A truly invasive plant eventually becomes so prolific that it dominates. It may seem to stop at the boundaries of your yard--the fence, for example, but without effective physical barriers it will find its way off of your property and escape. Invasive plants often "naturally" form a monoculture--a single species crop--that requires diligent control management.
To the casual observer, many invasive plants do not appear to be problematic but to the skilled eye it is often quite easy to notice invasive characteristics in a plant simply from personal experience and a little reading.
Let's take a look at six common ornamental plants that can cause regrets. All of these selections are popular and easily found at almost every nursery and big box store across the United States. The plants I discuss here are not native to the North America (with one exception) and it should be mentioned that although not every non-native plant is considered invasive, the worst offenders are non-native.
2. Verbena bonariensis. A very pretty, slender-stemmed purple flowered plant that is native to South America. Extremely popular as a summer annual or tender perennial all over the world (now considered a weed in the islands of the South Pacific). Verbena b. spreads by seed (volunteers) anywhere it can. The home landscape is ideal for its domination: many cultivated areas where the soil is often worked, including all flower and vegetable beds. It also loves gravel paths and walkways, or wherever there is a bit of grit between rocks and pavers. Nectar-loving insects adore Verbena b., which makes it appealing in the garden especially because the flowers can rise four to six feet--no bending down to observe the butterflies. The seeds will move great distances and volunteers are soon found in huge numbers all over your property, year after year. Fortunately it is not too difficult to pull by hand and it responds well to herbicide control, but once established it truly becomes a nuisance. Presently on the invasive species list in Washington State.
| Bishop's Weed|
4. Common Periwinkle - Vinca minor. Native to Europe and Western Asia. Another ground cover, Periwinkle will thrive in partial shade and moderate moisture but is surprisingly resilient. Again, the American tendency to heavily water and fertilize enhances its already aggressive nature in the garden. Planting Periwinkle beneath large shade trees is a common practice, as mistaking it for a good solution to erosion control (even the nursery retailers advertise it for this purpose). Because of this it ends up in woods and dunes where it forms a dense mat that excludes native vegetation. Very difficult to pull and tends to readily bounce back from herbicide damage. Invasives.org lists Periwinkle as a severe threat in South Carolina.
6. Obedient Plant - Physotegia virginiana. Aka False Dragonhead. OK, to be fair, I am including this attractive flowering perennial to the list. It is native to North America and widely distributed across the U.S. In mid-late summer it is erect, tall--four feet--and bursting with medium pink flowers. Obedient plant, unlike the previous five I have mentioned, is not an ecological threat but it is a threat to the gardener with limited space, which is most. Obedient plant forms large colonies, swiftly. After all, it's in the mint family, need I say more? In short, it's a space hog. If you like your perennial beds neat and contained, I recommend that you do not plant it. Pulling Obedient plant is moderately effective but root sections easily break off and regrow. Stay of top of it by both pulling and selective herbicide use.
Shane VanOosterhout is The Passionate Gardener.
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