No more frost for C-U? 'It all depends'
There's a better than 50 percent chance that Champaign-Urbana has seen its last freezing temperature of the spring, but it's not a certainty.
Even with a seven-day forecast from the National Weather Service showing temperatures no lower than 39 degrees, it's not a sure thing that a plant-killing frost won't occur in the next couple of weeks.
The latest-in-the-year sub-freezing reading in Champaign-Urbana since 1981 was 29 degrees on May 7, 1989. The latest sub-freezing spring reading ever: 29 on May 25, 1925.
Last year's final freeze — 29 degrees — was on April 20.
But the median date for the last freeze in Champaign-Urbana is April 13.
"Based on historical data, adding two weeks to that date gets you beyond most of the risk for frost," said state climatologist Jim Angel of the Illinois State Water Survey. "The forecast from the (National Weather Service) for the next seven days shows temperatures staying above freezing. But that could change, especially when you go farther out."
To be 90 percent sure that your tender plants won't be harmed by frost, plant on or after April 27, the water survey advises. That's the date when, in 90 percent of the years 1981-2013, the last freeze had already occurred.
Sandra Mason, a horticulture educator with the University of Illinois Extension, urged antsy gardeners to be cautious.
"I had an old professor who always said, 'It all depends.' And it all depends on what you're going to plant," she said. "It's not too early for things like peas, carrots, mustard, the leafy things like lettuce and spinach. But it is too early for things like tomatoes, peppers, cucumbers, sweet potatoes.
"A lot of the leafies tolerate frost just fine, and even things like broccoli, cauliflower, cabbage."
Mason said she's waiting to plant — even with the optimistic weather forecast.
"What I generally do is wait until we get the first of May and then I look at the week's forecast for that week," she said. "Usually, by the time May 10th rolls around, we're in very good shape. So we're just a tad bit early for some things.
"If it's cool, the plants aren't going to grow well anyway, Those warm-season ones aren't going to grow well anyway. But if it looks like it's going to stay warm, be warm and they're willing to cover those plants for a night or two, then they should be OK. It just depends on how much people want to baby them along.
"So see? It all depends."