Eradication of invasive plants (2): perennials

This is the second of two posts on invasive plant control. Last week’s dealt with biennials.

The table at the end provides an overview of key perennial weeds and a summary of control methods.

Perennial weeds may start as seeds or from underground roots or rhizomes. The life span of perennials varies widely. Some last only a few years whereas others may last indefinitely. Because of their root systems, it is unlikely that a perennial will be eradicated by digging the plant up. Thus, permanent control of perennials almost always requires the use of an herbicide.

Eradication of most perennial weeds requires great effort and extended time periods, generally over a multi-year period. Neither fire nor mowing will eradicate any perennial weed, although these procedures may find good use along with herbicide.

Most of the weeds in the table are dicots and hence susceptible to broad-leaf-specific herbicides such as 2,4-D, clopyralid (Transline), metsulfuron methyl (Escort), and triclopyr (Garlon). The one monocot in the table, reed canary grass, is controlled by a grass-specific herbicide such as clethodim (Intensity). Under certain conditions it may also be possible to use a nonspecific herbicide for these weeds such as glyphosate (Roundup). Details on the manner of use are given in the herbicide label specifications.

In the rest of this post, I give our own experience for successful eradication.

Spotted knapweed This is more of a problem in dry, sandy areas. However, when our gravel road was redone about 6 years ago, the contractor brought in gravel that was contaminated with seeds of this species. Each year for the next few years we waited until it was in flower and then spot-sprayed each flowering plant along the whole roadside with aqueous Garlon 3A as a foliar spray. This was effective and by year 4 this species had been eradicated.

Spotted knapweed growing in contaminated gravel brought in for road repair

Canada thistle This is one of the few weeds that is listed by the State of Wisconsin as a noxious weed. As far as I know, it is the only thistle species that is a perennial. It forms an extensive underground rhizome system. However, it does not compete well with prairie plants so that it is often only a problem in the early years of a prairie restoration. In the early years of our restoration work, small or larger patches of this weed popped up in planted prairies or in open savannas. Some of the smaller (i.e. less than 6 feet) patches could be controlled just by mowing consecutively for several years at the time of flowering. Larger patches were controlled by mowing at flowering time and returning later to spray the resprouts with glyphosate or triclopyr. We haven’t seen Canada thistle at PVC for at least six years.

Crown vetch This is a very aggressive plant which is very difficult to control. It has an extensive rhizome system and the seeds, like many other legumes, are very long-lived in the soil. The key is to not let it get started. US Highway 18/151 is heavily infested (probably planted when the road was rebuilt) and it is now moving off onto some of the nearby county highways. I watched one nice prairie remnant over which I had no control get taken over by crown vetch over a period of ten years, despite frequent burns. At PVC we had a single patch of crown vetch pop up on the lower part of our south-facing slope. I have no idea where it came from. It is easy to spot when flowering, and I sprayed it immediately with Garlon 3A. It quickly died and has so far not returned.

Leafy spurge This is another plant that is troublesome along roadsides and is a potentially serious invader of natural areas. It has a deep tenacious root system and spreads rhizomatously. Fortunately, we have never had to deal with this nasty plant, although we see it spreading along US 14 between Middleton and Cross Plains. Currently, the recommended herbicide for leafy spurge is imazapic (Plateau), spraying twice in the growing season, late spring (before seeding) and in the fall just before a killing frost.

Birds-foot trefoil This legume was once planted extensively agriculturally and because of its long seed life (as bad as sweet clover!) it can be a problem in prairies that are being established on former cropped fields. It is not rhizomatous but forms a deep tap root from which stems arise that spread out along the top of the soil. Because of its low-growing character, it is hard to see until it flowers. We have been dealing with this in Toby’s, the Pocket, and Ridge Prairies since they were first established. Our procedure is to find the center of the plant and give it one or two brief spritzes with 20% Garlon 4 in oil. The plant should be dead within a week. Details of its ecology and control can be found in my earlier post.

Reed canary grass (in upland sites) Reed canary grass is a major problem in wetland systems but does occur occasionally in upland sites. We have been dealing with this in some of our savannas for at least 15 years. It generally occurs as relatively small patches which can best be found at the time of flowering. Spray each patch with Intensity. If that herbicide is not available an alternative is to cut with a hand clippers each plant about six inches to a foot above the base, tie the stems together in a bundle, and use a spray bottle to treat the cut stems with 50% Roundup. This is a fairly time consuming procedure but is effective.

Motherwort This perennial is primarily a problem in heavily disturbed areas. It appears fairly early in the spring, before most native species are still dormant, and should be sprayed with foliar glyphosate or a broad-leaf specific herbicide.

Latin name	Common name	Growth	Severity (1)	Control methods
Centaurea maculosa	Spotted knapweed	Biennial or short-lived Perennial	Especially dry prairies and sandy habitats; more serious in western states; don't let it get started!	Spring and fall spray with broad-leaf-active herbicide
Cirsium arvense	Canada thistle	Perennial	Moderate; generally outcompeted in well established prairies; don't let it get started!	Spring spray with broad-leaf-active herbicide; summer mow; fall spray with broad-leaf-active herbicide
Coronilla varia	Crown vetch	Perennial	Now a potentially serious invader of prairies and savannas; don't let it get started!	Spring and fall spray with broad-leaf-active herbicide
Euphorbia esula	Leafy spurge	Perennial	Now a potentially serious invader of prairies and savannas; don't let it get started!	Spring and fall spray with broad-leaf-active herbicide
Leonurus cardiaca	Motherwort	Perennial	Low	Broad-leaf herbicide
Lotus corniculatus	Birds-foot trefoil	Perennial	Moderate	Broad-leaf herbicide; longlife seed bank; difficult to eradicate
Lythrum salicaria	Purple loosestrife	Perennial	Low	spray with broad-leaf-active herbicide when found
Phalaris arundinacea	Reed canary grass	Perennial	Low	Spring and fall spray;summer cut followed by spray of cut stems
Trifolium repens	White clover	Perennial	Low; outcompeted in established prairies (except along edges)	Spring spray with broad-leaf-active herbicide
(1)    Severity refers to prairies and savannas only. Wetland habitats will be different.