Invasion of the bawdy snatchers

The biggest plant growing in most gardens is invisible, though its roots underlie every inch of soil and its boughs overshade every other bean and blossom, boxwood and butternut. Being invisible, some gardeners don’t even realize it’s there; nonetheless, it is present — and ignored at considerable peril.

Have you guessed it yet?

I’m talking about the evergreen/flowering/fruiting decision tree, that endlessly branching product of dreams, dollars, time and sweat — fertilized, hopefully, with a bit of common sense — that makes a garden what it is and determines what it will become.

Some treat this ubiquitous tree as a hedge, clipping it back to predetermined dimensions every year, planting the same annuals, and tending the same perennials through season after season. (“If it ain’t broke, don’t fix it.”) At the opposite extreme are the inveterate experimenters — constantly trying new annual varieties, moving old perennials around or pitching them in the compost pile, laying out new beds or putting down pretty brick walks where posies once bloomed. (“Life is change.”) Most of us fall somewhere in between.

Of course, any decision a gardener makes can have positive or negative results (the year I was cultivating 80 4-foot-tall tomato plants and lost them all to wilt in the space of four days comes readily to mind). Still, there is a class of choices best described by Mark Antony in Shakespeare’s immortal drama: “The evil that gardeners do lives after them; The good is oft rototilled with their bone meal.”

I am speaking of the introduction of invasive plants — those non-native, often-beautiful, ever-bawdy species that escape from gardens, out-compete local plants and gradually derail native ecosystems. There are more than 70 species deemed to be a severe or significant threat by experts working with the North Carolina Native Plant Society — a nonprofit working to preserve local flora.

The Society labels the following 23 species “Rank 1,” meaning “the worst of the worst.” If you have the merest shred of green in your conscience you won’t even think about planting: alligator weed, aquarium watermoss, Asian bittersweet, Asian spiderwort, autumn olive, beach vitex, Chinese privet, Chinese wisteria, common reed, English ivy, fragrant honeysuckle, garlic mustard, hydrilla, Japanese honeysuckle, Japanese knotweed, Japanese stilt grass, kudzu, mimosa, multiflora rose, parrotfeather, princess tree, Russian olive and the fatally misnamed tree-of-heaven.

The “Rank 2” plants, though less scary, can nonetheless create a weed problem and may crowd out the natives. And since it makes sense to do all we can to preserve our extant ecosystems, we should also avoid: air potato, Amur bush honeysuckle, arrow bamboo, bicolor lespedeza, shrubby bush clover, bigleaf periwinkle, Bradford pear, burning bush, Chinese silver grass, common chickweed, common cocklebur, common periwinkle, common privet, coralberry, creeping water primrose, crown vetch, Eurasian water milfoil, exotic bamboo, Gill-over-the-ground, ground ivy, hairy jointgrass, hardy orange, henbit, hybrid bush honeysuckle, ivyleaf speedwell, Japanese barberry, Japanese climbing fern, Japanese privet, Japanese spirea, Japanese wisteria, Johnson grass, lady’s thumb, leatherleaf clematis, Makino jetbead, Morrow’s bush honeysuckle, nandina, Oregon grape, Oriental false hawk’s beard, Oriental lady’s thumb, paper mulberry, poison hemlock, purple loosestrife, spotted knapweed, tropical soda apple, water hyacinth, white mulberry, wineberry and winter creeper.

Confession time: I am guilty of Rank 1 misdemeanors on three counts. Fortunately the mimosa died (because it was overshadowed by mature trees), but the English ivy and knotweed are spreading. The ivy was planted to stabilize a bank, but it doesn’t know when to quit. The knotweed was planted as an easy edible (the shoots are a lemony substitute for asparagus) and gorgeous fall ornamental. Happily it’s in a bad location and isn’t prospering as it might.

Now that I know better, I will try to remove them — but that is far, far easier said than done.

Take the ivy (please!). There are no known natural controls for this emphatic plant. In its native haunts (Europe, western Asia, northern Africa), it was presumably held in check by something (everything that has evolved is held in check by something in its native locale — it’s nature’s way). But whatever that something was, it didn’t get on the boat with the Brit who toted the ivy here. To make matters worse, nurseries sold the stuff as an ornamental — and still do.

The glossy-leafed vine overshadows everything in its path, quickly out-competing other ground dwellers. Then it takes to the trees and kills them, either by blocking the sun or through sheer weight (the stems can grow to 1 foot in thickness, which amounts to some serious weight). A tree burdened with masses of English ivy is also more likely to go down in a storm.

Every inch of the stem grows roots, and any piece of a stem will re-grow if conditions are favorable. Experts advise cutting the vines and pulling them out of trees to whatever extent this is possible. When the ivy leaves wither, a tree’s foliage will recover. Then all of the vine on the ground must be pulled up by the roots and prevented from re-rooting. No composting unless the plant material is completely dried and dead. It can even be a good idea to burn it or send it to the landfill in plastic bags — both choices that stick in an organic gardener’s craw.

The same is true for Japanese knotweed. All parts of the bamboolike plant can grow — making it a sorcerer’s apprentice sort of deal, in which chopping the plant into little pieces just makes more. Worse still, much of the plant consists of subterranean rhizomes that can travel more than 20 feet before sending up shoots. Once established, it’s all but impossible to remove without resorting to chemical warfare. The best that can be hoped for is to beat it into near submission and keep pulling up sprouts year after year for the rest of forever.

A weird sidelight on knotweed is that it is dioecious (that is, there are male and female plants). It has been determined that all of the plants in the United Kingdom are female offshoots of a single individual. A substantial portion of the plants in both Europe and the United States are offshoots of that same plant, and given that it has spread into millions of acres, there is little question that it is the largest living individual of any species on the planet. Earth mother, indeed.

The enormity of Ms. Knotweed is the reason I used the word “most” in my opening sentence. She is arguably bigger than any one gardener’s decision tree, the cumulative result of bad decisions made by countless gardeners over the course of centuries.

For more information on invasive weeds, visit the North Carolina Native Plant Society Web site (www.ncwildflower.org/invasives/invasives.htm) or the Southeast Exotic Pest Plant Council (www.invasive.org).