A weed is a plant out of place. The weeds of natural areas often differ from those affecting agricultural fields. The ag weeds are frequently annuals which colonize bare ground. Weeds of natural areas are often perennials or biennials that are capable of invading established natural areas and are hard to get rid of.

Weed control is a critical, perhaps central, part of habitat restoration. Weeds are always with us, and weed seeds are in the soil. Opening up the habitat by removing established vegetation creates sunny open areas where weeds can flourish. The restorationist must be prepared to spend lots of time and money on weed control, especially in the early years of a project.

At Pleasant Valley Conservancy we have dealt with weeds in our prairie remnants, our restored oak savannas, and in the prairies we have planted in former agricultural fields. Each situation presents different problems and requires different solutions.

Shovels for weed control

Many weeds can be mowed or pulled by hand, but some of the most important weeds must be dug with a shovel. Although almost any shovel can do in a pinch, for extensive weed control, the type of shovel is critical. The photos here are of a shovel that we have found especially suitable for weed control in natural areas. The narrow blade causes less disturbance to surrounding plants. The "D" handle ensures a firm grip. The metal/composition shaft is strong and does not break when pried carefully.

The unmodified shovel is an Agrimaster that can be purchased at agricultural supply stores for around $20. The blade is cut down into the shape shown using a metal cutter such as an angle grinder. When the blade becomes dull it should be sharpened with a flat file or grinding stone. The design is modified by one devised by Nick Faessler of the Prairie Enthusiasts.

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Major weeds

Prairies and savannas under restoration are generally weed infested. The weed seeds were probably originally transported on the feet of grazing farm animals. During the many decades before restoration began, these weeds have had ample time to grow and flourish. Restoration enhances the opportunity for weed growth because of increased sunlight to the soil and the creation of bare patches. Any bare area is an invitation for weed growth. Although these bare areas should be immediately planted with native species, the weeds grow faster (which is why they are weeds). Thus, the restorationist can count on starting weed control almost immediately, and continuing it for the indefinite future (that is, for ever). It is vital to start weed control immediately, because uncontrolled weeds will produce copious seeds which will exacerbate the problem.

Many of the most serious weeds are roadside pests, and are spread by mowing equipment or by vehicles on the highway. Because of the opposition to herbicide use on highways, these weeds are much more serious now than they once were. They can still be controlled on highways by judicious and well-timed mowing, but due to budgetary constraints, weather, or other factors, they may not be mowed at the correct time. If a highway is mowed too late, seeds will have set and the mower will transport them further.

The Invasive Plants Association of Wisconsin (IPAW) has developed a list of the major weeds causing problems in natural areas in Wisconsin. From that list, I have extracted those that are most likely to cause problems in our savannas and dry to mesic prairies. They are listed in the table in order of our greatest concern. The last three in the table are not present at Pleasant Valley Conservancy, but are widespread in our area and could easily get established. We monitor for their presence continually.

Scientific name	Common name	Habitats	Growth form
Melilotus alba	White sweet clover	Prairies/savannas	Biennial
Melilotus officinalis	Yellow sweet clover	Prairies	Biennial
Pastinaca sativa	Wild parsnip	Prairies	Biennial
Cirsium arvense	Canada thistle	Prairies/savannas	Perennial
Lotus corniculata	Bird's foot trefoil	Prairies	Perennial
Verbascum thapsis	Mullein	Prairies/savannas	Biennial
Daucus carota	Queen Anne's lace	Prairies	Biennial
Torilis japonica	Japanese hedge parsley	Savannas/woodlands	Winter annual
Campanula rapunculoides	Creeping bellflower	Prairies	Perennial
	Minor problem weeds at Pleasant Valley
Cirsium vulgare	Bull thistle		Biennial
Nepeta cataria	Catnip		Perennial
Leucanthemum vulgare	Ox-eye daisy		Perennial
	Serious weeds but not at Pleasant Valley
Euphorbia esula	Leafy spurge		Perennial
Coronilla varia	Crown vetch		Perennial
Centaurea maculosa	Spotted knapweed		Perennial

 

All of the weeds on the above list are exotics, either of European or Asian origin. They were introduced either accidentally or intentionally in the 19th century, and are now well established throughout our range. Note that most of these are not the common agricultural weeds. The ag weeds are generally annuals, and rapidly colonize bare ground. They grow and set seed quickly, and then die back. The ag weeds are less serious problems now than they once were because of the use of Roundup-resistant crop varieties.

