W.A. Skroch and D.A. Bailey
The use of wildflowers
in the landscape has increased since Lady Bird Johnson first promoted
them in the late 1960's and early 1970's. Wildflowers were further
popularized by the "Meadow in a Can" seed collections that were
marketed in the early 1980's. A number of books have been written
that describe methods for planning and planting wildflowers, however,
few recommendations are available regarding maintenance and long-term
weed management. In wildflower plantings, weed management is a complex
system that requires knowledge of the specific wildflowers and weeds,
environmental conditions, and control methods. Therefore, the objective
of this leaflet is to discuss weed management strategies that can
be applied to the planning, establishment, maintenance and renovation
stages of a naturalized wildflower planting.
In North America
extensive plantings on the highways have given wildflowers high
visibility and further promoted the concept for use in other landscapes
including golf courses, office parks and private residences. Naturalized
wildflower plantings such as meadows are being recognized and appreciated
as landscape designs. Wildflowers attract birds, butterflies and
other wildlife as well as provide color when planted in lieu of
large expanses of turf grass. When allowed to naturalize, they can
reduce the maintenance required by a more formal landscape design.
objective in planting a naturalized wildflower area is to develop
an attractive, permanent planting that will provide flowers year
after year with self-seeding annuals and perennials. However, wildflower
plantings that are left unmanaged will eventually revert to the
composition of plant species in the original plant community through
a process called succession. Succession is a gradual change over
time in the species composition of the plant community. The species
that appear/disappear during this time change can generally be predicted
based on historical knowledge of the planting site.
will essentially follow this successional process because they are
given very little maintenance after the initial establishment period.
In addition to successional changes, other we problems can be anticipated,
based on knowledge of the site selected. A primary source of weed
contamination is from seed that have been deposited in the "soil
seed bank" over a long period of time. These seed often remain dormant
for many years but will germinate when tillage lifts them to the
soil surface where moisture, air, and light are favorable for germination.
Research has found as many as 98 to 3,068 viable weed seed per square
foot in the top six inches of soil. This quantity of seed, if allowed
to germinate, would present a formidable weed population in a new
planting. Therefore, in selecting a site for a wildflower planting,
knowledge of previous weeds and past use of the land will allow
insight into potential weed problems in the future.
THE SITE FOR POTENTIAL WEED PROBLEMS.
wildflower site for potential weed infestations from the weeds present,
the soil seed bank, the surrounding area and anticipated successional
changes, is the first step toward developing a weed management strategy.
The current weeds on the site should first be identified. If it
is infested with difficult-to-control perennial weeds such as nutsedge
(Cyperus spp.) or white clover (Trifolium repens), it may be best
to select another site for the wildflower planting or weed pressure
may be too great for wildflower establishment. If the site must
be used, it may be advisable to spend several seasons dedicated
to controlling and removing as many perennial and reseeding annual
weeds as possible before planting wildflowers. Weed species and
populations in surrounding areas should also be considered in the
site selection process. Seed dispersal mechanisms that allow wind
and water to bring in seeds from outside the planting area will
add to the site's weed population. Germination times, such as fall
or spring for annual weeds, will also affect wildflower growth if
weeds are established and large enough to compete when the wildflowers
germinate. After potential weed species and their sources have been
identified, their growth habits and characteristics should be studied.
Knowledge of these weeds is essential in determining methods of
control. (Weed identification references are included in the Reference
THE DESIRED WILDFLOWER SPECIES.
The second consideration
in the planning phase is the selection of the wildflower species.
The species planted will affect weed management at the site. The
objective in selecting a variety of wildflowers is to develop a
plant community that will be attractive and compatible in terms
of growing requirements, that will flower over extended periods
of time, that will reseed the site for generation of new plants,
and that can be held and maintained at the flowering successional
stage. In order to be successful in a particular location, the wildflower
species must be compatible with and adaptable to growing conditions
at the site. They must also be competitive with other species present,
including weeds. Native species generally are most adaptable to
local growing conditions and are usually the most competitive with
native weeds that will be present.
species that are to be included in a wildflower seed mix, consideration
should be given to the germination times and growth characteristics
of the species. Most wildflower mixes are a combination of annuals
for flower color the first year and reseeding annuals or perennials
for flowers in the second and succeeding years. The objective in
using a mix is to develop a wildflower community of compatible species.
Species that germinate and emerge rapidly after planting will become
better established, provide ground cover and, consequently, will
help reduce the number and growth of weeds. Early establishment
allows the wildflowers to develop better root systems and to capture
available resources such as water and nutrients so they can successfully
compete with weeds that germinate at later times. A careful selection
of wildflower species will help with weed management at the site.
