In J.R.R. Tolkein's "The Two Towers," two hobbits get lost in an ancient forest and discover beings called Ents, who have lived in the woods so long, caring for trees, that they have almost turned into trees themselves. Ultimately, the Ents go to war to defend the forest.

That's the kind of long-term relationship you have with a big shade tree. Plant an oak today, and, with the best of care, it will take two or three generations to reach its full majestic height of 50 feet or more. By that time, the grandchildren probably won't be able to imagine the place without it.Fall is a good time to plant trees. But planting a shade tree is a big commitment, and it's nothing to rush into. Here are some things to think about before you reach for a shovel:

Will it fit? It's hard to visualize just how tall and wide a full-grown tree can get. And that can lead to buyer's remorse if the tree dies because it was planted in the wrong place. "You don't want to drop a large shade tree into a confined space where it's going to run into concrete and blacktop," says Dennis Rohr, grower for Hinsdale Nurseries in Yorkville.

Before buying, find out the mature size of each tree cultivar you are considering, get out the tape measure and plot out the tree's spread in your yard. Remember that the root zone of a mature tree extends twice as far as its branches, says Jim DeHorn, organizer and recruiter for the TreeKeepers program, which trains volunteers to care for trees in the city.

If your site is small, you may have to choose a tree with a more vertical habit instead of a wide-spreading one, says Ed Hoffman, director of production at Clarence Davids Co., a tree-care firm and nursery in Matteson. And on a tight, shady urban lot, you may be able to plant only smaller understory trees such as American hornbeam, DeHorn said.

You can see a mature example of most species of big trees that grow around here at The Morton Arboretum in Lisle . That will give you a chance to absorb the grown tree's full height, breadth and shape. To pinpoint the location of a tree at the arboretum, see the the Web site catalog at redwood.mortonarb.org.

What might the tree run into? Look around for conflicts before you commit to a site. If planted near a sidewalk, will the tree start to push it up in 10 or 20 years? Will it invade sewer pipes? Interfere with power or telephone lines? Well in advance of digging, check for buried utility lines by calling JULIE (the Joint Utility Locating Information for Excavators) at 800-892-0123.

How will it change the yard? The shade of a tree will alter your landscape, Hoffman points out, so you will have to choose different garden plants. Some trees, such as many maples, cast shade so dense no grass will grow under them. Honey locust will allow some light to filter through, Hoffman says.

How much work will it be? Deciduous trees will drop leaves on the lawn and in the gutters; an evergreen will drop needles and cones. Some trees drop fruits or seeds. In general, faster-growing trees such as silver maples have weaker wood, lose more branches and require more maintenance than slower-growing kinds such as oaks, says Jim Skiera, associate executive director of the International Society of Arboriculture in Champaign.

What tree will do well in your yard? To choose a species and cultivar that has a good chance of success, have your soil tested. If you have very heavy clay soil, consider species that are native to flood plains, such as silver maples, ashes, disease-resistant cultivars of elms and swamp white oak, says Kris Bachtell, director of collections and ground at the arborteum.

Consider your neighbors' trees as well as your own, says Bachtell. "You want a mixed population," he says, not a monoculture, with the same kind of trees, all susceptible to the same disease or pest.

Start small. Choose a young tree, with a trunk diameter of 1 1/2 or 2 inches and a root ball light enough to lift. "It's always easier and less expensive to start with a small tree," Bachtell says.

Don't plant too deep. Dig a wide hole and plant the tree so the curve of roots to trunk is just above the soil. Water the tree regularly for its first few years, but don't drown it, Bachtell says.

Most important, make sure you like the look of the tree you plant.

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How to find the right tree

Here are some good sources for information on choosing shade trees.

The Morton Arboretum in Lisle has three good brochures: "Large Deciduous Trees for the Home Landscape," "Large Evergeen Trees for the Home Landscape" and "Native Trees of the Midwest for the Home Landscape." Each lists the height, shape and cultural requirements for several dozen cultivars. A helpful booklet is "Selecting and Planting Trees." The brochures and booklet are $2 each and are available at the arboretum gift store, Illinois Highway 53 at Interstate Highway 88, Lisle. Or order by calling 630-719-2454; postage will be an additional charge.

The Chicago Botanic Garden in Glencoe lists many tree varieties on its Illinois' Best Plants Web site, bestplants.chicago-botanic.org

Chicagoland Grows, a cooperative venture of the arboretum, the botanic garden and the Ornamental Growers Association of Northern Illinois, recommends tree and shrub cultivars that have been shown to do well here. For a list, see chicagolandgrows.org.

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More trees to consider

Windy City white ash: A Chicagoland Grows selection, this tree grows 50 to 80 feet high and 50 to 70 feet wide with an upright to rounded shape. Fall color can be either purple or a combination of red and yellow.

Accolade elm: Another Chicagoland Grows selection, this is a disease-resistant alternative to the American elm. Reaches 40 to 60 feet high and 35 to 40 feet wide, with an upright to vase-shaped habit. Fall color is yellow. Starts out lanky but becomes graceful as it ages.

`Marmo' maple: Another Chicagoland Grows selection, this is a tall, upright, fast-growing tree that reaches 45 to 70 feet high and 35 to 40 feet wide. Fall color is red, maroon and green.

Thornless honey locust: A broadly vase-shaped tree that can grow 40 to 70 feet high and wide, with small leaves that turn yellow in fall. Casts light shade.

ebotts@tribune.com