University of Vermont Extension 
Department of Plant and Soil Science

Anytime News Article

Dr. Leonard Perry, Extension Professor
University of Vermont
We're not talking here about gardeners that are bullies, rather plants that many refer to as thugs or bullies.  Knowing which to avoid, or proper placement, can prevent the gardener turning into a bully on these unwanted plants and perhaps losing the battle.
The problem with our anthropomorphic characterization of these aggressive plants is that it really isn't their fault, but ours.  Plants aren't inherently evil, they didn't grow up trained or conditioned to be bullies.  They are just responding to their genetic makeup and environment.
All plants have a right place or habitat, the problem is when WE put them in the wrong place or they don't respond as WE would like where they are planted.  So the key is to realize what we want out of a plant, then choosing the right plant, or if we have a plant to know its "behavior" or growth needs and to place it properly. An example is the aggressive root spreaders which, when put in a garden, can overtake weaker plants and become the garden.  However, place these same plants in a poor site, or between a rock and a hard place literally (such as surrounded by pavement) and they will stay in place.
Plant thugs can be placed into at least 4 groups, according to consultant Kerry Mendez (  If not thugs against other plants, they may be against our time and desires, so I like to refer to them as "plants with issues".  These groups include those that reseed, those that spread by roots, those that take lots of care and time, and those that are short-lived so need frequent replanting. 
Reseeding can be good if you want more of a cottage garden look (just weed out the unwanted seedlings), or a naturalistic garden. (Or if you end up with such plants and their results, use these themes with those viewing your garden.)  Yet if you want a neat and tidy garden, reseeding can be an issue. Cultural controls begin with removing flowers once done, before they go to seed, but this can be time-consuming.  A heavy layer of mulch, perhaps with a ground fabric beneath in the case of annual flowers, prevents seeds from reaching the soil.  Of course you can use good old-fashioned weeding if the time.
If a plant reseeds into a wild area, displacing desirable native plants, it is said to be invasive.  This happens with herbaceous plants such as purple loosestrife, and several woody ornamentals. Keep in mind, too, that depending on climate and soil, some plants may reseed in one area and not another.  A good example of both these points is the Miscanthus grasses, which go to seed in warm climates (such as below the New York city area) but not in colder climates where they are not invasive.
Some potential perennial reseeders include the 'Golden Jubilee' hyssop, some rudbeckias, tall garden phlox, bluets, blue cardinal flower, spurge, yarrow species (not many cultivars and hybrids), and mallows.  Some common annuals that may reseed are amaranths, the balsam impatiens or touch-me-not (touching the seed pods makes the seeds explode out), cosmos, cleome, larkspurs, old-fashioned petunias (not the hybrids you usually find), marigolds (in some areas), the tall vervain, flowering tobacco, some black-eyed daisies, and violas. Foxgloves are biennials that often reseed.
In another group are those that spread by roots.  My definitions are if they spread and can be controlled by a yearly hoeing, they are spreaders.  If they need several yearly hoe attacks to keep in bounds they are aggressive.  If once you plant it you have it forever, I call this an invasive root spreader.  Examples of the latter include goutweed, Japanese knotweed or bamboo, and silver banner grass.
Other perennials most consider too root invasive to plant in gardens are the yellow and gooseneck loosestrifes, blue lyme grass, mints, spotted bellflower, ladybells, Chameleon plant, Japanese anemone, plume poppy, snow-in-summer, perennial periwinkle (more in warm climates), spotted deadnettle (can make a nice groundcover in shade), carpet bugle, artemisias and some evening primroses.  A red flag word to watch for in catalogs and plant descriptions is "vigorous".  Watch the cultivar descriptions as some cultivars may not be spreaders.  While the yellow loosestrife is aggressive, 'Alexander' with its variegated leaves is a clump former. While most artemisias spread, 'Silver Mound' is just that and a non-spreader.
As with the reseeders, some root spreaders are less a problem in some areas, as in poor soils with low fertility.  Placement is a key point with these.  I have a plume poppy for instance, which placed between a mown path and a brick wall is no problem and the birds love it for perching near the adjacent feeders.  (The bluish leaves look great too against the red brick wall.)  Another key point is if weeding them out, don't throw just anywhere.  Such plants can be thrown on top of a mature field, and end up taking hold and becoming the field.  Definitely keep from waterways where the smallest root pieces can wash down stream, establish, then take over.
One method many use to contain such spreaders as mint and bee balm that you may want, is to plant them in wide pots, then sink the pots in the ground.  Roots will reach the pot sides, then begin to circle.  This can go on for a year or two before plants die out, so if you want to save them you'll need to repot every year or two.  Beware of the holes through which roots (such as of the mint) can escape their confines and colonize the surrounding bed (as I learned the hard way).
The third group includes those that take more time than we want to spend on them, such as with frequent dividing, staking, and deadheading (pinching off) spent flowers.  The latter just may be aesthetic, or it may relate to preventing self-sowing.  Luckily, most new annuals are "self-cleaning", dropping their flowers naturally and not needing to be deadheaded as in the past.  Instead of cultivars that need staking, look for shorter versions.  Or prune late-season bloomers like asters, false sunflower, even tall garden phlox, back about a third in early summer to promote branching and lower growth without staking.
Related in issue to the last group are those perennials that are short-lived, requiring more time to divide or replant than we want or have to give.  Some in this group include delphiniums, cardinal flower, blanket flowers, some pinks or dianthus, lupines, and columbines.  As with other groups there are exceptions, both in plants and from climate and soil conditions.
Of course if a plant starts getting out of bounds, chemical herbicides can be used.  There are synthetic and organic ones that just burn back the leaves-- good for annuals.  If you're trying to control the roots too, you may need a systemic herbicide-- one such as glyphosate that is translocated through the plant system.  If using these "big guns", be sure and follow all label directions, keeping in mind they are usually non-selective.  This means they can kill desired plants just as well as the target ones if not applied properly. For invasive root spreaders, several applications over several years may be needed to keep them under control.
If such spreaders have taken over your garden, you may want to consider either removing the desired plants and giving in to the thugs (easy solution, as I've done in some beds), or getting in a backhoe to remove and replace all the soil (extreme solution).

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