Thursday, November 18, 2010

Six Naughty Garden Plants

     In my last post I discussed the familiar topic of trouble-makers in the garden, plants that spread aggressively, crowd and displace more favorable plants.  Mostly we describe these types of plants as invasive, but just because a particular plant is good at reproducing or colonizing on your property does not automatically make it invasive.  More than once I have overheard a gardener calling a plant invasive simply because it established well and prospered while other beloved plants nearby diminished.  This is not the correct definition of invasive.  Sometimes a plant is simply a space hog and will out compete its meeker neighbors.  In the wild, these plants would not be growing next to each other in the first place.
     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. 

Chameleon plant
     1.  Chameleon plant  - Houttuynia cordata.  Native to Southeast Asia.  Planted here as a ground cover due to its rapid growth over large areas, especially where there is ample moisture.  The abundant amount of water and fertilizer that we apply to our gardens and yards gives it an extra shot in the arm.  Like many ground covers, Chameleon plant at first seems the perfect thing for a spot we wish to see "filled in" without a lot of hassle.  Unfortunately, Chameleon plant's stolons plow through soil with tenacious speed, forming a colony so dense that it successfully engulfs a garden in a few years, taking no prisoners.  Anything not rugged enough to stand up to it will soon be smothered.  Digging helps it to reproduce by chopping its roots, and its tough, waxy leaves provide resistance to herbicides.  Categorized as "difficult to control" by the Global Invasive Species List.

Verbena b.

     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

    3.  Bishop's Weed - Aegopodium podagraria (aka gout weed, snow-in-the-mountain, ground elder, and herb gerard). Native to Asia and Europe, Bishop's Weed is a favorite landscape plant because it thrives in shade--an extremely common complaint of homeowners who are flummoxed by lack of sunlight on their property.  Like Chameleon plant it spreads underground, in this case by stolons, and just as aggressively.  The Invasive Plants Atlas lists Bishop's Weed as invasive in six states including Michigan.

Common Periwinkle

     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. lists Periwinkle as a severe threat in South Carolina.

English Ivy

     5.  English ivy - Hedera helix.  Native to Europe, Western Asia and Africa.  Known for covering old neoclassical buildings on college campuses, English ivy is diabolical in its ability to grow horizontally and vertically.  It invades garden beds,  hedgerows, fields, woodlands and swarms tree trunks, eventually pulling them down.  On the forest floor it prevents other plants from germinating by casting dense shade.  English ivy can cause serious poisoning in humans and animals if ingested.  Control English ivy aggressively with herbicide sprays.  Cut the trunks near the ground and cover the fresh wound with herbicide.  An area that has been invaded will require more than one year of intervention.  The Plant Conservation Alliance's Alien Plant Working Group lists English ivy as an ecological threat.

Obedient Plant
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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