Invaders in the Garden

As a committed National Public Radio junkie, I was delighted to here this story on last Thursday's episode of All Things Considered addressing the role of the horticultural industry in the proliferation of invasive plant species. In this case, the plant in question is Lythrum salicaria (Purple Loosestrife), a species that has been wreaking havoc on wetlands in the northeast United States for decades. Yet it is still readily available in many garden centers. Whats more, dozens of other commonly used landscape plants are either known to be invasive already or are waiting in the wings.

There are two prime modes by which invasive plants come to new ground. They can be introduced accidentally, such as with Microstegium vimineum (Japanese Stiltgrass) which arrived as packing material for shipping containers. More commonly they are brought in purposely for use as landscape or garden plants. There is a very good reason for this, or rather there is a very good reason why plants desirable for landscaping so frequently become invasive. Many of the qualities people seek in landscape plants are the very same that make them so excellent at slipping out into natural areas.

The landscape bed is a tough place to live. The soil is heavily disturbed, often coated with mulch or sometimes (I wish I was making this up) shredded tires. Pesticides are used to suppress undesirable plants. Good landscape stock has to tolerate such stresses, and species are selected and bred such that they can. Further, a garden can be anywhere and subject to any number of conditions. Dry, wet, shady, sunny, disturbed by salt, etc....the better a plant can tolerate such stresses the more it will sell.

Such is the story of Berberis thunbergii (Japanese Barberry), a lowly member of the Berberidaceae with little to define it except for a crop of bright red berries and willingness to be pruned into any shape desired without consequence. The secret to this unassuming plant's popularity, to the point where you are hard pressed to find a corporate office landscape display without one, is the fact that it will grow anywhere. You can put one in a five foot diameter island in the middle of a parking lot and completely cover its roots with bright white gravel (another landscape convention I will never understand) and it will do just fine. This uncanny ability for survival is exactly what has allowed it to flourish in New Jersey woodlands. Rising densities of white-tailed deer slowly depleted native shrub layers, and Japanese Barberry moved in to take its place. The deer would not eat it (yet another excellent quality for both a landscape plant and an invasive) and continued targeting dwindling native plant populations. The end result is acres upon acres of woodland where Japanese Barberry is virtually the only species in the shrub layer.

The same scenario has more or less played out for other invasives. The driving factor may not always be herbivory, but the basic issue is always the same; a fundamental alteration of some natural ecosystem process that presents these 'generalist' introduced species with a competitive advantage. If you consider biological invasions from a Darwinian perspective, the notion does not make complete sense. After all, how can an outsider plant out compete a species adapted to live exactly where it is living; fine tuned by millions of years of evolution to fit perfectly into its niche? The answer is simple, through much of the world humans have changed all the rules.

In New Jersey, nearly all of the forest we have is less than one hundred years old. Prior to that, the land on which is stands had been cleared and committed to agriculture. Whats more, these secondary woodlands did not rise to reforest all of the areas on which trees stood prior to human habitation. Instead the state has become a mosaic of woodlands, farm fields, and the lawns and landscape beds of housing developments. Clearly the conditions in these new woodlands were not the same as the old. Drastic changes in soil chemistry, herbivore communities, and landscape composition shattered the native plant's advantage. This is not the forest to which they are adapted and the generalists, most of which are non-native, have been allowed to run wild.

This gets to the heart of the greatest challenge of modern environmental stewardship; how do we foster whole, functioning ecosystems on a landscape that has been so fundamentally altered? There is no single answer to this question, but one issue most of the ecological community agrees is at the forefront in terms of importance is the on-going spread of invasives. The sale of such plants threatens the welfare of our natural areas and renders the hard work of many land managers a losing battle. Lacking good regulation, the best way we can move the horticultural industry in a more earth friendly direction is through education. There are many excellent resources that will help you identify invasives that may be lurking in your garden or at your local plant supplier. I suggest starting with the The Nature Conservancy's Global Invasive Species Team page. Learn what to watch out for and you'll be on your way to being an ecologically responsible gardener!

Here's a few bad ones for NJ that are "under the radar", so to speak. To the best of my knowledge, all of these can be readily purchased and some I've noted are heavily used.

Miscanthus sinensis (Chinese Silvergrass)- This ridiculous looking plant can be found on every other front lawn from Bergen County to Cape May. Though it is not widely naturalized, where it has stepped out of the garden it has shown a disturbing tolerance to a variety of conditions and habitat types (a great trait from a landscape plant, remember?). If you want to see a particularly bad infestation of this stuff, head for Duke Farms in Hillsborough. In the interest of full disclosure, I work at Duke Farms in their Environmental Stewardship department. So make sure you come quick before we eradicate it all.

Ranunculus ficaria (Lesser Celandine, Fig Buttercup)- A delightful little yellow flower that makes great early season groundcover and is capable of completely (and I mean COMPLETELY) dominating a floodplain herb layer. Its rather bizarre life history; the plant is capable of reproducing by seed, tuber, and bauble; allow it to sweep new areas quickly.

Aralia elata (Japanese Angelica Tree)- Personally I don't know why anyone would use this plant in landscaping, unless you're going for the 'JRR Tolkien thorny forests on the slopes of Mordor' effect. It is truly nasty looking stuff. However, it is becoming a larger threat in the Mid-Atlantic Region.

Rhodotypos scandens (Jetbead)- This shrub has been used in American horticulture for decades, but only recently has been identified as an invasive species. Just goes to show you how difficult identifying the potential for a plant as an invasive can be.

Also, if you feel as good about that NPR piece as I do, head over the comments section of NPR's website and write Michele Norris some fan mail. She did decide to pull her Purple Loosestrife out, after all!