The Invasive Problem, Choosing Alternatives – Part 2

Our personal choices sometimes have consequences beyond the confines of our private gardens. While most ornamental plants, such as catmint, iris, and hydrangea, are noninvasive, some species jump from gardens to natural areas and do too well there, causing long-term ecological consequences. (See “Part One, Understanding the Issue,” in the January 2008 issue) Invasive species alter the structure and function of ecosystems and displace native plants. When we design a landscape, aesthetics often rule our choices, but it is equally important to consider each plant’s potential to invade.

Sarah Reichard, exotic-species expert at the University of Washington, estimates that only four percent of ornamental plants are invasive—a fraction of the garden plants introduced to North America since European settlement. (Invasive and aggressive are not interchangeable concepts. Aggressive plants are garden thugs that spread rapidly in enriched garden beds, but do not necessarily escape the confines of the garden.) That said, it seems that nearly 65 percent of invasive plants nationwide were introduced for ornament. In short, while the total number of invasive ornamental plants is low, most invasive species were intentionally introduced to this continent.

Choosing substitutes for invasive plants is simple. First, consider what makes any plant “ornamental.” Why do we choose linden viburnum over maple-leaf viburnum? Albizia over Amelanchier, or vice versa? Most people are drawn to flowers, often a plant’s most conspicuous and colorful attribute. Foliage is enduring and compelling, the most consistent character during the growing season. Berries and dried seedpods appeal as autumn and winter interest, and bark, whether rough and furrowed or smooth and glossy, figures prominently when we choose trees and shrubs. Woody plants lend shape and pattern through their architecture. Plants also fulfill roles outside of aesthetics in our private and public landscapes. They mark boundaries and define garden spaces. They serve as safety barriers and windbreaks, control erosion, build soil, and help repair damage caused by environmental degradation. Beauty and utility, in varying degrees, influence all our decisions.

Choosing Alternatives

If your garden design includes an invasive plant, you can find a pleasing alternative. Determine the main feature for which the plant was chosen in the first place, and look for an alternative with a similar trait. Whether you’re looking at flower color, season of bloom, overall shape and size, or foliage color and texture, it is often easy to find a suitable substitute—one that falls in line with more than one of these details. While it is not always possible to match every attribute, the compromise is usually minimal. Consider care, as well. Many invasive plants came to be widely used because they are tough and serviceable. If a proposed alternative needs coddling to succeed, it is unrealistic to think it will serve as a viable substitute.

When looking for alternatives to invasive exotics, the plants native to this continent are a logical place to start. However, just as not every invasive plant is truly invasive in all parts of the country, or in all ecosystems within a region, not all native plants are suitable for all areas. Locally native plants make especially good alternatives because they are often as easy and dependable as their invasive counterparts, being naturally adapted to the local climate and conditions. Native plants from outside of your location can work; just be sure to think about the plant’s native range and the specific site conditions of its natural habitat. It is not enough to say a plant is native to Virginia. Look at the actual place in which it is found, be it forest, field, or wetland, as well as its favored or tolerated exposure and its requirements for soil texture, pH, and moisture content. If these match the specifics of your garden, the plant will likely thrive.

Here is an example of how I choose a suite of substitutes. Purple loosestrife (Lythrum virgatum/salicaria) is a popular ornamental that is widely invasive. It bears rosy pink to purple flowers in summer, in spikes three to five feet tall. As substitutions, I chose six widely different species that give the overall look of loosestrife. All are native to moist, open areas. The closest match is fireweed (Chamerion (Epilobium) angustifolium). Its rose pink flowers are larger and showier than those of purple loosestrife, and they are borne in terminal spikes three to five feet tall. Fireweed blooms for a shorter period of time, but overall it is an excellent substitution. Prairie blazing star (Liatris pycnostachya) is another good choice, with pink-purple summer flower spikes three to five feet tall. Plants with similar flower color but different flower form include swamp milkweed (Asclepias incarnata), sweet Joe-Pye weed (Eupatorium purpureum), and queen-of-the-rairie (Filipendula rubra). Anise hyssop (Agastache foeniculum) has the form of loosestrife, but its spiky four-foot inflorescences are blue to purple, rather than pinkish. The final decision depends on whether flower color or flower form is most important to the design.

Safe and satisfying alternatives exist for all the invasive plants commonly grown as ornamentals, but the responsibility for making these choices in your garden rests on your shoulders. Seeing the multiple attributes of landscape plants makes finding other options easier. Train your eye to notice and value the plants around you for all their traits, and you’ll come to make logical substitutions.

Opting out of invasive ornamentals

1. Plants with showy flowers

A number of invasive species, particularly herbaceous plants, contribute colorful flowers,

and to a lesser extent, interesting foliage or decorative seed heads to the designed landscape.

Invasive Plant - Native Alternatives

Wisteria floribunda - Wisteria frutescens, W. macrostachys

Hesperis matronalis - Phlox carolina, P. paniculata

Ranunculus ficaria - Caltha palustris, Chrysogonum virginianum, Senecio aureus, Zizia aptera

2. Plants with decorative or colorful foliage

Both woody and herbaceous plants are used for foliar effect in the landscape. Foliage may be valued in summer, as are the curious handlike leaves of five-finger akebia, or in autumn, as are burning bush and many viburnums.

Invasive Plant - Native Alternatives

Acer ginnala - Acer spicatum, Carpinus caroliniana, Cornus alternifolia, Prunus virginiana

Berberis thunbergii - Fothergilla gardenii, Myrica pensylvanica, Ceanothus americanus, Ilex verticillata

Ligustrum japonicum - Ilex decidua, I. glabra

3. Plants with decorative fruit

Exotic shrubs with decorative fruits are often invasive. While most of the offenders below have fleshy fruits dispersed by birds, some, such as Amur maple, are wind dispersed.

Invasive Plant - Native Alternatives

Ampelopsis brevipedunculata - Ampelopsis arborea, Parthenocissus quinquefolia

Celastrus orbiculatus - Celastrus scandens

Viburnum dilatatum - Callicarpa americana, Ilex verticillata, Viburnum acerifolium, V. nudum, V. trilobum

4. Plants as problem solvers

These plants are seldom used in ornamental setting. Instead, they are employed to remedy landscape problems,

such as erosion, high winds, or and lack of privacy.

Invasive Plant - Native Alternatives

Rhamnus cathartica - Aronia melanocarpa, Crataegus species, Rhamnus caroliniana

Coronilla varia - Arctostaphylos uva-ursi, Apocynum androsaemifolium, Asclepias verticillata, Pteridium aquilinum

Burrell provides two lists of additional plants to be wary of.