Ask the Clark Gardener

Invasion of the Alien Species

Roellyn Armstrong, MG

Lest you think that this article will carry a political message advocating for a southern border wall or serve as a trailer for a new sci-fi docudrama, rest assured that neither will be the case. Rather, I am writing to give some clear thoughts about invasive plant species in our environment and what we can do about them.

While the vast majority of the plants that have become invasive are not indigenous to the continental United States and were introduced either unintentionally or deliberately, there are native plants that can become invasive, if they threaten an area where they did not originate.

First, it is important to define what is meant by invasive. Some of these plants have already been banned from sales, while others are on an official watch list. Both New York State and the USDA classify invasive plants and do-not-sells, with some plants on one list but not on the other. In all such cases, the most restrictive rule applies, meaning either list which has a plant ban governs its sale or distribution, either in the entire state on just on Long Island.

When we talk about native plants, we must realize first that we must define the region or area to which we refer. For plants native to North America may not be native anywhere else in the continental US. And if we are purists, we will want to know which plants (and animals) are native to Long Island. For when we talk about invasive species, even a native plant can be invasive if it inflicts damage outside its native area. To talk about an invasive plant, it is important to also consider plant provenance, that is, the area from which the plant arose naturally. And exotic plants (those not native) may eventually naturalize in their new habitat. For how long must a plant inhabit an area to be considered native. And what about nativars—plants deliberately or unintentionally cultivated from native species?

Invasive plants are called bad exotics whose presence in the local flora has the potential to cause damage. They are generally so successful that they pose a horticultural, as well as an economic risk, not only to the flora and fauna but also to the very diversity of the local ecosystem. The generally accepted characteristics of invasives are: it must be non-native to the area of concern; it must have naturalized in that area; and it must cause measurable damage to the environment, both horticulturally and or economically.

The biological traits that define invasive plants are: the plant is non-native and was introduced either intentionally or unintentionally into the ecosystem; the plant produces abundant fruits and seeds; seed dispersal is effective; there is rapid growth of the plant; and the plant is an aggressive competitor for resources.

The impact of invasive plants on the ecosystem has both ecological as well as economic consequences. In the ecosystem, they are responsible for the loss of biodiversity; they impede forest regeneration; they cause changes in soil dynamics, and they have grave, sometimes irreversible impacts on animal groups. Economically, invasive plants cause damage to farmlands, can present obstacles in water transportation and commerce, adversely affect recreational areas, and costs associated with their removal can be staggering.

So, now that we have some understanding of invasive species, we can separate those plants from the other more vigorous plants in our environment, which may or may not become invasive in the future. In the next installment on this topic, I will offer some alternatives to the many invasive plants we have typically used in our landscapes.

Resist the Alien Species; Alternatives for our Landscapes

Roellyn Armstrong, MG

Last issue we discussed invasive plants and the threat they may pose to their adopted environment. In this issue, we will examine some of the more attractive invasive plants, that although possessing some desirable ornamental attributes, pose a serious enough threat to our native ecosystems to warrant their being placed on watch lists or do-not-sell lists for nurseries. Since many of these plants were intentional introduced by the horticultural industry itself, it is that industry that is working toward identifying attractive and good performing alternatives Most, if not all of these introduced non-natives are garden tough and represent a significant commercial value to the industry. In the trade, along with the search for alternatives that offer most of the same favorable attributes as the plant it might replace, plant breeding is working toward creating sterile versions of these plants. Those plants which are now banned or “watched” include the industry-introduced Celastrus orbiculatis, the attractive Oriental bittersweet, seen wrapping itself around trees along our parkways; Euonymous alatus, burning bush, whose brilliant fall color is on display each year along the Meadowbrook Parkway overpass of the Southern Parkway heading south; Lonicera spp. the sweet smelling vine known as honeysuckle; and the barberry, Berberis thunbergii. For the barberry, which is now banned from sale, a new sterile cultivar ‘Sunjoy Mini Maroon’ has been introduced. There are alternatives to these invasive plants available to the home gardener, and these should be considered when removing the invasives, if that is possible. (And remember, that although these plants are termed invasives, they may not actually behave that way in your particular environment (too dry, too cold, etc.) they are not to be trusted!)

So here are the alternatives for the plants listed:

Low Bushes

Berberis thunbergii – Japanese Barberry

  • Physocarpus opulifolius – Eastern Ninebark, ‘Monlo Diabolo’ prized for its deep purple hue; ‘Summer Wine’. Also Cottinus coggygria (Smokebush) ‘Grace’, Weigela florida ‘Fine Wine’
  • For yellow or gold, try Abeilia grandiflora ‘Golden Glow’, Forsythia intermedia, Lonicera nitida (golden boxleaf honesuckle ‘Baggesen’s Gold’)
  • For green foliage, consider Rhus aromatica ‘Gro-Low’ or Cotoneaster apiculatus.

Euonymous alatus – Burning Bush

  • All these listed have nice fall color, the attribute we covet in the burning bush: Aronia arbutiflora and A. melanocarpa, chokeberry; Itea virginica and Rhus aromatica
  • Ligustrum spp. – the over used privet hedge, which is banned locally Ligustrum alternatives include Ligustrum ovalifolium, the California privet and Myrica pensylvanica, the Northern Bayberry


Lonicera japonica – honeysuckle, Campsis radicans – trumpet vine, and Wisteria floribunda – Japanese wisteria
  • These vines can be replaced by Lonicera sempervirens (trumpet honeysuckle), Wisteria frutescens, Aristolochia macrophylla, Dutchmans Pipe, and Gelsimium sempervirens (Hardy Carolina Jessamine, a US native)


Acer platanoides – the Green Norway Maple, on the do-not-sell list for their ability to form dense stands which alter the understory

  • Alternatives include: Acer x freemanii – Freeman maple which is a cross between the red maple and the silver maple. Try ‘Jeffersred’ or A. x freemanii ‘Armstrong’ which is more fastigiate in form. Also good are Carpinus betulus, an upright evergreen hornbeam or American hornbeam, Carpinus caroliniana. And also Nyssa sylvatica, black gum or tupelo tree.
  • ‘Crimson King’ a beautiful deep red specimen of A. platanoides, is also banned; try Cercis canadensis ‘Purple Rosebud’, ‘Forest Pansy’, ‘Lavender Twist’ or ‘Ruby Falls.’, all with lavender or pink flowers along leafless stems in early spring. And also C. canadensis ‘Alba’ with early spring white flowers.
  • Pyrus calleryana – Callery or Bradford Pear, which is an emerging invasive, subject to trellis rust and limb brakeage as a street tree.
  • Consider instead Chionanthus retusus, the Chinese fringetree with white fringe-like petaled flowers or the native Chionanthus virginicus, Amelanchier, or Crataegus viridis ‘Winter King’, a hawthorn closely related to the apple tree, much like the crabapple.

So, it is easy to see that there are many lovely alternatives to invasive plants that, while attractive, pose a danger to our ecosystem.

Remember, if you have a comment or question, send them to me at And come for a visit!

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