Separating the Bad Weeds from the Really Bad Weeds
Some weeds are more than just a gardening nuisance. Some are dangerous, impacting the environment, the food supply, and even harming people, animals, and desired plants.
Some of Our Worst Weeds Once Were Favorite Plants
Not all of the weeds and invasive plants that plague U.S. landscapes came into the country by happenstance or accident.
Some of our worst invasives started out as intentional plantings – plants chosen for the very reason that they grow easily and sport strong survival traits.
It was only long after their introduction that gardeners realized some of those plants were too competitive. Some of them were so aggressive that they invaded and overtook habitats, elbowing diverse native species out of the way in the process.
Many of today’s weeds have their roots from the late 1700s to early 1900s when plant explorers were eager to bring new plants from around the globe to the U.S.
That was a time when invasiveness wasn’t on the radar and before federal and state governments developed robust plant- and pest-inspection programs.
This tree-of-heaven is loaded with mature seed pods that have the potential to create new colonies of this woody invader. Photo by George Weigel
The tree that’s not from Heaven
One of the earliest examples of a seemingly good plant gone awry is tree-of-heaven (Ailanthus altissima).
This small, dense tree with the pointed leaves was introduced to the U.S. from China in the 1780s as a durable, versatile, and appealing street tree that also had medicinal uses in Chinese medicine. It was widely sold and planted throughout the 1800s.
By the early 1900s, though, wild trees-of-heaven were popping up in sidewalk cracks and alleys, and spreading “suckers” were forming dense colonies that were dominating habitats. Seeds were sprouting new saplings rampantly, too.
Fast-forward to the present, and tree-of-heaven is now on most states’ Noxious Weed Lists, which bans its propagation and sale.
Most recently, tree-of-heaven also has been found to be a key host plant for another invasive pest – the spotted lanternfly.
Kudzu can grow 60 feet in a single season.
The “vine that ate the South” and others
Kudzu (Pueraria) is another example of a "wonder plant" turned weed.
This semi-woody vine from Asia was exhibited at the 1876 Philadelphia Centennial Exposition and soon became a popular ornamental vine, shading porches in the South.
The U.S. Soil Erosion Service even planted and encouraged kudzu as an excellent soil-erosion aid on Southern farms.
It wasn’t until the 1950s that governments and landowners realized this vine – which can grow 60 feet in a single season – was advancing out of control. As it invaded forests and climbed buildings, kudzu became known as the “vine that ate the South.”
Today it’s a potential tree-killer on the Federal Noxious Weed List.
Multiflora rose on a roadside.
Multiflora rose
Multiflora rose (Rosa multiflora) was imported from East Asia for erosion control and cattle fencing but is now a leading thorny invader that’s an important host for the rose rosette disease that plagues cultivated rose varieties.
The rampant invader Japanese honeysuckle came to the U.S. as a fragrant vine that gardeners planted. Photo by George Weigel
Japanese Honeysuckle
Japanese honeysuckle (Lonicera japonica) came to the U.S. in the 1800s as a fragrant ornamental vine that also was useful for erosion control. It’s now one of the country’s leading viny invaders and is on most states’ Noxious Weed Lists.
Japanese barberry. iNaturalist
Japanese barberry
Japanese barberry (Berberis thunbergii). This dense, thorned Asian shrub has been a popular landscape plant since the 1800s for its drought-toughness and deer-resistance, but it’s turned out to seed rampantly in the wild and is increasingly being added to state weed lists.
Tamarisk. iNaturalist
Tamarisk
Tamarisk (Tamarix). A small tree that produces plumes of pink flowers, tamarisk had been sold as a durable ornamental hailing from Eurasia and Africa. However, it’s been found to be a rampant seeder as well as a super-thirsty species capable of lowering an ecosystem’s water table.
Scotch broom blooming. iNaturalist
Scotch Broom
Scotch broom (Cytisus scoparius). Still sold in some areas, this European shrub produces spikes of fragrant yellow flowers. It’s also a rampant seeder that produces fast-growing colonies that out-compete native species.
Numerous other once-ornamental species are turning up as potential trouble-makers as both private and public organizations tackle the topic of invasive plants. Among ones that are still or recently sold: burning bush, flowering pears, purple loosestrife, privets, butterfly bushes, non-native honeysuckles, English ivy, and Norway maple.
A 2021 study by the University of Massachusetts Amherst found that despite state and federal efforts, nurseries and garden centers are still selling hundreds of invasive species – including 20 that are illegal to grow or sell nationwide.
It’s also possible for gardeners to buy plants in another state that are banned for sale in their own, bypassing efforts to curb the spread of invasives.
What to do?
Weed scientists say a front-line solution to stem invasives is for gardeners to know what’s potentially invasive in their region before buying.
Many states have invasive-plant “watch lists” in addition to already-banned Noxious Weed Lists.
Two good national resources are the U.S. Department of Agriculture’s National Invasive Species Information Center, and the Early Detection and Distribution mapping system (EDDmaps) that shows where specific plants are becoming a problem.
Another resource that keeps track of potentially invasive plants in seven states and U.S. regions is iMapInvasives, a non-governmental online platform that also includes details on identifying and controlling invasives.
A second strategy is identifying invasives already in the landscape and removing them before they seed or spread into the wild.
And a third strategy is preventing new seeding by
- Removing the invasive plants or at least their flowers before seeds have a chance to mature;
- planting dense, desired plantings that fill bare space and deny openings to weed seeds;
- covering bare ground with mulch, and/or
- using twice-annual treatments of a granular weed-preventer such as Preen Extended Control Weed Preventer.
