Outsmarting Invasives

By The National Garden Bureau
From the April 2024 Issue

Did the horror of kudzu removal introduce you to the term “invasive plant?” Or perhaps Japanese privet and its wide-ranging spread enticed you to rethink what shrubs you specify for privacy hedge. Or maybe you’ve seen the rampant reproduction of Bradford pears, those malodorous, weak, easily damaged trees, popping up in open fields.

After all you know about the horrors of invasive plants, does it lead to a certain trepidation— or at least a rabbit hole of research — when planning a landscape design? How do you balance a client’s desires for certain plant features with ecological awareness? What do you do when a client’s favorite plant is also an invasive species?

First, take a breath. Somewhere on the spectrum between the ecological ideal of all native plantings and the environmental nightmare of harmful invasives lies an area of compromise. While you’re right to avoid specifying invasive plants, it’s also important to understand exactly what an invasive plant is — and more importantly, whether or not it’s invasive in your area. It’s also important to understand the distinctions among invasive, aggressive, native, and sterile plants.

Select non-invasive versions of your favorite perennials, such as this Buddleia Lilac Cascade from Walters Gardens. (Photo: Courtesy of the National Garden Bureau)

What Defines “Invasive?”

Not all plants labeled invasive invade all USDA zones. Some plants are worse than others, with widespread bullying tendencies. But plants that may be considered invasive in USDA zone 10 may never spread in chilly zone 4. But what, exactly, defines an invasive plant?

An invasive plant is an aggressive, non-native species whose presence causes environmental or economic harm. Invasive plants grow and reproduce rapidly. Approximately 42% of threatened or endangered species are at risk due to non-native, invasive species — which include not only plants but also animals and other living organisms, such as microbes. Invasive plants compete with native species for water, light, nutrients, and space. They can out-compete native plant communities and degrade wildlife habitats. Many states ban invasive plants due to the harm they cause to local ecosystems. You can find which plants to avoid in your region or state by visiting the USDA National Invasive Species Information Center.

It’s worth noting that non-native plants are not synonymous with invasive plants. Many of the foods we love, like tomatoes, peppers, and lettuce, originated in other countries and many widely-adored ornamental plants grow beautifully in gardens without invading local ecosystems. If you have any concerns about what’s growing in your garden or what you intend to plant, check with your local university extension agent.

Invasive Versus Aggressive Plants

Along with some of the most publicized invasive plants, other widely used ornamentals may spread aggressively, if not invasively, depending on your region. Aggressive plants include species that, usually because of human intervention, spread rapidly and can outcompete other plant species. Aggressive plants can be native or non-native, and they may be aggressive in some climates — but not others. For instance, in southern climates, elderberry, Virginia sweetspire, fleabane, violets, and trumpet creeper grow aggressively, even though they’re North American native plants.

A sterile butterfly bush can be found in the Buddleia Chrysalis™ Series from Darwin Perennials, shown here in Blue. (Photo: Courtesy of the National Garden Bureau)

Native & Non-Native Plants

Native plants occur within a region as the result of natural processes and are adapted to local climate and soils. If the species was present before widespread European settlement, it’s considered a native plant.

These plants co-evolved with native insects and wildlife, making them critical to ecosystems. While some may consider them troublesome weeds, many of these aggressive native plants help prevent erosion along riverbanks, provide nectar for pollinators, serve as a food source for caterpillars, and feed birds, both with their seeds and by attracting insects beloved by our feathered friends. While aggressive native plants may irritate those who prefer tidy, perfectly manicured beds and borders, these plants provide benefits that many non-native invasive species don’t.

On the other hand, non-native English ivy, mahonia, nandina, lantana, buddleia, spirea, Japanese barberry, Asian beautyberry, and privet, among others, are listed as invasive species in some parts of North America. Yet don’t be surprised if you find many of these plants in a client’s yard. Many landscape designers and home builders included these non-natives because they grow quickly, bloom profusely, and look good, only to discover their wayward ways much later.

How To Outsmart Invasive & Aggressive Plants

If you discover aggressive or invasive plants in a client’s landscape, your options depend on several factors. If your state lists a plant as banned, you should remove it. And while some native plant advocates would urge you to remove all non-native invasives, it isn’t always realistic given the plant’s purpose (a mature privacy screen), the client’s attitude, or the lack of budget to remove and replace. In these cases, consider a little TLC to keep the plants in check. Practices include:

Deadhead flowers after blooming to prevent seeds from spreading.
Mulch areas around the plants to reduce the number of seeds that germinate, as they need direct contact with soil to grow.
Remove seedlings early when they’re easier to pull up.
Prune suckering plants to contain their growth so they don’t outcompete other plants for light and nutrients.
Dig and remove offshoots or runners to keep the plant’s growth in check.
Replace invasive plants with sterile cultivars.

Another color in the Cascade Collection: Violet.

Not Your Parent’s Bradford Pear

Many National Garden Bureau (NGB) plant providers offer new, sterile cultivars of plant species deemed invasive or aggressive. Sterile is defined as producing no or few seeds but nectar production is not negatively impacted. This allows you to enjoy the flowers and foliage in your garden — without fear of decimating your local ecosystem.

