Most people shopping for a tree probably look closer at the top 90 percent of a candidate than the bottom 10 percent, the roots.

EDWARD GILMAN/University of Florida

This tree's roots are girdled and rootbound, making it a poor choice when shopping for a tree.

That's a mistake, says Matt Grubisich, a regional urban forester with the Texas Forest Service. "A tree is only as good as its root system."

Now is the ideal time to transplant trees, because they are dormant. But how do you distinguish between a good tree and one that is likely to have problems after it is added to your property? It is most important to pull a tree under consideration out of its container or pot, or unwrap the burlap, to look at the roots. If the roots are an impenetrable mass on the outside of the rootball, the tree has been kept in its container too long. While you can cut into the dense mat and disentangle some roots, you're better off looking for a better specimen.

If the main roots circle around and around, thickly overlapping and interfering with one another, especially at the base of the tree trunk, virtually choking it, don't buy it. While the roots can be cut away, the growth of the tree is delayed and the long-term health of the tree may be damaged.

Mr. Grubisich offers tips for buying a good tree and avoiding the bad:

•Check for a solid, moist root ball that doesn't fall apart.

•A low-profile root ball (wide and shallow) does best in our area.

•Roots should be no larger around than a person's pinkie.

•Look at the form of the tree: It should taper from top to bottom.

•Look for a tree with a central leader (strong trunk). Trees that fork low on the trunk are weaker and need corrective pruning later.

•A tree should have good form. Branches should be evenly spaced around the tree, like a good Christmas tree, with no holes in the crown.

•Branches should be pliable.

•Look for evidence of insect infestation. For example, little bumps on branches could be scale.

•If leaves are present, make sure there is no wilting or burned appearance around the edges.

•Little branches on the lower trunk are not a bad thing. Most nurseries trim them off, but if a tree comes with them, leave them for a couple of years. It shows the tree is growing properly and getting a good shape.

•If a tree has been pruned, make sure wood has started to form over the cut, which means the tree is repairing itself. Also, check the quality of old pruning cuts. The cuts should be "O" shaped rather than oval-shaped, which indicates a bad cut.