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How to Deal with Plant Thugs

Aggressive Plants to Avoid

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You've probably heard the term right plant, right place. Well when you put a plant in the wrong place, it doesn't always die. Sometimes it grows too well. I'm not talking about plants designated as invasive. They should be completely avoided. Plant thugs are plants that grow too enthusiastically because they are planted in ideal conditions. How many times do we plant something that is a ground cover by nature, like Bishop's weed (Aegopodium podagraria) or Chameleon plant (Houttuynia cordata), on the edge of a garden, thinking it will make a nice border? How about a little bit of mint? Who's fault is it when it spreads out of bounds, the plant's or the gardener's?

Invasive versus Aggressive: Not every overly enthusiastic plant is invasive and plants can be invasive in one area and not another. Get to know which plants are aggressive in your area and especially your growing conditions, which greatly affects how well a plant grows. If you have full sun and sandy soil, you are not going to have a problem with Petasites, which spreads with abandon in boggy areas.

Profuse Self-seeders: Aggressive self-sowers tend to be more of a problem in warm climates, where they are not kept in check by long, frozen winters. However there are several that will spread even in colder zones, like Miscanthus (maiden grass) and and butterfly weed.

Runners: It's the thugs that spread by rhizomatous rhizomes that present the biggest problem. A good tip off that a plant is a potential thug is when it's described as vigorous. (Or when a fellow gardener has a lot of it to give away - every year.) Ask anyone who has ever planted running bamboo or ribbon grass. Some of us will still want these aggressive spreaders in our gardens. If you decide to give them a try, controlling them can be an ongoing chore. The following techniques will help control them, but they won't contain them entirely.

  1. Move them where they will not thrive

  2. Deadhead self-sowers, before they go to seed

  3. Use them in containers, as annuals

  4. Put some sort of border in the ground, so the roots cannot spread.

  5. Choose hybrids that are either sterile, so they do not self sow at all, or that at least do so less vigorously.

  6. Choose variegated varieties, which tend to be slower growers

The list of aggressive plants is long and, as I mentioned, varies from area to area. It includes trees, shrubs, ornamental plants and edibles. I can't list them all, but here are 10 common garden plants that you might not have suspected. And a whole lot more, submitted by readers who regret having planted them. Feel free to add your own. What I wish I'd Never planted.

1. Bee Balm (Monarda didyma)

Bee Balm (Monarda)
Photo: © Marie Iannotti
Bee balm is one of those plants you always see at plant swaps. A small plant quickly becomes a large clump. If it stayed a clump, it would not be on this list, but its roots tend to wander far and wide, establishing still more clumps. Plants that spread by rhizomes are almost impossible to rip out, because any tiny piece of root left in the soil will resprout.

Still, it's a very attractive plant, with flowers in pinks, reds, purples and near white. Many varieties are prone to powdery mildew, so give them plenty of room for air flow.

Bee Balm goes by many names, including Oswego Tea and Bergamot. The leaves are indeed, used to make a tea. I've read that the colonists began drinking it when tea became scarce, after the Boston Tea Party. A New York Indian tribe, the Oswegos, introduced it to them. Butterflies and hummingbirds enjoy the flowers.

Plants tend to look a bit ragged, after blooming. Don't be afraid to cut them back, almost ot the ground. They will regrow and probably bloom again. USDA Hardiness Zones 4 - 9.

2. Bellflower, Spotted (Campanula punctata)

Campanula 'Cherry Bells'
Photo: © Marie Iannotti

I have to confess, I love bellflowers. But their delicate charm masks a will of iron. The rhizomes of this plant will push up through asphalt. They keep coming out with new varieties and they often say they are better behaved, but so far, that has not been the case. If you love it and plant it, you will be ripping it out by the hand fulls. A better idea might be to grow it in containers. Since it's hard down to zone 4, it should over-winter, with a bit of protection. Or try a different genus of Campanula, like the peach-leaved Campanula persicifolia. They make excellent cut flowers. USDA Hardiness Zones 4 - 9.

3. Chinese Lantern (Physalis alkekengi)

Chinese Lantern Plant, Strawberry Ground Cherry (Physalis alkekengi)
Photo: Lars Sundström / Getty Images.

It's hard not to be fascinated by the flowers on this plant. They look like bold, orange tomatillos and they are very close relatives. The fruit inside the pod is edible and supposedly sweet, but I haven't tried it myself. I generally cut them and use them in displays. Cutting them is a good idea, because you don't want to compound the problem of its over exuberance in the garden by letting it self-sow. This is yet another plant that spreads by rhizomes and it is very hard to eradicate. So keep cutting. They also dry nicely. The orange lantern turns beige, but it gets a lacy look that allows a peek at the seed pod inside. USDA Hardiness Zones 3 - 9.

