Melissa Burdick is the curator of herbaceous plants at the Norfolk Botanic Garden.

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Invasives in the Trade
by Melissa Burdick       #Invasives

Plant exploration has been an alluring and exciting facet of the horticultural world for millennia. Centuries ago, exotic plants moved along the Silk Road between Europe and Asia. During the age of sailing, individuals paid a king’s ransom for rare specimens for their glass houses and royal estates. During the Victorian era, the up-and-coming wealthy class paid fortunes to botanists to collect and deliver the newest, most fantastic plants as a symbol of their rising social status. Even today, the zeal for plant exploration is strong, with adventurers traveling farther and higher into unexplored nooks and crannies of the planet for the sheer purpose of discovering beautiful or bizarre new specimen. 

English ivy (Hedera helix, seen at left) is a so common in gardens in warm zones that it’s hard to imagine a world without it. However, it can easily climb a 50-foot tree and weigh its branches down so heavily that the tree will fall over in a mild wind. Choose native options, such as Virginia creeper (Parthinocissus quinquefolia, seen at right), instead.1

But the allure of exotic plants from around the world is not without its dark side. Besides the risk of collecting and exporting these plants, some are so rare that they are considered considered endangered in countries that have few resources to protect their environments from such pillaging. There is also the risk of introducing something with the potential to wreak botanical havoc in their new environments. 

You can’t pick up a garden magazine these days without finding at least one article about the merits of gardening with natives. This fervor for native plants is, in part, a backlash against the trouble that gardening with exotics has caused. But it’s not quite fair to make the blanket statement “exotics are bad” without knowing just exactly where that sentiment comes from.

Japanese wisteria (Wisteria sinensis) is one of the most beautiful garden vines you’ll ever see, but it will easily escape the garden setting and can smother natural woodlands in warm regions.2

Most exotic plants commonly used in the gardening trade are lovely, innocuous plants that only add to a garden’s appeal. However, there are “bad apples” that have ruined the reputation of non-native plants beyond repair. Most of us have heard the story of kudzu, an Asian vine that was introduced in the South to stabilize soil on slopes. The vine took to its job so readily that it quickly began taking over. With no natural predators or diseases to keep kudzu’s growth in check, the great Land of Dixie is now smothered under a heavy veil of green. Sure, kudzu was imported as an environmental aid and not as an ornamental, but did you know the same thing is happening with wisteria? Japanese and Chinese wisteria (Wisteria sinensis and Wisteria floribunda) are quintessential garden vines with lush foliage and euphorically fragrant, pendulous, purple flowers. Unfortunately, in the warmer zones (7b and up), these garden vines have escaped cultivation and started taking over natural woodlands. 

But if it’s good in a garden, why isn’t it okay for natural areas? Because plants that can be kept tidy in the home garden will have no one to trim them or pull their weed seedlings on a regular basis out in the woods. Birds, wind and rain transport seeds or rootable pieces of the plant from our gardens into natural areas, where they readily establish themselves. There are no native animals or diseases to keep the interloping plant in check either. Their growth quickly outpaces that of endemic species and crowds them out. The wildlife that used to feed on the foliage and fruit of those native species goes hungry and either dies or evacuates the area. In turn, the wildlife that preyed on those animals goes hungry as well. Eventually, the whole forest or natural refuge becomes a monocultural “dead zone” with little chance of recuperating. 

Beach vitex (Vitex rotundifolia) is a delightful shrubby relative of the chaste tree (Vitex agnus-castus), but unlike its tree-form relative, the beach vitex can devastate natural ocean or bayfront ecosystems.3

I’ve painted a dire picture of the potential future of American wildlands. With all this in mind, you might be wondering, “If these plants are so awful, why is it still legal to sell them?” Well, here’s where the matter gets tricky. Not all plants are invasive in all areas. Wisteria isn’t hardy in more northern climates and so it won’t survive if seeded out into the wild. So it’s a perfectly fine and desirable plant in more than half the country. Another favorite garden plant, beach vitex (Vitex rotundifolia), is a charming garden addition, but it has recently been shown to utterly destroy native beach grass populations along southern coastlines. Because it’s only a problem in regions near beaches, it’s still legal to buy and sell the plant.


