"If you can't control it, don't plant it."

That's the advice from Backyard Invasion, a new exhibit at the Carnegie Museum of Natural History in Pittsburgh, featuring images of invasive and endangered Pennsylvania plants.

Pittsburgh is a little out of my way this week, but here are some key points that are worth sharing no matter where you garden:

Non-native plants — also known as “exotic” plants — can be classified as “invasive” when they exhibit rapid and aggressive growth. Around here, that would be the Cytisus scoparius (Scotch broom) and Cortaderia sellowiana (pampas grass) that blanket our hillsides each spring and fall, respectively. (I find it particularly ironic that the Scotch broom is in full flower all around the Cow Palace just when the San Francisco Flower & Garden Show is on.)

When they take over a garden, park, or planting area, invasive plants endanger the surrounding ecosystem. Obviously, if a weedy species such as pampas grass is consuming a site's water and nutrients, and producing shade, other plants can't thrive there. But it's not just the plants that are forced out: along with them go propagators such as birds, bees, and butterflies; mycorrhizal fungi, lichens, and other symbiotic microbes; and macro- and micro-nutrients that otherwise would support a diversity of flora. And, nature being nature, there's a whole chain of parasites, predators, relatives and beneficiaries that follow all of these out of the ecosystem.

The invasive plants are likely to encroach upon regions previously occupied by native plants, many of which are rare, if not at risk for extinction. One reason may be because, while native plants have evolved very particularly to the conditions of their locale—e.g. serpentine soils—invasives will grow damn near anywhere. The second fold of this problem is that, because of their specific adaptations, natives have very few places to naturally relocate once displaced. If you're growing native plants in your garden, weed often and thoroughly: your native micro-habitat is vulnerable to the invading hordes.

Able to adapt quickly to new environments and produce seeds in short cycles, invasive plants can be extremely hard to eliminate. Any removal effort is certain to cost money as well as time. Again, a strong case for early detection and treatment (and why it's not so easy to just dispatch throngs of highway workers to machete the Cortaderia and Cytisus into submission).

Because they create such high-maintenance headaches, it's a lot easier to just look the other way when invasive weeds start sprouting. But if they're left unchecked, entropy dictates we will become a planet of weeds. To learn more about the invasives threatening California, check out Encycloweedia, as useful as it is cleverly named. And make sure you, or your garden designer, really knows what's going into your garden—before that cute little "Tree of Heaven" becomes the noxious weed from hell.