What can a tree do for your overworked air conditioner?

The shade of a tree can cool you on a hot day — and it can do the same for your air conditioner.

The cooling system won’t have to work as hard if shady trees keep your home from heating up as much in the first place, according to Beth Corrigan, Community Trees Program specialist at The Morton Arboretum in Lisle. That can save you money.

If you have central air conditioning, trees and shrubs are a good way to shade the compressor — the big, blocky, sometimes noisy piece of machinery outside your house.

A compressor that sits out in the hot sun has to cool itself as well as the air it draws from inside the house. A shaded compressor stays cooler, so it will work more efficiently at a lower cost and have a longer life. And when you’re sitting out on the patio, green plants are a more pleasant sight than a big metal box.

That doesn’t mean you should surround your compressor with a thicket of shrubs. Make sure it’s easy to reach the unit for regular service.

It’s also essential not to interfere with the airflow around the compressor. The unit disposes of heat by blowing out hot air. If that flow is blocked, the system won’t work well.

The plants need some space to avoid being harmed by the hot air. “If they’re too close, they’ll be scorched,” Corrigan said.

To avoid clogging the compressor’s machinery, choose a tree or shrub that doesn’t drop a lot of seeds or pods. The Plant Clinic at the Arboretum can help you select an appropriate plant.

A nice, big tree that can shade both the house and the compressor is ideal, if you’re lucky enough to have one. A smaller tree, such as a redbud or kousa dogwood, can provide some shade too. Even a large shrub, planted so it blocks sun from the east or south, can make a difference in the compressor’s performance. In a pinch, Corrigan said, you might try a tall, upright ornamental grass, such as Karl Foerster feather reed grass (Calamagrostis acutiflora “Karl Foerster”) or Northwind switchgrass (Panicum virgatum “Northwind”). Grasses can tolerate the hot air.

“You’ll need to keep trees or shrubs pruned, so they don’t grow branches or suckers too close to the unit,” Corrigan said. It’s always worth it to have large trees checked regularly by a professional and pruned if they need it. Regular maintenance will prolong trees’ lives, and the cool shade and savings they provide.

“Shade is one of the biggest benefits we get from trees,” Corrigan said. Both by shading the air conditioner and by shading the house, “a large, mature shade tree can provide significant energy savings.”

The larger the tree, the more valuable shade it provides — one reason that big, old trees are so worth preserving.