(The Reference Section includes resources for selecting wildflower
PREPARATION AND ESTABLISHMENT
The second stage
of weed management in wildflowers is site preparation and establishment
of the planting. Fall (October/November) is the recommended planting
time for wildflowers. Generally, the soil is cool (40-60 F) and
moist at this time of year which promotes seed germination and establishment.
Winter annuals and some perennials will germinate and overwinter
as small seedlings. In addition, the soil is cool enough to hold
summer annual seed in a dormant state for early spring germination.
The most effective weed management during site preparation is to
kill as many weeds and viable weed seed as possible to prepare the
site for wildflower planting. There are essentially two methods
that are effective: 1) the use of a systemic herbicide and 2) fumigation.
A. SITE PREPARATION
control with a systemic, non-residual herbicide such as glyphosate
(Roundup (TM) and other trade names), is the approach most often
used in establishing . Glyphosate is often thought to kill all plants,
however, it has selectivity for some plants depending on the time
of year it is applied and the growth state of the plant. Proper
timing of the application is, therefore, very important to achieve
maximum weed control results.
In order to
plant wildflowers in October or November, site preparation must
begin in late summer (August/September). Total preparation time
is approximately four to six weeks using the following method:
- Be sure
the area has NOT been mowed so grass and weeds will be the proper
size at the time of spraying as specified on the glyphosate label.
- Spray only
when the plants are dry. When the site meets these conditions,
uniformly spray the area for coverage but do not wet the plants
- Allow at
least six hours of drying time for maximum plant kill with glyphosate.
The timing of this first application should coincide with active
weed growth because the optimum susceptibility to glyphosate for
perennial weeds is when they are actively growing. In addition,
they are most susceptible when they are not stressed.
- Seven to
ten days after the initial spray when the grass and weeds are
dying, till the site and prepare it for final planting. This should
be accomplished by mid-September for planting in early October.
- If weeds
emerge, re-treat with glyphosate two to four weeks after the final
tilling and bed preparation. Tilling will bring new weed seed
to the surface. The soil will be warm and if sufficient moisture
is present, many annual weed seed may germinate. These seedlings
will die within seven days after treatment and the site will be
ready for planting.
B. SITE PREPARATION
A second technique
for site preparation and establishment of wildflowers is the use
of fumigation. Fumigants kill most weeds and dormant weed seed except
those with hard seed coats such as Carolina geranium (Geranium carolinianum),
white clover, and nutsedge. Fumigation is a temporary weed control
method and new weed seed will germinate as they are introduced into
the planting area. The advantage of fumigation is that wildflowers
become better established prior to weed emergence. Research has
shown that fumigation can result in increased plant size over non-fumigated
sites. Although increased size can benefit wildflower growth, the
cost off fumigation is high and must be considered.
There are several
fumigants available for use in wildflowers including methyl bromide,
metham and dazomet. Methyl bromide is a gas and for large areas
requires the use of specialized equipment and a licensed applicator.
Metham (Vapam and Sectagon) are water soluble liquids and are less
active than methyl bromide, but they are easier to use. Dazomet
(Basamid Granular) is a granular product that is most effective
in cool soil temperatures. The metham and dazomet fumigants remain
in the soil for longer periods of time than methyl bromide. To ensure
the soil is safe for planting a soil bioassay should be conducted
after any fumigation. However, because of the potential for prolonged
soil activity with metham and dazomet, a bioassay is essential when
these fumigants are used. If the soil still contains the fumigant,
planting wildflower seed will result in the death of the seed.
is chosen for preparation of a wildflower planting site, the site
should be tilled and prepared for planting in late summer to early
fall (August/September). Fumigation requires adequate soil moisture
for movement in the soil and is generally very effective in September
to early October when soil temperatures are still high. The site
can be planted when the bioassay shows the soil is safe for planting
(late October/ November).
time, loosen the soil surface slightly to enhance seed contact.
At the time the soil surface should be disturbed as little as possible
because most annual weed seed that germenate are within the top
one-fourth inch of soil. If the soil is deeply disturbed, additional
weed seed that are capable of germinating and competing with the
wildflowers will be moved to the soil surface.
wildflower seed evenly in the planting area to give each seed adequate
space and resources for germination. Even seed distribution allows
maximum coverage of the soil by new seedlings and provides the most
benefit in weed suppression. After sowing, either tamp the soil
or water the area to ensure good seed-soil contact. A light, seed-free
mulch, such as wheat straw, can also be used for seedling protection.
The mulch should be lightly and evenly dispersed with no heavy clumps.
A heavy, uneven mulch reduces wildflower growth and development
and results in areas with no plants.
The third and
final stage in the development of a weed management strategy for
wildflowers is to develop both short and long-term maintenance programs
The best approach
for developing these programs is to:
weed problems, such as successional changes, before they occur;
seedlings quickly as they grow
- take corrective
weed control action as soon as possible. In general, weed seedlings
are easier to control before they mature and establish good root
systems. Optimum size for best control is usually four inches
or less. Early control will reduce weed competition with young
wildflower seedlings giving them maximum growth conditions.
