Aggressive plants are plants that grow too enthusiastically, usually because they are planted in their 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 believing there is such a thing as a little bit of mint? Whose fault is it when it spreads out of bounds, the plant's or the gardener's?
Invasive Plants vs. Aggressive Growers
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 affect 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.
There are two basic ways plants spread out and take over your garden.
Profuse Self-seeding Plants
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 maiden grass( Miscanthus) and butterfly weed.
Plants that Spread by Runners
It's the aggressive plants that spread by rhizomatous rhizomes that present the biggest problem. A good tip-off that a plant is a potential aggressor is when it's described as vigorous. Ask anyone who has ever planted running bamboo or ribbon grass. Some folks love these plants to still want these aggressive spreaders in their 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.
- Move them where they will not thrive.
- Deadhead self-sowers, before they go to seed.
- Use plants that spread by rhizomes in containers, as annuals.
- Put some sort of border in the ground, so the roots cannot spread.
- Choose hybrids that are either sterile, so they do not self-sow at all, or that at least do so less vigorously.
- Choose variegated varieties, which tend to be slower growers.
The list of aggressive plants is long and varies from area to area. It includes trees, shrubs, ornamental plants, and edibles. There isn't room here to list them all, but here are 10 common garden plants that you might not have suspected.
01 of 10
Bee Balm (Monarda didyma)
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 re-sprout.
Still, it's a very attractive plant, with flowers in pinks, reds, purples, and near white. Butterflies love them, but many varieties are prone to powdery mildew, so give them plenty of room for airflow.
Bee balm plants tend to look a bit ragged, after blooming. Don't be afraid to cut them back, almost to the ground. They will regrow and probably bloom again. USDA Hardiness Zones 4-9.
02 of 10
Bellflower, Spotted (Campanula punctata)
Bellflower's delicate charms mask 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 hardy 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.
03 of 10
Chinese Lantern (Physalis alkekengi)
It's hard not to be fascinated by the flowers on this Chinese Lantern plant. They look like bold, orange tomatillos, a very close relative. The fruit inside the pod is edible and supposedly sweet, but they are generally cut and used 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.
04 of 10
Evening Primrose (Oenothera biennis)
If you can think of Evening Primrose as a ground cover and keep it out of your flower bed, you may not think of it as an aggressive spreader. 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.Continue to 5 of 10 below.
05 of 10
Loosestrife (Lysimachia sp.)
Purple Loosestrife (Lythrum salicaria) is on the invasive lists of several states, but Lysimachia plants, although not necessarily invasive, sure can travel. To be fair, not all species are troublemakers. Several are too tender to make it through colder winters. But two species to 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 flowers 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.
Purple Leaved Loosestrife is perhaps the worst offender, of the two. It looks very pretty when the purple leaves first 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.
06 of 10
Hollyhock Mallow (Malva alcea)
This is another plant many gardeners refuse to think of as aggressive because they love it so much. Hollyhock mallows 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.
07 of 10
Lamb's Ear (Stachys byzantina)
Lamb's Ear is a favorite in children's gardens, partly because of the name and partly because they 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. Some gardeners use this plant as edging and that's just asking for extra work. It does not spread uniformly and it tends to die 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.
08 of 10
Lily-of the-Valley (Covallaria majalis)
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. It even does well under trees, where nothing else grows. The pink variety is a bit more finicky and much less aggressive. USDA Hardiness Zones 3-9.Continue to 9 of 10 below.
09 of 10
Obedient Plant (Physostegia virginiana)
What a misnomer. The Obedient Plant has a mind of its own. The obedience 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 begun breeding better-behaved varieties, like 'Miss Manners' and you'd be well advised to look for them. Take every advantage you can.
10 of 10
Wormwood (Artemisia ludoviciana)
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.
On the plus side, wormwood is a medicinal herb with many uses, including as a de-wormer. It supposedly repels other insects, including slugs and moths.
Invasive Plants: Specific Methods of Control. UNH Extension