The biennial weeds grow the first year as inconspicuous rosettes. In order to form flowers they need a cold period. The second season they form flowers and seeds and then die back. Thus, if they are removed by pulling or digging before they set seed, their life cycle is broken. The biennial weeds can also be controlled by mowing, although the timing is critical. If the patch is mowed too early, the plants will resprout. If mowed to late, seed set will already have begun. If mowed at just the right time, resprouting will not occur and the roots will die. For very large patches, mowing is the only economical solution.

The perennial weeds are the most difficult to control. Once established, they continue to grow and make seeds indefinitely. Some of these perennials form rhizomes and spread not only by seeds but by underground growth. Eradication of perennials by digging or pulling is difficult or impossible. The most effective removal is the use of an herbicide. In the absence of herbicide, the only way to remove them is by "outcompeting" them with native plants. If the perennial weed is mowed at the time of flowering, when most of the nutrients are up in the stems, the roots can be starved and seriously set back. If "good" plants can be established in these areas, they will shade out the weed and further set it back. Mowing is not a quick solution, but over the years the weed may be gradually eliminated.

Sweet clover

White sweet clover. White sweet clover is the major problem in our prairie remnants. In some areas we also have yellow sweet clover, and sometimes both are present in the same unit.

Sweet clover, a non-native legume, is a biennial which grows the first year as a vegetative plant. In the second year, the plant flowers and sets seed. The second-year plants are often tall and bushy. Seeding is prolific and if the heads are not removed the seeds fall to the soil. The seeds are long-lived in the soil, so that once an infestation has occurred, it does not go away quickly. In contrast to many other invasive plants, sweet clover is actually favored by fire, which stimulates the seeds to germinate. Sweet clover is an aggressive plant and very difficult to eradicate.

Sweet clover became a problem on the south slope in 2001 and became worse in the next few years. Removing shrubs and trees and opening up the habitat to more sunlight promoted sweet clover development. Controlled burns stimulated seed germination and exacerbated the problem. Because of the desirable prairie vegetation, use of herbicide was not an option. Although labor intensive, hand pulling is the method of choice. Pulling works best if the ground is moist. Plants can be pulled as soon as they are visible, even before flowering, but they are most easily seen when in flower. It is important to pull up the whole plant, including the large tap root. If the root is too firmly entrenched to remove, a sharp shovel should be used, cutting the root off below the soil surface. Once the root is broken, the plant can be pulled. Pulling is a major activity for us from late May through July and we must return to each area at least once a week to get new growth.

For clover patches too large and formidable for hand pulling, we use a brush cutter with a sharp-toothed (triangular) blade, cutting at the time of peak flowering but before seed set has begun. We return to the area a few days after the cutting and pull any plants missed. Some cut plants will resprout from the roots and produce flowers. These are more difficult to remove because the resprouts are shorter than original plants and harder to see among the prairie vegetation.

Timing is critical. Once seed set begins, cut or pulled plants must be bagged and removed from the field. We place the plants in large garbage bags and take them to a landfill. (Composting should never be done!)

A strategy that has worked well is to pass through each prairie at least once a week, pulling all visible plants. If a crew of workers is available, they should spread out and move in a straight line through the unit. (The procedure is almost like a survey of a crime scene looking for signs of evidence.) The crew then moves in a swath through the unit and returns to the starting point in a second swath. This procedure is continued until the whole unit has been traversed. Further passes must be carried out on subsequent weeks, to remove stragglers. (In some of our most seriously infested areas, we are monitoring weekly from early June until the end of July.)

Because of the long viability of sweet clover seeds in the soil (up to 30 years), this species must be managed on a nearly continuous basis.

Fortunately, sweet clover is not a major problem in most of our savannas or planted prairies. Units 1, 2, 3, 5, 6, 7, and 9 are the most seriously infested, although small patches are present in savanna Units 8, 10, and 11. We also have minor infestations of yellow sweet clover in the Valley Prairie. The source here was almost certainly Unit 6, which is uphill of the Valley Prairie. Unit 6 is the only unit on the south slope that has yellow sweet clover.

Sweet clover control is a major but essential expense in prairie restoration. Also, once started the process must be continued, because if any sweet clover plants are left to set seed, the initial control efforts will have been wasted.

Since we began serious sweet clover control in 2001, we have seen a gradual reduction in intensity of the infestation. Areas that were once so bad they had to be mowed can now be hand-pulled. Areas that once required hours of hand-pulling can now be pulled in less time. However, every area where sweet clover was initially present still has a few stragglers that must be dealt with. We spend much time in June and July pulling sweet clover, and monitoring the sites.

Wild parsnip

Wild parsnip is also a biennial plant with a long tap root (see photo below). As with sweet clover, control is by hand pulling second year plants. An alternative to pulling is digging, using a small sharp hand shovel. Wild parsnip produces a chemical that causes a serious dermatitis to the skin, especially after it is exposed to sunlight. For this reason, it is essential to wear long-sleeved clothing and to avoid touching the plant with any part of the skin.