After a wildflower
planting is established, there is no single approach or "magic formula"
to manage encroaching weeds. The problem is further compounded by
the fact that most wildflower plantings are a mix of species, thereby
reducing the number of herbicides that can be safely used for weed
control. Similar plants, such as those in the same family, generally
have similar tolerance to selective herbicides. For example, many
popular wildflowers are in the Compositae family and so are many
of the troublesome weeds. As a result, if an herbicide is safe for
the wildflower, it will probably have no effect in controlling closely
related weeds. With a mix of wildflowers from various families,
it is unlikely that one herbicide will be completely effective for
weed control without damaging some wildflowers. Therefore, weed
management programs must be developed to use a combination of cultural,
chemical and mechanical weed control techniques.
plantings are not static but change annually and seasonally due
to the mix of species. A good mix will provide flushes of seasonal
flowering from spring to fall. Annual changes will occur based on
the number of perennial plants and those species that have reseeding
potential and the extent to which that potential is realized. Reseeding
is essential for natural regeneration of plants in order to perpetuate
and extend the life of the planting. Weed control techniques should
be carefully selected in order to optimize wildflower reseeding
potential at the site.
weed management approach that incorporates multiple weed control
techniques and is site specific will be the most effective for wildflowers.
Avoidance of the weed problem by preventing regeneration is often
easier than trying to establish weed control at a weed infested
site. In selecting the techniques to use, remember that small seedlings
are the easiest to control. Also, control of weeds at the site will
prevent weed seed dispersal and the development of other weed propagation
structures such as rhizomes and tubers. Good control will minimize
future weed pressure. A range of weed control techniques are discussed
below. The manager must evaluate the site on a continuing basis
and select those techniques that will be most effective on a long
term basis for a given location.
of weed control that is often overlooked is competition. Seedlings
that emerge first are often able to capture more space and resources
and this gives them a competitive edge over latter germinating seed.
Young plants are the most susceptible to damage from weed competition.
Therefore, if wildflowers are planted in a properly prepared, weed-free
site, they will be quicker to germinate and establish than weeds.
Early development of the wildflower canopy in the spring will also
help suppress weed growth. An even distribution of seedlings at
this stage will allow maximum growth of the wildflowers. If the
wildflowers are heavily planted, they will compete with themselves.
This will cause dieback and open areas within the site which promotes
weed establishment.. Understanding weed and wildflower biology is
essential in order to make maximum use of growth characteristics
for weed suppression.
plantings are maintained with an annual mowing. Mowing prevents
development of pines and hardwood trees and arrests the successional
development at the herbaceous plant stage.
be timed to meet three objectives:
- to removal
weeds before they flower and develop viable seed;
- to disperse
wildflower seed for reseeding within the site; and
- to remove
dead plant material and improve the appearance of the planting.
Mowing is an important management tool and timing is essential
to maximize weed control and wildflower reseeding.
C. HAND PULLING.
is a viable alternative for weed control in small wildflower planting
sites. Any weeds that are pulled from the site before seed mature
and disperse will contribute to future overall weed control. One
weed can produce thousands of seeds. Therefore, even on the smallest
scale, the contribution of hand pulling should not be overlooked.
control programs are based on herbicide selectivity and plant tolerance.
The objective with chemical control is to find an herbicide that
wildflowers can tolerate but which is detrimental to the weeds present.
As discussed earlier, similar plant species have similar tolerance
to herbicides. For this reason, herbicides are often classified
as broadleaf and grass (graminicide) herbicides based on the weeds
they control. Herbicide labels list the weeds that are controlled
as well as tolerant, desirable species. At present, there are very
few wildflowers included on broadleaf herbicide labels. Therefore,
there are no broadleaf herbicides that are known to be safe for
a wide range of wildflower species. On the other hand, most weedy
grasses can be controlled in wildflowers with a graminicide such
as sethoxydim (Vantage, Poast) or fluazifop (Ornamec, Grass-B-Gon)
without damaging the broadleaf wildflowers. However, use of a graminicide
will also control any native or ornamental grasses that may be included
in the wildflower planting.
to the broadleaf and grass classification, herbicides are also categorized
by their activity as either preemergence or postemergence herbicides.
