Invasive Plants Can Ruin Your Garden—Here’s How to Avoid Them, According to an Expert

As gardeners, we want our plants to thrive. However, some plants succeed a little too well, to the detriment of other plants in our gardens and beyond. When such plants escape cultivation and negatively impact the wider ecosystem, they're classified by government agencies as “invasive.”

Invasive plants endanger native species, both plants as well as the insects and birds that depend on them. They also decrease biodiversity and degrade agricultural soil quality. According to the United States Department of Agriculture (USDA), invasive species have contributed to the decline of 42% of U.S. endangered and threatened species.

Here's what every gardener should know about invasive plants, how they become problems, and what you can do to keep them out of your yard.

What Are Invasive Plants?

The term "invasive" is often used as a synonym for aggressive growth, like when you ask the nursery staff, “Is this mint plant invasive?” But in legal or statutory terms, invasive means that the plant is a significant, fast-moving threat.

The USDA defines an invasive species (including plants and pests) as a non-native species to the ecosystem in question whose “introduction is likely to cause economic or environmental harm or harm to human health” per Executive Order 13112. Some local agencies use the alternate term “noxious” or “noxious weed” in the same way.

Invasive species are different from weeds in a home garden. A weed is generally any plant that is growing where it isn’t wanted. Some gardeners may consider a milkweed popping up in a flower bed a weed, but it is a native species and the host plant of the monarch butterfly, so it is by no means an invasive plant.

Invasive Species Lists

To help keep track of invasive species, the USDA maintains federal, regional, and state lists. For plants specifically, “When we’re looking at what goes on the list, we do a risk assessment,” explains Anne Le Brun, National Policy Manager of the Noxious Weeds Program at the USDA.

“We look at different aspects like its ability to establish and spread,” she says. “In growth habit, does it form dense thickets or is it able to climb like a vine? We also look at the reproductive potential of the plant species. If it has a shorter generation time or more ways to reproduce, that gives it the potential to take over an area faster.”

Once the Secretary of Agriculture’s office declares a plant invasive on the federal level, it can restrict its sale or transport across state or federal borders through permits and regulations. It works with state forest and agriculture agencies to enforce and manage control plans.

How Invasive Plants Get Introduced

Sometimes, seeds of invasive plants quietly hitchhike over from other countries in shipping containers, or on the shoes, clothing, and luggage of travelers. The rest of the time, cuttings or seeds are intentionally brought into the country or moved from state to state by individuals before the plants are known to be invasive.

A major part of the puzzle is that plants, as well as natural systems, don't always behave predictably. Plants that aren't a problem in one place can quickly grow out of control in another area without the natural competition or environmental limitations found in their native habitats.

“When we are talking about invasive plants or pests, we make our best guesses here. Anytime they are in a new place, we are going to learn something new,” Le Brun says.

Invasive Landscape Plant Examples

There are about 1,000 invasive plant species in the United States. These include shrubs like burning bush, and numerous vines, groundcovers, grasses, and other perennials that were introduced for their ornamental qualities. Some are present in almost every state, and others are limited to a region.

Take the Bradford/Callery Pear (Pyrus calleryana) for example. It was once the darling of landscapers and urban planners for its fast growth and abundant spring blossoms, so now it lines countless U.S. streets. Instead of staying small as expected, these trees can grow 40 to 50 feet tall. The branches tend to be weak and prone to dangerous breakage. Plus the flowers have a funky fragrance.

These traits alone don't make this tree invasive. But then, the trees began cross-pollinating with other pear species, whose offspring create thorny thickets that smother native plants and habitat. To help slow the spread of the species, some cities will pay you to remove a Bradford pear or provide you with a native tree to plant instead. It is also banned for sale in several states. And while there are now sterile varieties, these reportedly can revert and produce fruit.

Cogongrass (Imperata cylindrica) is another invasive plant that stands out for Le Brun because it is extremely tough. “Currently growing in the Southeast, it spreads by wind-dispersed seeds and underground rhizomes. I’ve seen an image where a rhizome penetrated a pine tree root,” says Le Brun. It grows extremely fast, too. In an existing infestation in the Florida panhandle, it spread more than 200 square feet per day in drought conditions.

Butterfly bush (Buddleia) (except newer sterile cultivars) and miscanthus ornamental grass (Miscanthus sinensis) are other examples of popular garden perennials that have turned out to be highly invasive, especially if the climate and soil conditions are ideal for their spreading.

Tips for Avoiding Invasives in Your Garden

Le Brun vets any new plant for her home garden carefully. “I’ve started taking those work lessons with me at the garden center,” Le Brun says. Use these tips to avoid introducing a potentially invasive plant to your garden.

• Find out how it spreads. Plant tags that say “good naturalizer,” or “reseeds easily” could be red flags. To make sure you aren’t adding a future interloper to your garden, ask the nursery staff how the plant reproduces or research it online.
• Check state watchlists. Plants can be an issue locally before they are declared invasive or noxious on the federal level, so it’s helpful to check on your state watchlists.
• Consider similar climates. If the plant isn't on your state list, “Try to see how it’s behaved elsewhere,” Le Brun advises, especially in nearby states with climates and growing conditions similar to your garden.
  
• Order online cautiously. When ordering plants online, it’s vital to check the source of the plant material, Le Brun stresses. “Check if it’s coming from the U.S. or not, and if it comes from outside the country, check the requirements for importing.” You don’t want to become an unwitting importer of restricted plants.
• Report potentially invasive species. If you see a plant that has been declared invasive and seems to be new to your area, report it to your state (search for “invasive species reporting” and your state).