When I think of invasive plant species, I think of kudzu, that leafy green vine that has been known to cover houses and cars in a matter of days.

I generally don't think of those lovely, white pear trees that cover the hillsides like snow drifts at this time of year in Maryland as invasive.

The Callery pear originated in China and was brought to this country in the early 1900s as possible root stock for cultivated pears.

Soon, it was widely used to landscape residential developments and public areas because of its graceful beauty.

It has a dark gray bark against which its white spring blooms look striking. When blooming is over, it has shiny green leaves. In the fall, those leaves progress from red to gold. And it has a compact, pyramid shape that is attractive. And, at first, it produced no seeds or fruit.

By 1982, it was the second most popular tree in this country.

However, cross pollination with other varieties of Callery pear trees inevitably occurred, and the tree began to produce small, olive-brown fruits that do not mature into pears.

It is those fruits that are the problem.

Birds love them, eat them, and deposit them all over the place, and Callery pears, pears spring up, driving out less robust native plant species. That's why the hillsides look almost white this time of year. That is waaaayyyy too many pear trees.

"It is not the Bradford pear in and of itself," said Kerrie Kyde, invasive plant ecologist for the Natural Heritage Program at the Department of Natural Resources.

She was using the name commonly associated with these pear trees. "It is the Callery pears as a group. Bradford pears are self-incompatible. You can have just one. But as soon as we started producing all kinds of different cultivars -- there might be 50 of them -- you got a hybrid, and that is what is proliferating.

"The trees completely encircle the Washington beltway," she said.

And I can testify to its profusion on Interstate 97 and U.S. 50.

"It is knocking out space for all the things that would be there, like redbuds and dogwoods, and you can see how incredibly spready it is."

As prolific as they are, they are not strong and do not live long. They can grow as high as 60 feet but the tree often splits, breaks up or comes out by its roots during storms, causing damage in neighborhoods and along highways.

"That state highway administration is trying to control them," Kyde said.