4. Evening Primrose (Oenothera biennis)

Evening Primrose (Oenothera biennis)
Photo: © Marie Iannotti

If you can think of Evening Primrose as a ground covers and keep it out of your flower bed, you may not think of it as a thug. Be sure to give it plenty of ground to cover, because it lasts for years. You will often see it happily taking over the side of the road. This is a carefree plant - as far as growing it. It can cause a lot of consternation if you decide you want to remove it. But most will bloom for weeks, throughout the summer. It is a native wildflower with several medicinal uses. USDA Zones 3 - 11.

5. Loosestrife (Lysimachia sp.)

Gooseneck Loosestrife (Lysimachia clethroides)
Photo: © Marie Iannotti

I'm not referring to Purple Loosestrife (Lythrum salicaria), which is on the invasive lists of several states. Lysimachia plants may not be invasive, but they sure can travel. To be fair, not all species are trouble makers. Several are too tender to make it through colder winters. But 2 species I would warn against are Gooseneck Loosestrife (Lysimachia clethroides) and Purple Leaved Loosestrife (Lysimachia ciliata 'Purpurea').

Gooseneck Loosestrife can look like a gaggle of geese, when it's in bloom. The flower curve down, then up, like beaks. It usually takes a few years before it starts sending out its rhizomes in earnest, but don't become complacent, it will happen eventually.

I find Purple Leaved Loosestrife the worst offender, of the two. It looks very pretty, when the purple leaves fist come out in the spring. The yellow flowers aren't much to speak of and the purple colored leaves quickly fade to green. And it spreads. And spreads. And spreads. USDA Hardiness Zones Gooseneck Loosestrife 3 - 8 / Purple Leaved Loosestrife 3 - 9.

6. Hollyhock Mallow (Malva alcea)

Hollyhock Mallow Photo (Malva alcea)
Photo: © Marie Iannotti

This is another plant I am loath to call aggressive, because I love it so much. Hollyhock mallow are perfect plants for cottage gardens. They bloom profusely and self-seed with abandon. Many of the newer varieties do not seed as freely as older types and may not become a nuisance. Just be sure you love it, before you find out how it does in your garden. USDA Hardiness Zones 4 - 8.

7. Lamb's Ear (Stachys byzantina)

Lambs Ears
Photo: © Marie Iannotti

Lamb's Ear is a favorite in children's gardens, partly because of the name and partly because they really are as soft as a lamb's ear. While in bloom, they look lovely. But shortly afterward, they go into decline and you have to cut them back hard and wait for new growth to make them look presentable again. I've seen gardeners use this plant as edging and that's just asking for extra work. It does not spread uniformly and it has a tendency to down out in the center of a clump quickly. If you're mostly interested in the leaves and don't care much about the flowers, you should try 'Helen Von Stein'. This variety is sterile, so there will be no self-sowing. It doesn't produce that many flowers to begin with, but the leaves are larger than other types and they stay attractive longer. USDA Hardiness Zones 4 - 10.

Groud Covers You Can Walk On

8. Lily-of the-Valley (Covallaria majalis)

Lily of the Valley
Photo: © Marie Iannotti

For a few weeks in May, Lily-of-the-Valley can fill your yard with the most glorious perfume. After that, it will simply fill your yard. Once again, this is a good plant - in the right place. Lily-of-the-Valley is not for the garden border. It should be considered a ground cover and a darn good one. Once established, it can thrive in dry shade or the best of soils. I've seen it too very well under trees, where nothing else grows. The pink variety is a bit more finicky and much less aggressive. USDA Hardiness Zones 3 - 9.

9. Obedient Plant (Physostegia virginiana)

Obedient plant has snapdragon like flowers that bloom from the bottom of the stalk up.
Photo: © Marie Iannotti

What a misnomer; the Obedient Plant has a mind of its own. The obedience actually refers to the stem's ability to be bent into shapes, kind of. This is another rhizomatous runner and it will pop up years after you thought you had removed it all. They have become breeding better behaved varieties, like 'Miss Manners', and I would advise you to look for them. Take every advantage you can.

10. Wormwood (Artemisia ludoviciana)

Wormwood (Artemisia ludoviciana)
Photo: Ayla87 / Stock.xchng (http://www.sxc.hu/profile/Ayla87)

The delicate silvery foliage of Artemisia plants makes them deservedly popular with many gardeners. They blend beautifully with most flowers and look good all season. They also like to spread out by runners and elbow out other plants. In less than ideal conditions, they aren't terribly aggressive and can be controlled - if you stay one step ahead. Wormwood is happiest, and most vigorous, in full sun and well-drained, moderately rich soil. It's easy to start from seed or root cuttings. If you really want more plants, simply lift the suckers and transplant elsewhere. USDA Hardiness Zones: Zones 3-9.

Wormwood is a medicinal herb with many uses, including as a de-wormer. It supposedly repels other insects, including slugs and moths, but I haven't had a chance to test it out.

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