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March 2, 2011 / Dave Gorham

One Thing Gardeners Want to Know… Has It STOPPED FREEZING YET??

When will the last North American frost of 2011 occur (or of the 2010-2011 winter season)? Well that’s certainly a question on every gardener’s mind these days, especially with spring just around the corner! I’m sure you’re probably wondering whether or not the last frost has already occurred in your area or if another surge of cold air will dive southward. Yesterday, temperatures across parts of Montana were 40ºF below average, while afternoon highs in parts of North Dakota were as much as 25ºF below normal. Here in the Houston area (and across parts of the Southeast) over the past several days we’ve been enjoying the spring-like weather with highs in the 70’s. I could definitely get use to weather like this! This is also the type of weather that gets gardeners fired up about planting their crops; however, the key is know when to plant. If you plant too early (despite how it may feel right now) and your area gets an unexpected late frost, your crops could be damaged.

Map of composite temperatures anomalies for February-March-April. Image: CPC

From the map (shown above) there are three possibilities that can be considered. The first is that there will be a better than normal chance of an early spring with an early end of the frost season from the Southern Rockies and Texas eastward across the middle portion of the Deep South to the Ohio Valley and Mid-Atlantic states. There will also be a higher than normal chance of a late spring and delayed growing season from the Pacific Northwest, eastward across the Northern Rockies, Northern Plains and upper Mississippi Valley. Lastly, odds are that a normal arrival of the frost-free season will arrive from the Central Plains to the mid Mississippi Valley and over a large part of California and the Southeast U.S. this year.

Average dates for the last spring frost across the U.S. Image: Better Homes and Gardens

Notice parts of the Northeast, Great Lakes, Midwest, Plains and West typically don’t see their last frost until May; whereas those of us living along the Gulf Coast see our last frost between January 30 and February 28. However, we all know this can vary year to year and you can’t just rely on trends or average weather conditions. This is where gardeners can make a mistake. It’s best to research which crops will flourish in your environment and may be hearty enough to withstand an unexpected late frost. Cool season crops, such as cabbage, broccoli, potatoes, and lettuce can tolerate a light frost. However, warm season crops like squash, corn, tomatoes, and cucumbers will be killed by frost. It’s best to plant warm season crops after the last frost date for your area.

Cool season crops. Photo: PRIAPI GARDENS

Tomatoes are warm season crops. Photo: Sunset

Even though it’s been feeling like spring here in Houston, StormWatch Manager Fred Schmude thinks this pattern could change in March. “For the Houston area I am concerned that we will see another significant frost or may even a light freeze sometime in between the 5th and 20th of March…long range data continue to hint at several more thrusts of colder air blasting south across the Lower 48. In addition note that the source region of cold air over western Canada remains well below normal (see below). Only a slight change in the flow pattern could bring down a significant piece of colder air right down to the Gulf Coast region. I would say the risk of another frost or freeze sometime during this time period is in the 20-30% range, with the best chance of course farther away from the coast. Typically in Houston we are done by the 3rd week of March, but you cannot totally rule out a light frost or freeze threat up to the first week of April. It happened back in the early 1970s, but fortunately this year we don’t have the same type of flow pattern.  For this year I would say here in the Houston area we are done by the 21st of March…I would be very surprised if a frost or light freeze occurred after that time period.”

Well-below average temperatures remain over western Canada. Image: NOAA

For more information on your specific state and city’s average last frost dates, visit the National Climatic Data Center.

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