Invasive plants (weeds) eradication (1): biennials

This is the first of two posts on weed control. It deals with biennial plants. The second post on perennials will be made next week. The table at the end of this post provides a survey of all the invasive plants that are potentially a problem in the Upper Midwest area.

It seems unfair that as soon as nice plants are blooming in our prairies and savannas that invasive plants are also thriving. Unfortunately, we can’t let the invaders get ahead of us!

Biennial plants grow from seed their first year but do not flower. The cold period during overwintering induces flower bud formation and the second year they send up flower stalks and set seed.

Whether biennials are a problem in restoration ecology depends to a great extent on whether a seed bank exists and how long their seeds remain alive in the soil. Although there is little information on seed bank longevity, for many purposes it may prove best to assume that the life in the seed bank is extended. Thus, it makes sense to always assume that there will be an overwintering seed bank and that new plants will be appearing the following growing season. If they don’t show up, consider that a bonus!

The control cycle for a biennial like garlic mustard runs something like this:

First-year plants

1. First-year plants at the seedling stage may be visible sometime in April-May. While they are still at the cotyledon stage they can be killed by fire. This is best accomplished with a propane torch carried on a backpack.
2. First-year plants past the cotyledon stage are generally mixed in with native species and will be hard to find until late in the summer/early fall. They remain green long into the fall, making them visible when native species have senesced and turned brown (late November or anytime in December before snowfall). Since there are no green native plants at this time, spraying with glyphosate is preferred since this herbicide has no soil residual. The plants will not show any response to the herbicide, but will not come up the following spring. This is a very effective time to deal with first-year plants since all first-year plants controlled in the fall will not have to be dealt with the following year.

Second-year plants

1. Most effort on garlic mustard focuses on second-year plants. Spray new growth in early spring as soon as the plants are large enough to find, using a broad-leaf active herbicide.
2. The earlier spraying can be initiated, the better.
3. Return at weekly intervals, or more often if good growing conditions exist (depending on the weather).
4. Flowering plants that were missed, or flowering plants discovered too late to spray should be mowed with a brush cutter or hand-pulled before they set seed. Arguments exist about whether pulled plants will go on to make viable seeds if they are just laid on the ground. Most cautious people assume that flowering plants will go on to set seed and thus bag them for disposal in a land fill.
5. Once plants start to form seeds they should definitely be bagged.
6. The area should be canvased at weekly intervals until the end of June, removing all flowering plants.
7. It is vital to prevent any plants from making and dropping viable seeds.
8. Experience has shown that eradication of garlic mustard requires many years, but it can be done.

Wild parsnip

Wild parsnip is generally easier to eradicate than garlic mustard. The same techniques are used, although the timing is different.

Sweet clover

            Sweet clover is probably the most expensive weed that we deal with in prairie restoration work. In contrast to garlic mustard, the control cost per person hour per acre is quite high, and most infested areas are almost impossible to eradicate. It is found primarily in prairie remnants, as a legacy of their former use as pastures. The seed bank can remain alive for many years, and seed germination is stimulated by fire. I discuss the history of sweet clover in the UpperMidwest in this post:

First-year plants

1.     Very difficult to find early in the growing season, as they are small and delicate.

2.     In late fall, after native plants have senesced, they will still be green and can be sprayed with glyphosate. This is very effective, but areas sprayed must be monitored in subsequent years.

Second-year plants

1.     Begin control as soon as significant patches are found, usually mid-June or early July in the Upper Midwest. From then on, sweet clover will probably continue to appear throughout the summer.

2.     Scattered plants should be hand-pulled or dug using a shovel (a Parsnip Predator is ideal). The whole site should be canvassed biweekly until fall.

3.     Sweet clover is often found in large patches, even up to several acres, making hand pulling not really an option. Wait until all plants are in flower and mow, either with a brush cutter or a tractor. Timing of mowing is critical. If too early, plant stubs can resprout and flower. If too late, the cut flowers lying on the ground can set seed.

4.     Return to mowed areas within a week or two and hand pull those plants that were missed.

5.     Sweet clover forms a long tap root, and when hand-pulling, it is important to get the whole root. There are dormant buds at the base of the stem which will start growing if the stem is broken without getting the root.

6.     A strategy that has worked well is to pass through each unit at least once a week, pulling all visible plants. In seriously infested areas, monitoring weekly from early June until the end of July, then returning in September when there is often a resurgence of smaller flowering plants. Never allow seed formation to occur.

7.     Once sweet clover control has started, it must be continued annually, because if any sweet clover plants are left to set seed, the initial control efforts will have been wasted.

Japanese hedge parsley

About 20 years ago this was considered a newly emerging noxious weed. Unfortunately, it is now well established, at least in the southern part of the Upper Midwest. Hedge parsley is probably more difficult to eradicate than garlic mustard, but is less noxious and hence is often overlooked.

Hand-pulling is the preferred control method although spraying with a broad-leaf-specific herbicide will also work.

The severity given in the table below is for prairies and savannas. Wetlands are different!

Latin name	Common name	Growth	Severity]	Control methods
Alliaria petiolata	Garlic mustard	Biennial	High; mainly in shadier areas	Spring and fall spray; hand pull; don't let it get started!; eradication takes years
Arctium minus	Common burdock	Biennial	Low	Spring spray with broad-leaf-active herbicide
Cirsium vulgare	Bull thistle	Biennial	Low	Hand pull, dig or spray with broad-leaf-active herbicide
Daucus carota	Queen Annes lace	Biennial	Moderate; generally outcompeted in well established prairies	Hand pull, mow, or spray with broad-leaf-active herbicide
Dipsacus laciniatus	Cut-leaved teasel	Biennial	Low	Hand pull, mow, or spray with broad-leaf-active herbicide
Dipsacus sylvestris	Common teasel	Biennial	Low	Hand pull, mow, or spray with broad-leaf-active herbicide
Hesperis matronalis	Dames rocket	Biennial	Low	Hand pull, mow, or spray with broad-leaf-active herbicide
Melilotus alba	White sweet clover	Biennial	High	Hand pull, mow, or spray with broad-leaf-active herbicide first-year plants in fall; long-life seed bank; eradication very difficult
Melilotus officinalis	Yellow sweet clover	Biennial	High	Hand pull, mow, or spray with broad-leaf-active herbicide first-year plants in fall; long-life seed bank; eradication very difficult
Pastinaca sativa	Wild parsnip	Biennial	High	Hand pull, mow, or spray with broad-leaf-active herbicide; can be eradicated with hard work
Verbascum thapsus	Mullein	Biennial	Moderate; establishes on bare ground; generally outcompeted in well established prairies	Hand dig or pull; late fall spray with broad-leaf-active herbicide or glyphosate
Torilis japonica	Japanese hedge parsley	Biennial or winter annual	Moderate; mainly in shaded areas	Hand pull, mow, or spray with broad-leaf-active herbicide; difficult to eradicate
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