(MONEY Magazine) - If you've spent any time at all reading about real estate here or elsewhere, you probably know that attractively landscaping your yard can be one of the most cost-effective ways to boost your home's curb appeal.

But you can plant all the geraniums you want -- at the end of the day, trees are what will set the tone. And no aspect of your yard is trickier to get right, or potentially more expensive.

As a practical matter, the payoff for planting a tree -- prices range from $10 for a sapling to $10,000 or more for a mature specimen -- is difficult to quantify. In the eyes of an appraiser, trees typically don't add value to your home the way a kitchen or bath renovation might.

While diseased trees may cost you at the closing table, realtors don't put a price tag on every dogwood and elm when calculating what your house is worth. Still, the overall effect can be critical to selling your place, not to mention enjoying it while you live there.

Follow these four steps when you shop.

Shade trees are the blue-chip stocks of your yard -- the long-term core holdings that you build around. The most valuable shade trees are the ones that take the longest to grow, like stately oaks and elms.

But who has the time to wait 50 years for a pin oak to achieve its regal 100 feet of height, or the $10,000 it takes to buy a decent-size specimen?

What you want is a hardwood tree that is reliable and disease-resistant and grows fast enough (one to three feet a year) that it can be enjoyed by you, not just by your grandkids. How big a tree you start with depends on your time frame.

If you have little time to watch your prize grow -- say you plan to move in a year -- Washington, D.C. landscape architect Sunny Scully suggests spending about $1,000 for a tree with a four-inch caliper (that's trunk diameter). That's big enough to make a statement, but not so big that you'll need a second mortgage to pay for it.

Keep in mind that nurseries typically remove most of the roots from tall trees (more than six feet, with a caliper of an inch or more). Those trees will spend as long as three years throwing down new roots rather than growing up.

If you have more time, save a few hundred with a sapling. Among the pros' favorites are oaks (chinquapin, willow and scrub), birches (Heritage River) and red maples.

Wandering around a nursery, you can easily be taken in by good looks, especially when they come cheap. Lots of apparent bargains are anything but.

You may find yourself drawn to the Bradford pear tree's oval shape and lovely blooms, but Texas landscape architect W. Gary Smith warns that one ice storm can bring down the branches.

Silver and Norway maples have shallow, aggressive roots that can damage the lawn or sidewalk, notes Dean Hill, a landscape architect in Indianapolis. While maples are valued for their beautiful fall foliage, their leaves are so dense that it's tough to grow grass beneath them.

Remember Dutch elm disease, the nasty fungus that in the '60s and '70s destroyed a tree favored for its graceful boughs and tolerance of air pollution? The way to protect the health of your garden -- as well as your wallet -- is to have a diverse population of trees.

Once you've picked your shade trees, sprinkle in ornamentals, screens and evergreens. Among ornamentals, hot picks today include fringe trees (their white springtime blooms look like fringe), the redbud, the serviceberry and even the old-fashioned crab apple (the fruitless variety).

Good choices for screens (who wants to see the neighbor's pool?) are the dense Amur maple, the Willowwood viburnum and the fast-growing Leyland cypress. For something different, Scully suggests a Japanese cedar, also called cryptomeria, which is charmingly puffy.

Just as you can overspend on your house by adding steam rooms or cupolas that buyers won't pay for, so too can you overdo landscaping.

Frank Gregoire, a St. Petersburg appraiser, recalls a homeowner whose ranch failed to attract buyers even though it was across the street from the beach. The $50,000 worth of trees and shrubs planted around the backyard pool made the property feel like the set of Dark Shadows. "A number of buyers thought the landscaping made it dark and gloomy," says Gregoire.

Before you buy, picture what your trees will look like in adulthood. Oaks reach 80 to 100 feet, while ornamental trees can hit 25 feet. Plant shade trees 20 feet from the house; ornamentals can be closer.

Give all your trees space to do well, and they'll do well by your home.

Gerri Willis is host of CNN's Open House. Write her at real_assets@moneymail.com.