The Invasive Plant Issue

The Invasive Plant Issue

Euonymus alatus compactus in 5Rivers Metroparks near Dayton, Ohio

The invasive plant issue is not going away any time soon. While some industry professionals are sensitive to this horticultural product liability issue, others are hiding under a burning bush, hoping it will all blow over. Massachusetts and some other states have already banned a plethora of nursery production plants, including burning bush, Japanese barberry and Norway maple. Other states, like Connecticut, use the same assessment protocol, which is endorsed by the American Landscape and Nursery Association (ANLA), and put many of the same plants on a list. These states then work with the nursery industry to determine what to do with the plants.


Ohio is the most recent state to develop an assessment protocol and related procedures to evaluate invasive plants. The Ohio Invasive Plants Council spent four years on the project, working with other stakeholder groups such as the Ohio Department of Natural Resources, The Nature Conservancy and Ohio Nursery and Landscape Association (ONLA), to create a 22-question, science-based tool to generate a new official list of invasive plants. Many of the same nursery production plants as in Massachusetts and Connecticut may end up on the list over the next couple years.

The Ohio process will concurrently address nursery cultivars and a separate list of alternative plants, which will include native plants, as well as non-native, non-invasive plants. The five-person assessment team responsible for addressing these issues includes two horticultural researchers designated by ONLA. Potential regulation of invasive plants, however, remains to be addressed in Ohio.

What Invasive Means

When it comes to defining what an invasive plant is, there tends to be confusion. Some growers may ask, “Don’t we all want plants that are somewhat aggressive and tenacious? Isn’t that what we refer to as garden performance?”

Think of it this way: We’ve all seen weeds that proliferate in our nurseries and greenhouses. They form monocultures in our fields, containers and non-crop areas, and we spend big-bucks preventing or removing them. We’ve also seen pests and pathogens like Asian longhorned beetle, emerald ash borer and boxwood blight invade our region with huge potential impacts. Likewise, invasive plants have the potential of taking over natural areas like parks, forests and preserves. These are not to be confused with the “garden thugs” that disrupt our cultivated areas. There is a difference between weediness and invasiveness.  
In order to be considered invasive, a plant must:

1. Jump over some form of spatial gap to escape from cultivation or initial infestation
2. Invade a natural area
3. Reproduce in the natural area
4. Expand its foothold over time.

The bar is set fairly high, which is why most non-native horticultural introductions are considered non-invasive.  

Why Invasive Is Problematic

Why should we care if a plant takes over a natural area? The easy solution is to pull them or simply let them go. Isn’t this part of a grand process called nature? Isn’t the movement of invasive plants around the globe part of a natural process? We just happen to accelerate it a little with our collecting and breeding. It may even seem as if we’re making our world better.

The answer lies in the importance of biodiversity to our local ecologies. When a plant takes over a natural area and creates a monoculture, it tends to sterilize the environment. Natural systems that have evolved over millennia will no longer function. Research shows these new arrivals take a minimum of 500 years — often much longer — to evolve into hard-working ecological citizens. Eventually these natural systems stop working, and the constructive interactions between plants, insects, birds, beneficial fungi, nutrients and countless other factors are affected.

For example, according to Douglas Tallamy’s Taking Nature Home, non-native bush honeysuckles are rapidly becoming The Shrubs That Ate Southern Ohio. This occurred in less than fifty years — the ecological blink of an eye. Common reed (Phragmites australis) has accomplished the same feat in Northern Ohio.

Common reed was not introduced by the nursery industry, but other plants, such as Asian bittersweet, demonstrate a similar potential. Research shows non-native plants take an average of 125 years to demonstrate invasive potential, but Japanese honeysuckle remained in-bounds only for about 80 years before leaping from cultivated areas into the woods. All of these plants have had a tremendous negative impact on the natural ecosystem they invaded.

Stopping The Spread Of Invasives

We call ourselves the green industry. As conscientious stewards, shouldn’t we guard the doorway between cultivated and natural areas? The Voluntary Codes of Conduct for Nursery Professionals adopted by ANLA, ONLA and others in 2002 resolved to “ensure that invasive potential is assessed prior to introducing and marketing plant species new to North America [and] phase out existing stocks of invasive species.”

Ten years later, there is still a lot of work to do. Why not evaluate invasive potential of perennials and shrubs in connection with our nationwide plant trials? When clear and convincing evidence exists on plants in the trade, such as Lythrum virgatum ‘Morden’s Pink’ and ‘Morden’s Gleam,’ why not voluntarily remove them from production? With so many great under-used plants on our palette, we can avoid over-reliance on so few. Non-invasive alternatives to common nursery plants represent a profitable value-added product. Now, think about how can we encourage our research partners and plant breeders to develop them faster.