For the purposes of maintaining established wildflower plantings,
preemergence herbicides would be applied to the soil prior to the
emergence of the target weeds. Postemergence herbicides are applied
after the emergence of the target weeds.
products are applied uniformly to the soil to prevent germination
and growth of the weeds controlled by the herbicide. This will require
tolerance by all the wildflowers in the planting or it may damage
susceptible species. In an established bed, these herbicides may
be safe for existing plants. However, there is little known regarding
the effects of preemergence herbicides on the reseeding potential
of wildflowers. If the preemergence herbicide is detrimental to
the wildflower seeds, regeneration of new wildflower plants would
be reduced. In a planting with a good, established population and
a growing weed population, the use of a preemergence may be warranted
to reduce the weeds, even though reseeding will be reduced for a
period of time. Managers should consider these possibilities when
making the decision to use preemergence herbicides.
herbicides are usually sprayed on actively growing weeds. One way
to circumvent damage to the wildflower planting is to select a method
for application that directs the herbicide to the targeted weeds
and avoids the wildflowers.
application methods can be used for this purpose:
- Spot spray
- A small sprayer is used that can be directed to individual or
small groups of weeds so the spray will not contact desirable
- Wipe-on application
- An application device is used to wipe the herbicide on weeds
that are taller than the wildflowers. Equipment may be either
a rope wick or roller type applicator. This technique may be particularly
useful for reducing tall weeds such as horseweed. Inexpensive
hand wicks are available.
- Hand-held clippers that dispense a thin layer of herbicide onto
the blade can be used to selectively cut weeds. This method applies
the herbicide to the cut surface of the stem so it is translocated
into the weed to kill the total plant.
any chemical control program, the user has the responsibility yo
determine the most effective herbicide to use, timing of the application
and the proper rates. Herbicide labels must always be carefully
read and followed for the best and most cost effective results.
produce chemical compounds that inhibit the growth of other nearby
plants (allelopathy). The toxic substances may be released from
the roots or leaves and through plant residues on the surface that
are leached into the soil with rainwater. Weeds are known to have
allelopathic (detrimental) effects on some crops and vice versa.
Some of the weeds that are known or suspected to be allelopathic
are bermudagrass (Cynodon dactylon), johnsongrass (Sorghum halepnese),
yellow and purple nutsedge, pigweed (Amaranthus spp.) and sunflower
(Helianthus spp.). Horseweed is known to be allelopathic to itself.
This most likely is the reason it declines in the successional changes
in North Carolina's Piedmont. However, even though it will decline
over several years, if it is left in most wildflower plantings,
it will severely reduce the wildflower population by out-competing
more desirable plants.
allelopathic effects will never become a total weed control method
for wildflowers. However, by observing differences in wildflower
plantings and surrounding areas, managers may see differences in
plant growth that can be effectively used in developing a weed management
decline over time because many herbaceous perennial plants often
have a limited life span. Decline may also be attributed to weed
pressure, allelopathy, site, and environmental growing conditions.
Renovation of an existing planting may be more cost effective than
starting with a new bed if the weed pressure is first brought under
New seeds can
be planted into existing wildflower plantings that are relatively
weed free. The soil should be disturbed as little as possible to
prevent additional weed seed germination. This can be accomplished
either by raking or using a slot seeder. Since the seed cannot be
tamped without damaging existing plants, water them in to provide
adequate soil contact. Renovation should be completed following
the same timeframes used for initial seeding.
is an ongoing challenge. As wildflowers become more popular, additional
knowledge and experience will generate improved management techniques.
The primary objective for wildflower plantings should be a naturalized
planting with seasonal changes and interest rather than a weed-free
groomed area. This can be achieved with properly applied weed management
and E.B. Smith. 1981. Weeds of Arkansas - A Guide to Identification.
Cooperative Extension Service, University of Arkansas, Fayetteville,
and A.E. Smith, Jr. 1990. Evaluation of Wildflower Plant Species
and Establishment Procedures for Georgia Road Sites. GDOT Research
Project No. 8604 Final Report. University of Georgia, GA.
1986. The Wildflower Meadow Book. East Woods Press, Charlotte, NC.
Department of Transportation. 1989. Wildflowers on North Carolina
Roadsides. N.C. Department of Transportation Landscape Unit, Raleigh,
Botanical Garden. The University of North Carolina. Box 3375, Totten
Center. Chapel Hill, NC 27599-3375.
1985. Growing and Propagating Wild Flowers. The University of North
Carolina Press, Chapel Hill, NC.
and J.F. Derr. 1992. Weed Control Suggestions for Christmas Trees,
Woody Ornamentals, and Flowers. N.C. Cooperative Extension Service
Publication AG-427. N.C. Cooperative Extension Service, N.C. State
University, Raleigh, NC 27695-7609.
T.J. Monaco and A.D. Worsham. 1989. Identifying Seedling and Mature
Weeds Common in the Southeastern United States. AG Ex nsion Bulletin
208. The N.C. Agricultural Extension Service, N.C. State University,
Raleigh, NC 27695-7603.
Editor. 1984. Landscaping with Wildflowers and Native Plants. Ortho
Books, San Francisco, CA.
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