If there was a synonym for the month of March, it’d have something to do with see-saws and rollercoasters. Winter starts to lose its grip across the country, and at times it may seem spring has arrived for good, but freezing temperatures almost always make a comeback after these warm spells. Most spring blooming shrubs and trees can handle brief drops below freezing and sail through light freezes, but tender summer crops like tomatoes, squash, basil, and so many others will wilt and die at the first signs of frost. The magic number is 32F and a single degree could make the difference between life or death, regardless of how warm the weather was prior. With this blog post, I’ll share a few helpful ways to determine the frost and freeze risk for your home garden so you never have to deal with an unexpected spring frost again.
Average Last Freeze Dates aren’t always useful
It takes just one night below freezing to ruin an entire summer garden, and that’s why the average last freeze date is one of the most important things gardeners pay attention to. Contrary to belief, the average last freeze date is not considered the “safe date” for planting tender plants. The last freeze date is a 30 year average. Even in “normal” years (whatever that means!) the last freeze of the season can fall before or after the average date. The last freeze of winter will vary year after year, and forecast numbers are only accurate for a few days out. Fortunately, weather does follow a “pattern” and that can give an idea of the risk for a future freeze beyond the 7 day forecast.
Dry and warm weather patterns are one of the most risky in early spring for early plants. Long spells of dry weather comes with a lot of sunshine. This can give warm weather plants a great start to the growing season during the daytime, but calm winds and clear skies at night can drop temperatures very quickly. In this scenario garden temperatures may drop dangerously close to that devastating 32-degree Fahrenheit reading. March 2017 was my first spring in North Florida. We had 3 days reach 89F, but the coldest morning of the month was a bone chilling 25F. That temperature came 2 weeks after the average last freeze. The dry weather was the reason for the huge temperature swing – we saw less than an inch of rain for the whole month. Anyone who has spent time in Florida knows rain is the state’s specialty!
March is no calmer at my new home in Columbia, SC where temperatures have historically ranged from a balmy 94F to an unbelievable 4F. While record high temperatures are challenged much more frequently in this warmer world than record cold, anything is possible in spring. The average last freeze date is March 23rd, but 30% of years in Columbia experience at or below freezing temperatures in April. I started tomatoes and eggplant in late March last year because the weather pattern appeared mild into April (and it was!), but I knew there was still a risk.
I suggest never placing tender plants in their garden spots permanently until about 2 weeks after the average last freeze. This way gardeners can keep an eye on temperature trends before committing to planting cold sensitive crops in the ground. Typically the first week of April is a safe time to plant for summer in Columbia, but it’s important to keep the long term forecast in mind because freezing temperatures in mid-April are not unheard of.
What can you plant before the average last freeze?
Gandy’s Garden at WLTX is filled with so many greens this time of the year. It’s easy to start fast maturing greens like spinach, lettuce, arugula, radishes, turnips, and snow peas from seed now. These make fantastic potted plants. They’re beautiful, tasty, and hardy! Broccoli, Cauliflower, Kale, brussels sprouts, parsley, and cilantro can be planted nearly assoon as the ground thaws out. They easily survive 25F temperatures without damage, but will not grow until afternoon temperatures are regularly in the 50sF. Most of these plants overwintered for me here in South Carolina, but my mom in New York City just sent me a picture of her parsley that survived the entire winter outside in New York City. It was buried under 2 feet of snow at one point!
Spring in the southeast can quickly become too hot for cool loving greens. I consider March to be too late to plant broccoli, cauliflower, kale, brussels sprouts, parsley, and cilantro from seeds in South Carolina because there’s only about 8-10 weeks left before our temperatures regularly reach the 90sF. window. Once warm weather arrives in May, it will force these greens into bloom and they’ll lose their flavors. These longer lived cold loving greens can be started from seedlings in the southeast in March and reach full maturity before the blazing heat of summer arrives.