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Expert Gardeners Work To Make Plants Thrive In Summer Heat

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An orchid sits in the shade of a fig tree.
Sabri Ben-Achour
An orchid sits in the shade of a fig tree.

So if the dashboard of a car sitting in the sun can reach temperatures of 200 degrees Fahrenheit — that's how hot it gets in there, according to the National Oceanic Atmospheric Administration — how do greenhouses manage to maintain the moderate growing climates plants love so much?

As it turns out -- surprise, surprise -- the greenhouses are actually a lot more meticulously calibrated than your average hatchback. Jim Kaufmann who helps run the nation's green house at the U.S. Botanic Garden in Southwest D.C. explains how in the garden's orchid house. 

Kauffman points up at the leafy canopy above his head.

"That's natural cooling at its best," he says. "When the temperatures recently have been spiking into the hundreds, this house has been the least of our concerns." That's because the fig canopy takes up the entire greenhouse, shading the sensitive, prized orchid collection below. 

But sometimes even the Botanic Garden needs extra help. In the larger rooms, vented glass ceilings allow hot air to escape, naturally drawing up cooler air from underground ducts below.  

Water, purified through reverse osmosis, is sprayed out through misters. Shade cloths block the worst of the sun's heat, and a computer, programmed by season, keeps everything regimented.  

But what's an Average Joe gardener to do?

But most people don't have those things. So what's the average gardener supposed to do to keep plants alive during the sweltering D.C. summer? The simple answer, watering, is not necessarily a good idea, says Kauffman. 

"Because sometimes some plants get so hot they just stop taking up the water, they just shut down," he says. "Just like anybody else, it gets so hot they're not working anymore."

The problem is hot soil, and when hot soil is wet and it can end up drowning the plant.  

"You get the heat coming off your house, heat coming off of the street, that's a lot of heat just cooking  the soil," he says. The only way to know if those plants are boiling, he says, is to feel the soil. 

"You reach in right around the roots and you just take a fistful," he says. "If you can take that soil and squeeze it in your hand and it retains that shape of your fist you're at a good soil moisture and you probably have a good soil texture in there."

But if the water drains out when the soil is squeezed, there's too much water.  "And if it just crumbles apart you're probably too dry," he says. 

One way to keep the soil cool is to use mulch -- 2 or 3 inches of it -- but again, he says, there is such a thing as too much of a good thing. 

"The typical thing I see a lot of around here is what I call the mulch volcanos where folks will take shredded mulch and build it up a foot or more above the base of the tree. You never want to do that," he says. Mulch is made up of bark, and what rots mulch, could rot the bark on your tree. 

Native plants, especially heat-lovers, are key

But fundamentally, when it comes to beating the heat, maybe the best strategy is to not try so hard.  That means picking plants wisely. Outside the greenhouse, Kaufmann uses a lot of native plants: long leaf pine, Echinacea, sedums, portulaca and ornamental grasses.

"The echinaceas range, you'll see them in purples, whites, green. That's a really good all around perennial, a real easy one to grow," he says. In addition, thyme is a really tough plant and a good bet, as are rosemary and a lot of other herbs, he adds.

For annuals, there are some really resilient ones. Lantanas and coleus are two examples. 

 "They go from oranges to yellows, multi colors," he says of the lantanas. "The coleus ... there's a variety of texture leaves, like duck's web feet to black leaves."

It's impossible to beat Mother Nature, says Kaufmann, so it's better to figure out ways to work with her.  So the best way to beat the heat is to use plants that love it.

[Music: "Heat of the Moment" by DJ Manian from Barracuda Summer 2005 Vol. 2]

Photos: Plant Survival

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