Invasive Garden Plants: Is that a weed? Back »

Above: (Left to Right) Pale pincushion flower, shasta daisies, and yellow archangel are just a few of the many aggresive plants that can take over a garden if left unattended.

If there is one thing that can be said about a garden it is that there are almost always weeds to pull. One of the classic definitions of a weed is “a plant that is growing out of place”. While we most generally consider the common plants like crabgrass, dandelions, purslane and many others as weeds, just about any plant can be a weed at times and in the wrong location. Whether or not you personally would consider a particular plant a weed could also depend on how much of that particular plant you have already and how much of the garden that particular plant has already taken over.

Gardeners and garden writers have various names for some of these more “aggressive” plants, such as “spreading”, “invasive” or “thug” plus a few, more colorful terms, which some gardeners might use under more private circumstances. Then, just about the time you are about to eradicate a particular plant from your garden forever, a friend stops by and that is just the plant they are looking for to plant in a particularly difficult site. It is as if the old saying “one man’s trash is another man’s treasure” becomes “one gardener’s weed is another gardener’s pleasure”. So, here are descriptions of some of the most common garden perennial plants that you might have enjoyed at one time in your garden but now have come to regret planting because it is now a weed.

Shasta Daisies

Above: Shasta daisies are nice plants until they take over your garden.

Shasta daisies are in bloom right now and are generally a welcome sight in the spring garden. Their showy white flowers with the yellow centers are pretty and make great cut flowers. However, unless you have some of the more well-behaved cultivars, the typical species Leucanthemum supurbum is an aggressive spreading plant. What starts as a nice clump of a few plants can soon be a mass of hundreds taking over a fairly large section of your garden. Shasta daisies spread mainly by seed so if you cut off the flower stalks when the flowers fade, you will reduced the chances of it spreading. But if you want to grow Shasta daisies, look for some of the named varieties which tend to either produce little viable seed or do not spread from the base like the species might.

Yellow Archangel

Above: Yellow archangel is a very aggressive spreader even in the shade.

Yellow Archangel (Lamiastrum galeobdolon) is a rapid-growing groundcover vine for shady places, including under trees. It has attractive silvery foliage all summer long but particularly in the spring. In early summer, small yellow flowers develop at the base of each leaf giving a nice display of color. However, this is an aggressive plant. Like the Aegopodium that is described below, this plant can take over if you are not careful. In somewhat warmer climates it has pushed out native species and covered large areas of the forest floor where it can be difficult to control. Like the Leucanthemum, there is at least one cultivar called ‘Herman’s Pride’ that is not aggressive at all. Instead it forms nice clumps of upright stems with the same yellow flowers. Its leaves are about ¼ the size of the species form but it is much better behaved.

Pale Indian Plantain

Above: Pale Indian plantain is another aggressive spreader.

Pale Indian Plantain (Cacalia atriplicifolia) is a native, tall grass prairie plant that is sometimes planted into gardens in our area. It grows to form a basal rosette of leaves that may spread to a foot wide. In late spring a flower stalk grows up that can reach 5’ or more producing a branched panicle of pale, greenish white flowers. After the flowers mature they produce little seeds with a white tuft attached that makes them easy to be carried by the wind. New plants are easily produced around the garden. While this is an interesting plant, it may not be interesting enough to warrant the need for deadheading and then digging out the unwanted baby plants.

Garden Valerian

Above: Valarian has pretty white and fragrant flowers but spreads everywhere.

Garden Valerian, Valeriana officinalis is a native plant that is coming into bloom at this time of year too. It grows about 2-3’ tall and has a white flower head. Some people think the flowers have a pretty vanilla fragrance but in large quantities some say the flowers smell like feet, and not in a good way. I think they look and smell good in small quantities, but that is where the problems come in. It is hard to keep them in small quantities. They spread readily by seed and at the base so that soon you can have them popping up in fairly large areas, particularly in shady and moist areas of your garden. They are easy to pull out with a fairly shallow but rather dense root system. Deadheading will also help to keep them at bay.

Carpenter Plant

Above: Native plants like this carpenter plant can be invasive in a garden too.

Carpenter plant or cup plant (Silphium perfoliatum) is another native plant that can become weedy with seedlings showing up in beds, often some distance from the original plant. Silphium is a member of the Aster family so it has daisy like flowers that develops seed that can be wind-blown from the original plant. This is no small plant. A mature plant can reach 6-8’ in height with a 3’ spread so when it decides to settle down in amongst much smaller plants it is very noticeable. Mature plants have fairly large, ½” wide stems that are definitely square, hence the name of carpenter plant. The cup plant name comes from the way the bases of the leaves clasp the stem and can hold water after a rain. The seedling plants can be a little difficult to identify. Look for the roughly-textured, opposite leaves on a fairly large plant for a seedling. You may not notice the square stem or cupped leaves until they get a little larger. You may need a shovel to dig these guys out because they develop a pretty good root system at an early age.

Pale Pincushion Flower

Above: Pale pincushion flower is a biennial that has pretty delicate yellow flowers but it is indelicate in how much it can spread.

Pale Yellow or Cream pincushion flower (Scabiosa ochroleuca) is a biennial plant that is a garden thug, but a good natured one. It spends its first year as a small, unassuming clump of somewhat fuzzy leaves growing down close to the ground. It does not flower the first year, but rather stores up carbohydrates for the winter and next year’s floral display. By mid-summer of the next year the plant has grown up to 3’ in height and may reach 3’ in width by the end of the summer. It produces dozens of 1” wide creamy yellow flowers that are produced on fine, wiry stems. The anthers stick up on tiny little pin-like stalks giving the plant its common name. All of those cute little flowers produce lots of seed, which when ripe also looks like more pins stuck in a pin cushion. It soon falls free to spread around and plant next year’s crop of seedlings. The first year plants are easy to pull out but are often missed because they are not that showy in the garden.

Bishop’s Goutweed

Photo by: Anneli Salo (Own work (Own photo)) [CC-BY-SA-3.0], via Wikimedia Commons

Snow-on-the-mountain or Bishop’s Goutweed (Aegopodium podograria) is a favorite plant of mine, but only for the right kind of locations. Otherwise its cuteness soon wears off and you may wish you never planted it. This plant spreads mostly by underground stems called rhizomes, soon making a dense patch that will outcompete most other plants. It grows well in fairly dense shade and will even take dry shade but may brown up a bit during the heat of summer. The variegated form is the one that is most often planted. It has a 3-part dissected leaf with white margins along the edges. In early summer it produces a white umbel flower, like that of dill but smaller. The plant only grows about 8-10” tall, except when it is blooming. I like to use this plant under trees where it can just grow as far as it wants. Otherwise I would advise to contain its growth with some sort of barrier, like maybe concrete block, to keep it in check.

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