The Downside of Early Summer? One Cold Snap Could Make the Leaves Drop

Horticulture experts say that while trees will survive below-freezing temps — as will plants and grass — it could cause them to lose their leaves and force them to "re-leaf."

If you’ve been loving these 70- and 80-degree days, you can bet your trees and grass and flowers are loving them more.

They don’t think it’s spring, they think it’s June, said Russ Higgins, a DeKalb-based commercial agriculture educator for the University of Illinois Extension.

So, it’s going to be a mighty rude awakening if we head back to what we normally expect at this time of year – temperatures that sometimes hover in the mid- to high-40s during the day and can flirt with low 30s and high 20s at night, Higgins said. In fact, just two or three days of below-freezing temperatures overnight could be enough to make the the blooms to fall from the flowering magnolia and crabapple trees and for those baby green leaves to drop like it’s the start of fall, he said.

“It’s not going to kill the trees,” Higgins said. “But they’ll have to ‘re-leaf,’ and that will take 10 percent of the tree’s internal reserves. … When it uses up the reserves, it leaves the tree in a weakened state.”

Greg Stack, a University of Illinois Extension horticulture educator based in Lemont, said healthy trees will recover and you likely won’t see much of a difference once the second leaves emerge and the summer progresses, he said. But for trees that are older and perhaps already susceptible to illnesses, it could certainly speed their decline or demise, he said.

What can you do? Absolutely nothing, both men say.

Extra water and fertilizer won’t help, nor will doing things to provide some sort of covering or protection during cold snaps, they said.

“You can’t control it. You can’t do anything about it,” Higgins said.

With everything ahead a schedule by a minimum of four weeks, grass and perennial plants have also come out of dormancy and are taking full advantage of the heat blast, Stack said.

Fortunately, they will be able to withstand a cold snap with fewer problems, he said. You might see some brown spotting, he said, but they won’t die.

If you want to be safe, you can cover the ground around the plant with straw or mulch, which should be pushed back during the day so that water can reach the roots, Stack said. You can also consider covering them with some sort cloth at night, but stay away from plastic coverings, which cause humidity to build up around the plant, he said.

Interestingly, the up-and-down weather should not necessarily affect the insect population, which has also emerged far earlier than normal, Higgins said. While the common thinking might be that a cold snap will wipe out a certain number of bugs, in fact insects are very good at finding ways to protect themselves during spurts of inclement weather, he said.

That said, you needn’t fear a bumper crop of flying and crawling pests as a result of the abnormally warm temps, Higgins said. Their natural predators have also emerged early and will keep the population down in the same way they do in the summer, he said.

However, Higgins added that he, too, has been surprised by what he’s been observing this spring.

“I’ve been amazed at the diversity of insects I’ve already seen,” he said. “It’s realistic to say the one thing we’ll see are those insects that troublesome (Japanese beetles, June bugs) emerging sooner than normal.”  

One thing people should do is resist the urge to plant, Higgins and Stack said. Just because it feels like May or June, doesn’t mean it is, and you’ll be wasting money and time if you succumb to the desire to start putting out impatiens or petunias or tomato plants, they said.

Higgins said he’s heard that some farmers – who should know better – are planting crops far earlier that they should. One cold period, and they will lose their crops and find themselves replanting the same fields weeks later, assuming they can afford to buy a second round of seed, Higgins said.

If you really feel the need to put your hands in the dirt, plant seeds that germinate in the house and can be put outside on temperate days, they said.

The heat wave peaked in  with an 85-degree high Wednesday. Want to know how that compares with previous years? According to www.wunderground.com's historical date for the 60439 Zip code:

March 21, 2012: 85 high, 67 low

March 21, 2011: 60 high, 41 low

March 21, 2010: 44 high, 32 low

March 21, 2009: 62 high, 37 low

March 21, 2008: 35 high, 30 low

March 21, 2007: 71 high, 37 low

March 21, 2006: 33 high, 26 low


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