The cool shade of a big oak is a blessing for any yard. If you're looking for landscape plants to put under it, don't forget the biggest thing out there.

"If you've got a 30-inch tree, you're not going to replace it in one person's lifetime," said Kim Coder, a forester with the University of Georgia Extension Service.

"You can replace that azalea every year for 30 years," he said. "But if you mess up the tree, you're not going to get it back."

For that reason, tilling the soil under an old oak tree is "out of the question," Coder said.

"People will go in under a big oak and till up the place for azaleas or flower beds," he said. "And then they'll wonder why the tree starts to decline. Tilling under an oak destroys an awful lot of roots."

Tilling may be the worst of the things that "go desperately wrong," Coder said, when traditional Southern landscaping and big oaks come together.

Other things that can be just as bad, he said, are those that happen close to the trunk.

"People like to put raised beds or big planting boxes around the base of the tree," he said. "That often opens the door for root rot and other pathogens to get into the tree. It tends to suffocate the roots under the bed or planter, too."

The most common mistake is simply overplanting.

"A typical Southern landscape may have a few tall pines here and there, some oaks to provide the main canopy, a midstory of dogwoods, an understory of flowering shrubs and some bulbs and flower beds," Coder said. "And there's probably some ivy running up some trees.

"But most active roots are very shallow," he said. "We may have 60 feet of height in which to arrange plant parts above the ground. But in the ground, we've only got a foot to 18 inches of depth to stuff everything into."

The key, he said, is "the ecology of the system." Simply put, whatever is in your yard has to divvy up the water, nutrients and sunlight. And there's only so much of that stuff to go around.

"You can water and fertilize the place and increase its carrying capacity," Coder said. "But it still has a limit. If we try to grow too much out there, the whole landscape will end up looking pretty sorry."

That doesn't mean you can't plant anything under big oaks.

"Lots of things will grow under oak trees and survive and thrive," he said. "You just want something that's not invasive or aggressive -- something that will be polite and just sit there."

If you want grass, extension turf specialist Gil Landry said the best choices are St. Augustine in south Georgia and tall fescue in north Georgia. The second-best choice is zoysia statewide.

But don't fertilize it as much under the tree, he said. And don't mow it as short as you normally would. If two inches is the recommended height, cut it three inches under the tree.

There may be better choices than turf. Extension horticulturist Mel Garber suggests planting a ground cover such as Ophiopogon japonicus or periwinkle (Vinca minor).

If you want more striking color, he said, try impatiens, which thrive in low, diffuse light. They do need more care, though.

You may want to mulch the area with pine straw, then plant a few flowering shrubs and add container plants for summer color.

"Just don't treat the tree like a big umbrella," Coder said. "It's a living thing, with needs of its own. And it's the most valuable thing out there."