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.
Although most weeds are exotic species that have been introduced from outside North America, there are also native species that may be highly invasive and must be controlled.
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
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.
Herbicides for weed control
Herbicides play a central role in restoration ecology, although they must be used carefully and with full knowledge of their potential activities. Each herbicide sold comes with a label that provides essential information about the chemical, its activity, and precautions that must be taken when using it. It is mandatory that before using any herbicide its label must be read and understood.
Uses of herbicides for specific weeds and woody plants are given below and on other pages of this web site. This link provides a summary of the main herbicides used at Pleasant Valley Conservancy.
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.
||White sweet clover
||Yellow sweet clover
||Bird's foot trefoil
||Queen Anne's lace
||Japanese hedge parsley
||Minor problem weeds at Pleasant Valley
||Serious weeds but not at Pleasant Valley
||Native species that have been invasive at Pleasant Valley
||Pale Indian plantain
|Solidago canadense, altissimum,gigantea
||Canadian goldenrod group
Most 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.
The three listed weeds that are native species grow rapidly by rhizome extension and are capable of spreading rapidly into bare soil. Although not as difficult to control as the exotic weeds, they may still present problems under certain conditions.
All three of these native species have presented special problems at Pleasant Valley Conservancy.
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.
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
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. An alternate disposal method is to put the bags on a burn pile and burn them in the winter. (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. In certain "problem" areas, there is a resurgence of smaller flowering plants in September, so we return then for further monitoring. The important point is to never let seed formation occur.
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 were 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
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
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 gloves and 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 almost 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.
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 is 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
Canada thistle is a perennial with an extensive rhizome system and is one of the most difficult weeds to control. (It is probably for this reason that it is on the State list as a noxious weed.)
Most recommendations are for mowing
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
However, use of herbicide is essential for eradication Canada thistle. Our procedure is to revisit each clone about a month after mowing and spray all of the resprouts
with 2% Roundup (41% active ingredient or a generic equivalent).
Even after eradication, sites with Canada thistle are monitored yearly.
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
The best way to eradicate this plant is with herbicide. Since 2010 we have been controlling it very successfully by a special herbicide procedure. Since the stems of a bird's foot trefoil plant grow from a single tap root, the center of this tap root is given a brief "spritz" of 20% Garlon 4 Ultra in bark oil. This is all that is required to kill the plant, and damage of adjacent "good" plants does not occur. This procedure has been successful in eradicating bird's foot trefoil, although new plants still arise from the persistent seed bank.
For large plants, it may be necessary to gather outlying stems and trace them back to the central tap root. This can usually be done with a shovel.
At Pleasant Valley, bird's foot trefoil has been especially a problem in our upland planted prairies (Pocket, Ridge, and Toby's).
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, Garlon 3A, or Transline. Another procedure is to spray the center of the mullein plant with 20% Garlon 4 Ultra in bark oil (the same mixture used for basal bark treatment of woody plants).
the second-year plants have sent up tall flower stalks. These
are easy to dig with the shovel shown at the beginning of
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.
Outcompeting perennial weeds is a long-term solution. Once a prairie sod gets well established, and the weeds have been eradicated, most of the perennial weeds are unable to get established. However, the principal components of prairie sod are warm-season grasses, especially big and little bluestem and Indian grass, all of which require frequent fire. Annual burning of established prairies is strongly recommended.
Control of Smooth Brome (Bromus inermis)
Most agricultural areas in southern Wisconsin are overrun with smooth brome (Bromus inermis), a cool-season exotic European invader. Smooth brome reproduces both by seed and by rhizome, and can soon take over any abandoned field. At Pleasant Valley Conservancy all of our former agricultural fields had been left fallow long enough so that smooth brome dominated. Those fields that were to be planted to prairie were heavily herbicided so that all emergent vegetation was killed, but there would be underground rhizomes that survived, so that smooth brome eventually made an appearance. If the prairie became firmly established, smooth brome was usually out-competed, although some of the planted prairies faired better than others.
Where smooth brome developed, or persisted, a technique was used based on the fact that smooth brome develops before any prairie plants appear. Spraying at this time with glyphosate will kill any plants with leaves above ground, but will have no effect on other plants. Since glyphosate has no residual soil activity (it is inactivated when it touches the soil), prairie plants are not affected. Although timing of the spray process is critical, with careful attention glyphosate spraying is "specific" for smooth brome (or any other cool-season plants).
The use of this technique for control of smooth brome in planted prairies is discussed elsewhere in this web site.
The Pleasant Valley Road Cut When restoration began, the road cut, especially on the uphill side, of Pleasant Valley Road was virtually solid smooth brome. (Above the road cut, good prairie plants were present, and flourished as soon as fire was introduced.)
The co-op spray rig was quite suitable for spraying this road cut. The operator could raise the boom to avoid small trees or signs, and immediately lower it afterward. The whole road cut was sprayed in about 15 minutes, a task that otherwise would have taken hours.
|Spraying the road cut of Pleasant Valley Road to kill smooth brome (April 2005).
|Spraying was very effective. Smooth brome had died and left mostly bare soil. A week after spraying, the whole road cut was planted with a mixture of prairie grasses and forbs. Five years later, the site was dominated by little bluestem, and almost no smooth brome remained.
|The Pleasant Valley Road cut in the fall of 2010, five years after spraying. A fine stand of little bluestem dominates the site.
areas at Pleasant Valley Conservancy
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).
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
thistle (Cirsium arvense),
and Queen Anne's Lace (Daucus
carota). We have been hand
pulling ox-eye daisy in the prairie planted in 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