds. Packets might say "start indoors 6 weeks before Last Frost"; that usually means start the seeds in indoor pots six weeks before the 50 percent last frost date. For direct sowing outside, packets might say "Plant after all danger of frost has passed"; for this I would use the 10 percent date.
Every locale is different and microclimates will affect growing conditions too. Know your last and first frost dates as determined by your state Extension or NOAA. Use those as a guideline in determining the actual dates in your own garden. Try keeping a gardening journal and track temperatures in your backyard. If your "official" last frost is May 6, but you notice year after year that a frost hits after that date, adjust your planting schedule accordingly.
I know there are many numbers here and it can become a little confusing. Once you know your specific dates, use them. I'm comfortable with my Memorial Day planting because I've never lost a plant to frost after that date. I am willing to fudge that a little and plant earlier if I see a weather forecast with sustained warm temperatures leading up to that day. On the other end I plan on the first frost hitting about October 1. In my garden I plan for a frost-free growing season of a little more than 120 days.
It's all about matching the plant's required growing season with the season available in your garden. With a little research you can be more successful in matching the two.
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