However, many know of the Bradford Pear conundrum. In the past, while “sterile” cultivars may not have been self-fertile, like the Bradford pear, some still cross-pollinated with wild species. For instance, Bradford pears easily hybridized with other Pyrus calleryana cultivars, as well as P. betulifolia, also known as Asian pears, producing fertile fruit — and filling fields with invasive seedling forests despite supposed “sterility.”

Today, NGB members’ cultivars undergo rigorous testing and trials. New breeding efforts ensure that sterile cultivars don’t cross-pollinate and spread in order to protect native ecosystems. These stringent requirements have resulted in inter-specific hybrids that don’t face the same restrictions in states where an invasive species is banned. Hybrids can be sold in these states if they’ve been trialed and shown to produce less than two percent viable seed, often referred to as “low seed set.”

New Cultivars

Buddleia, commonly known as butterfly bush, is beloved for its quick growth, beautiful arching limbs of purple or white flowers, and its prolific display of butterflies and hummingbirds that come to flit among the flowers. Unfortunately, the invasive nature of buddleia has made many shy away from the popular shrub in recent years. Yet today, clients can enjoy buddleia for butterflies without invasion — just choose a sterile, but still beautiful and beneficial, variety. Here are a few series to look for:

Buddleia Cascade Collection: ‘Lilac Cascade,’ ‘Violet Cascade,’ and possibly ‘Pink Cascade II.’ For areas where buddleia tops the invasive plants’ list, the varieties above provide safe cultivars to incorporate into a landscape design: after four years of trials, they produced no observed seed set. These large plants with graceful, cascading flowers make an ideal back-of-the-bed addition. (Lilac and Violet shown above. Zones 5-10).

Buddleia Chrysalis™ Series. Ideal for patio containers, window boxes, and even hanging baskets, this compact buddleia produces a mounding habit — as well as a very low seed set. Available in White, Cranberry, Blue (shown above), Pink and Purple. (Zones 5a-9a).

Buddleia isn’t the only sterile cultivar of well-loved — but sometimes invasive — plants. Nandina, an attractive heat- and drought-tolerant evergreen shrub known for its pretty foliage, spreads invasively in southern climates due to its prolific fruit. The fruit contains cyanide compounds, making it toxic to wildlife — especially for cedar waxwings, who ingest large quantities of it. While vigilant gardeners can prune off the fruit to avoid its spread, instead consider the colorful, sterile new cultivars available like Nandina Cool Glow®. This compact nandina showed little to no seed set during its four-year trial, but produces gorgeous Winter color. The plant is available in Lime, Peach, or Pomegranate to brighten the Winter garden. The dense, upright shrub adapts to many soil types and is ideal for foundation or mass planting. (Zones 6 – 9).

Along with buddleia and nandina, both Chinese and Japanese privet finds itself on many states’ invasive species lists. Fortunately, a sterile species provides the same benefits beloved by landscapers without the invasive tendencies. Privet Kindly™ is the first non- invasive Ligustrum japonicum. With the features that made the privet a favorite of landscapers, the rounded, upright habit, glossy evergreen foliage, and white fragrant flowers look lovely in the garden — but exhibit no female fertility. The classic hedge is well- behaved and seedless, producing lightly fragrant flowers in the Spring to Summer. Plus, like conventional privet, this new cultivar is deer resistant, as well as drought and heat tolerant. (Zone 7).

Choose a sterile variety of a popular landscape shrub if it is an invasive in your area. Here are some options. TOP: Nandina Cool Glow® MIDDLE: Privet Kindly™ BOTTOM: Spirea Double Play® Doozie. (Photos: Courtesy of the National Garden Bureau)

With its lovely blooms, pretty Japanese spirea also finds itself on several invasive plant lists— and even spread its marauding ways into the Great Smoky Mountain National Park. Consider a  if you want to incorporate spirea’s beautiful blooms into a landscape, such as Spirea Double Play® Doozie. It’s a classic landscape addition with a twist: this spirea produces no seeds, so you can plant it without fear of it invading other areas! Not only is it sterile, it produces gorgeous early Spring red foliage, followed by bright, purple-red flowers in late Spring…and all Summer long. Because it doesn’t produce seeds, the plant puts all its energy into flowering, meaning it’s a non-stop show all season. Spirea Double Play® Doozie is also low-maintenance. It’s drought tolerant, deer resistant, and needs no deadheading to reflower. (Zones 3-8).

Invasive species wreak havoc in some regions, so always check what plants are considered invasive in your zone and state — and avoid specifying those in a landscape design. If the invasive is a client favorite, suggest a comparable non-invasive native alternative or look for new, thoroughly tested sterile cultivars of the species.

The National Garden Bureau is a non-profit organization that exists to educate, inspire, and motivate people to increase the use of garden seeds, plants, and products in homes, gardens, and workplaces by being the marketing arm of the gardening industry. Our members are experts in the field of horticulture and our information comes directly from these sources.

Do you have a comment? Share your thoughts in the Comments section below, or send an e-mail to the Editor at cmenapace@groupc.com.