From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-21743-649812
I like to do a little research before bringing a plant home from the
nursery. It's not that I'm being snobbish; I just don't want to
unwittingly contribute to my neighborhood's collection of invasive
plants.
Scotch broom, purple loosestrife and Japanese knotweed have all
escaped cultivation here in the Pacific Northwest, and can be seen
covering hillsides, choking lakes, and crowding out native understory
in forested areas. At one time these plants were readily cultivated,
considered welcome additions to local gardens. Little did we know how
invasive these plants could become or what long-range problems they
would cause.
My first experience with a strong-willed plant was with Houttuynia
cordata 'Chameleon', an innocent-looking little snippet with
heart-shaped, tri-colored leaves and small white flowers. Sold as a
ground cover, this lovely little plant has a tenacious personality.
Quietly working underground, it popped up where I least expected -- 10
feet from the original bed!
I chose this plant for an especially difficult spot in my garden
because it grows in wet or dry soils in shade or sun, is not
attractive to slugs, and has no particular insect or disease problems.
Well, if it sounds too good to be true, there's got to be a catch. My
new Houttuynia behaved as promised and then some! Now that I'm more
familiar with its wanderlust ways, I'm prepared to keep it in line by
anticipating its escape from the bed. This plant is especially easy to
contain. All I do is pull it out, snake-like rhizomes and all.
Unfortunately, not all plants with aggressive characteristics are
quite so easy to manage.
Controlling Overzealous Plants
Most ill-mannered plants are attractive, making them hard to resist,
but I now realize that descriptions such as "grows in poor or average
soils," "drought-resistant," "makes a great ground cover" or "is a
low-maintenance plant" are actually tip-offs that they are potentially
invasive. These warnings may not prevent me from planting a specific
plant, but I'll make a note of it and watch the plant carefully. If
you choose to plant something with a questionable reputation, you can
try these steps to control a plant's unacceptable behavior.
Deadhead.
Remove all flower heads as they fade to limit the offspring of many
would-be parent plants. Coneflower (Echinacea) and black-eyed Susan
(Rudbeckia hirta) can produce thousands of seeds in a season.
Pulling out or hoeing seedlings, in combination with deadheading, will
usually control the majority of rampant self-seeders, including
yarrow, lambs' ears (Stachys byzantina) and spurge.
Cut Them Down.
Remove seed heads by mowing or shearing back hard just as flowering
has finished to temper creepers such as periwinkle (Vinca
minor), spotted deadnettle (Lamium maculatum cvs.),
soapwort (Saponaria officinalis) and sweet woodruff (Galium
odoratum).
Don't Spoil Them.
Most pervasive plants do well in poor soil with little maintenance,
but if you pamper plants such as spurge or evening primrose
(Oenothera spp.) with more water and fertilizer, they can
become invasive.
Pull, Pull, Pull.
The only way to control some plants is to physically pull out new
shoots, roots and all. Although it can be hard work, relentless
pulling will eventually eliminate even the most persistent of plants.
This is effective for controlling lambs' ears, dame's rocket
(Hesperis matronalis), mint, common violets, and English ivy
(Hedera helix).
Mulch.
A thick mulch will prevent seeds from touching bare soil and taking
root. If you cover the ground with a 3- to 4-inch layer of organic
material, such as hay, straw, grass clippings, or wood chips, any
seeds that germinate will be simple to pull, roots and all.
Relocate.
Find a place in your garden where a plant's rampant tendencies can be
an asset. Evening primrose, snow-in-summer (Cerastium
tomentosum) and spotted deadnettle may be too aggressive for the
perennial border but just right for a hot or dry area where little
else will survive.
Contain Their Roots.
Some plants should be grown in pots to contain their roots; herbs such
as mint, thyme and marjoram are good examples. To control horizontal
spreading of creeping plants, plant them in bottomless (for good
drainage) containers sunk into the ground up to rim level.
An Ounce of Prevention
Even the most innocent-looking plants can have bad habits. Before
bringing a new plant home from the nursery, it pays to ask a few
questions: Does it stay where it is planted or does it like to wander?
Will it pop up in unexpected places because birds or wind spread its
seeds? Will it creep, climb, or otherwise terrorize more civilized
plants in your garden? If so, do you want the responsibility of
keeping it under control or should you choose another plant instead?
Finally, before accepting a gift from someone else's garden, keep in
mind that if they have too many of whatever it is, chances are before
long, you will too.