Invasive Perennials …Beware!

centaurea montana invasive perennial

Invasive perennials become “weeds” in a garden when the environment is in the plant’s comfort zone. Rocky Mountain gardens where plants are not as vigorous grow slower and shorter so we get away with growing some of the invasive perennials that are notoriously difficult to control in other areas of the country. However, the invasive perennials we DO have really like to call our Mountains home.

Some Characteristics of Invasive Perennials:

They often grow an underground root called a “stolon” that moves underground to propagate.

They reseed readily.

They are extremely hardy and vigorous when growing in a certain environment.

Often, they are planted to solve a “problem” in an area where it is hard to get anything to grow, and then they migrate to other areas where they are unwanted.

Hybrids that are considered sterile but reseed anyway may be considered invasive.

Some Invasive Perennials You May Want to Avoid:

Perennial gifts from unknowing friends who have no idea that the perennials they are sharing are invasive is one of the most common ways to accidentally transplant an invasive variety into your garden. Your neighbors don’t mean to do you harm. They are only aware that this unknown plant is extremely hardy and grows so well that it must be shared with a fellow gardener. A perennial that incorporates two or more of these traits is doubly difficult to get rid of; for example consider the very beautiful Centaurea montana or mountain bluet.

centaurea montana invasive perennial

Centaurea’s bloom of intricate pinwheels of flamboyant colors is delightful and this hardy mountain perennial blooms both in spring and fall, so what is the problem?  Centaurea spreads both with stolons (underground stems that root), and seeds, which makes it so invasive that it shows up all over the garden. It also is very happy in a high mountain environment which gives it a vigorous growth habit.

Another very beautiful but very invasive perennial is the Campanula rapunculoides or adenophora known as ladybells:

adenophora or ladybells - invasive perennial
Beware the loving grandmother bearing gifts of invasive perennials!

This plant was a gift from our Grandmother over fifty years ago and believe me I’ve spent all of those 50 years trying to remove it from my garden. This perennial has fleshy taproots that resemble parsnips so the roots are difficult to dig and even one small piece of root will reproduce. Ladybells act as if herbicides are fertilizers and grow stronger, so spraying is not successful. The main problem is that when they bloom in the heat of late July, a gardener finds they do not want to part with them for they bring true blue colored clumps of tall spikes to a full sun, partial sun and/or shade garden. They are so vigorous they grow anyplace their roots or seeds land.

Many perennials may only be invasive in certain areas. For example, Asclepias or Butterfly Weed is a highly invasive perennial in my garden probably due to the highly amended soil. However it has one special talent for it is the Monarch butterflies’ home of choice for propagation. The larva hatch, feeding on Asclepias. Next, the caterpillars consume the plant prior to spinning a cocoon and repeat the process.

asclepias - butterfly weed - invasive perennialEvery gardener wants Monarch butterflies flitting around their garden so they keep the perennial. I chop it back every spring but can’t bring myself to part with the butterflies so I always leave a small start. By early summer, the perennials with their brilliant gold flowers on huge bushes are gigantic but so are the Monarchs so I suppose it is worth it? Organizations trying to protect the Monarch butterfly actually recommend that you plant these, but use caution.

A tight, tiny foliaged golden blooming succulent named Sedum acre or Goldmoss’s main goal in life is to fill cracks and crannies with bright spreading foliage.

sedum - invasiveTucked in a mortarless rock wall, Goldmoss will reproduce as a surface spreader, covering any gap in the rocks. And when it is done there, it just keeps on growing! Soon it is spilling everywhere and drops pieces of plants that root on whatever it touches; roadbase, cement, sunny or shady spots, it grows and spreads. In Idaho, Sedum acre is nicknamed “Star of Idaho” and in Utah it is called “Star of Utah.” If a gardener requires instant color between rocks or stepping stones (it is a walk-on-me perennial), Goldmoss is the candidate. Just know its rootless foliage becomes very unattractive after blooming due to the drying blooms. A fine use of sedum acre is to bury the green moss in planting holes as a soil conditioner for it becomes mulch almost immediately. Nice to grow your own mulch!

Tall garden phlox is a very popular well behaved late season blooming perennial but its evil twin with the same five-petal flower arrangement is highly invasive. Wild phlox lives forever, for my first start came from a gardener who gifted me back in the 1950s with my first root.

invasive perennials - wild phloxHowever, seed from hybrid phlox may also be culprits so deadheading phlox as soon as it finishes blooming may save a lot of troubling labor for wild phlox is almost impossible to curtail. The root system is so vigorous and strong that a backhoe may be the easiest method of removal.  The evil twin covered with its tall straight stemmed muddy white flowers is an attention-getter in a garden for white always draws the eye, but remove it you must for it will outgrow and outperform all other tall garden phlox.

Some members of families are not as neat as others. In the Lamium family are the well-behaved ‘Herman’s Pride, ’but an unrelenting cousin must be planted in a pot or inside a barricade or it will take over the shade garden.

invasive perennials - lamium
Lamium “Herman’s Pride’ is a wonderful addition to a perennial garden.
Other varieties of Lamium should be contained to a pot so they don’t overrun your garden.

When Hybrid perennials reseed, the seedling often reverts back to the original or Mother perennial. For example the gorgeously-foliaged lamium ‘Silver Beacon’s’ seedlings have reverted to a dark green leaf with a single stripe through it. In the picture below notice how the brilliant foliage of ‘Silver Beacon’ has changed into the original lamium seeded along the fence.

Invasive perennials have the ability to overrun any garden once they gain a toe hold. There are other invasives to watch out for and the best solution is to talk with other gardeners. Extensions offices are also an excellent source and often list perennials that are invasive to an area. The much detested Aegopodium or bishop’s weed shows up on practically every State’s invasive list.

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