‘Bradford’ pear (Pyrus calleryana ‘Bradford’) has been a trusty shade tree in landscapes for several decades, but in the last few years, seedlings from bird-transported seeds are nosing out native tree species.4
 

Burning bush (Euonymus alatus) is a classic shrub whose vivid red fall foliage makes it a particular favorite for landscapes, but it has been found replacing native understory plants in woodlands throughout the Northeast and Midwestern US  at alarming rates.5

Bamboo (Bamboo spp.) was often used as a fast and thick privacy hedge, but it rightly earned itself a horrible reputation for invasiveness — it could practically be the poster child for the movement! A little research will show that there are types of garden bamboo that are clumping, rather than running, that are much safer for garden planting.6

Mimosa (Albizia julibrissin) is an unmistakable small tree with hot pink powder-puff flowers in early summer. While they are a delight to look at, any gardener in the South will tell you that they seed like crazy through the garden. They’ll seed just as badly into wild areas as well, only there are no gardeners to weed them out. Thank goodness they are short-lived plants.7

Chinese privet (Ligustrum sinense) is a fast-growing, low-maintenance hedge shrub, but its easy garden requirements translate into a plant that grows completely rampant in the wild.8

To an extent, it falls on regional governments to monitor and regulate the movement of plants with invasiveness issues in their districts. As always, regional government’s resources are infinitely stretched, making this an incredibly difficult task and limiting it to only the most dangerous species in the most sensitive ecosystems. Homeowners along natural watersheds or woodland preserves may be required to have landscape changes approved by a government official on a county-by-county (or even city-by-city) basis. Some states, such as California and Florida, have such issues with invasive plants that they’ve instituted a complete ban on their importation, and anyone caught importing those plants faces serious legal trouble.

For the most part, though, the responsibility of regulating the planting of exotic species lies with the gardener. Each state or county should have lists with different classifications of plants with invasive potential. The worst culprits will be labeled as “Highly Invasive” and should be avoided at all costs. After these bad guys comes the “Moderately Invasive” plants that have proven to be invasive in some areas but aren’t running rampant yet. Then comes plants with “Low Invasive” potential, which doesn’t mean that they aren’t invasive — it just means that they’ve been found to seed themselves into natural areas here and there and could possibly become a problem in the future. 

As if having three different sets of lists, region by region and state by state, isn’t confusing enough, these official lists are usually difficult to find because they’re buried deep in government websites. They may be found under the parks and recreation department, natural conservation department, agricultural department… or who knows where. A home gardener’s better bet is to make contact with, or visit the websites of local native plant societies. While these native plant enthusiasts will work hard to win you over to the native-plant-only mentality, they will also provide you with a regionally specific “hot list” of plants to avoid. 

This heavy dependence on self-monitoring by the home gardener is part of what makes the prognosis so bleak and disheartens so many of those whose job or passion it is to protect our native wildlife and ecosystems. Those of you who have read this article are the rare exception in the world of home gardening. What it’s going to take is a movement of almost evangelical proportions to spread the word that just because something is being sold in the local garden center or big box store, it doesn’t mean it’s okay for your yard. “Research before you buy” is the essential key to making educated decisions about what to plant and what to avoid. Arm yourself with the knowledge of what plants can become problems in your region, and allow yourself to feel liberated in your ability to choose plants in addition to native species that will beautify your garden.

1. Photo ©istockphoto.com/mcfields (left). Photo courtesy of Margaret Gratz(right).
2. Photo by Jillian Pond (top). Photo by James R. Allison, Georgia Department of Natural Resources, Bugwood.org (bottom).
3. Photo by Randy Westbrooks, U.S. Geological Survey, Bugwood.org (left). Photo by D. Eickhoff, www.flickr.com/photos/50823119@N08/ (right).
4. Photo by Shane Darby.
5. Photo courtesy of Karen Ott Mayer.
6. Photo ©istockphoto.com/tadamee.
7. Photo by James H. Miller, USDA Forest Service, Bugwood.org.
8. Photo courtesy of Janet B. Carso.

From State-by-State Gardening November/December 2012.

 

Posted: 10/24/12   RSS | Print

 

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