Parsnip appears somewhat earlier than sweet clover and its seed bank is not as long lived. Although we have brought Pleasant Valley Conservancy under control, we cannot relax our vigilance, because all of the neighboring fields in the area are heavily infested. We can anticipate new contamination every year.

Other weeds

Here is how we control the other weeds in the table above.

Weed map We have prepared a detailed weed map of Pleasant Valley Conservancy on which the locations of all of our weeds are shown. Every site at which we have ever found the weed is marked, and we use this map in our monitoring work. Near the dates when the particular weed are most likely to be visible, we visit these various sites, shovel in hand. We also survey the whole property as much as time permits, and record any new locations for each weed.

Canada thistle is mowed with a brush cutter at the time of early flowering. This is a clone former, and it is important to mow all the plants in the clone. Since this is a perennial, mowing must be done very year.

Bird's foot trefoil grows low to the ground and is difficult to find once the prairie grasses and forbs are high. We look for the bright yellow flowers hugging close to the ground. A large number of stems emanate from a single root stock, usually spreading in all directions. We gather all of the stems together and lift them up in a bundle, then dig the root out. The whole clump can then be removed. Those patches flowering early in the season can be sprayed with clopyralid (Transline).

Mullein (photo below) is primarily a problem in bare areas, such as former burn pile scars, or sites where trees have fallen. Early in the season the previous year's plants, visible as rosettes, can be sprayed with glyphosate, 2, 4-D, triclopyr, or clopyralid. By July the second-year plants have sent up tall flower stalks. These are easy to dig with the shovel shown at the beginning of this page..

Queen Anne's lace (photo below) is another biennial that can present problems. It is an invader of bare areas and can be a problem in prairie plantings. However, once the tall grass prairie gets established, Queen Anne's lace ceases to be a major problem, although if we have time we will dig or pull all plants seen. Large infestations can be mowed at the time of flowering.

Japanese hedge parsley is a so-called emerging invasive, and is being found in small amounts in many parts of southern Wisconsin. Unfortunately, we have had hedge parsley since we first started restoration, and it continues to persist. This plant is considered a "winter annual", which means that its seeds germinate in the fall of the year, form a small first-year plant, and then overwinter. Growth resumes the following year and by mid- to late July it is in flower. It is easier to hand pull than sweet clover and a shovel is rarely needed.

Creeping bellflower is a perennial with a persistent creeping rhizome system. It produces dark blue flowers which to the uninitiated seem attractive. Because of this, interest in controlling it is sporadic. In fact, it is often planted, either intentionally or accidentally (mistaken for the native American bellflower). Digging will set the plant back, but does not remove most of the rhizomes, which can resprout. Large patches can be sprayed.

Weedy areas at Pleasant Valley Conservancy

Oak savannas. Weeds are less of a problem in our oak savannas, probably because they have been shady for so long that the really invasive weeds, which are mainly sun lovers, have not become established. There are a few places where catnip (Nepeta cataria), an introduced plant, is well established. Mullein (Verbascum thapsus, photo below) is a frequent colonizer of bare areas, such as scars resulting from the burning of brush piles. There is also the occasional wild parsnip and bull thistle. We hand pull mullein, parsnip, and thistle. However, in our savannas, our principal problem is woody plants, honeysuckle, buckthorn, and prickly ash, as well as brambles (Rubus sp.) (see link for details).

Planted prairies. The weed load in our planted prairies is considerably less than in the remnants because the fields were sprayed with herbicide before planting. The principal problems have been ox-eye daisy (Leucanthemum vulgare), Canada thistle (Cirsium arvense), and Queen Anne's Lace (Daucus carota). We have been hand pulling ox-eye daisy in the prairie planted since 1998 (Toby's Prairie) and between this and the controlled burns, this weed is almost under control. In the prairie planted in 1999 (Pocket Prairie), ox-eye daisy has been hand pulled, or, in a few places where the patches were heavy, mowed followed by treatment of the resprouts with the herbicide (2,4-D). When using the herbicide, care was taken to confine the spray just to the new growth, spraying on a windless day and keeping the wand close to the grown to avoid drift. 2,4-D has the advantage that it does not kill prairie grasses. Queen Anne's Lace has been hand pulled or mowed. After five to six years, the load of these weeds is considerably less.

Canada thistle is a potential problem because it is a perennial that spreads by underground runners. Because it is spreading through patches of good prairie plants, we have used herbicide very selectively. We deal with it primarily by mowing with a brush cutter, being careful to avoid cutting desirable species. Between mowing and controlled burns we hope to keep it under control. Competition from prairie plants should help.