Integrated Pest Management Manual
Weeds of Developed and Historic Sites
Weeds
are usually described as any plants growing where they
are not wanted. Any undesirable grass or broad leafed
plant species, from a small herbaceous plant to a woody
shrub, vine, or tree, may be considered a weed if it is
growing in a landscape bed or in a structure.
Dicotyledonous (broad-leaved) plants are those that have
two cotyledons in each seed. They are characterized by
broad leaves and often have woody stems. Some species
(e.g., sunflower) only become woody in old parts of stems
and roots; these are referred to as semi-herbaceous dicots.
Most weeds have little or no woody tissue (e.g., plantain,
dandelion) and are herbaceous.
Grasses are members of the plant family Gramineae. All
grasses are monocotyledonous and have long, narrow leaves
with parallel veins and fibrous root systems. Some grasses
produce underground stems called rhizomes (e.g., Kentucky
bluegrass, quackgrass) or aboveground runners called stolons
(e.g., creeping bentgrass), while others produce both
(e.g., bermudagrass).
It is important to understand the distinction between
monocots and dicots since the selectivity of many herbicides
is based on which type of plant they kill. Thus many herbicides
which would kill a dicotyledonous weed in turfgrass, which
is a monocot, could not be used in a landscaped area since
woody landscape plants are also dicots.
The description of each potential weed species is beyond
the scope of this report. Contact the Cooperative Extension
Service of the agricultural university in your state or
your regional Integrated Pest Management coordinator for
specific information on the most important weeds in your
region. A list of useful pictorial weed guides is included
in the reference section.
IDENTIFICATION AND BIOLOGY OF WEEDS OF DEVELOPED AND
HISTORIC SITES
Undesirable plants (weeds) invade man-made environments
such as landscape beds wherever they are established.
Weeds are often found where soil has been exposed or disturbed
by compaction, planting activities, or maintenance activities.
They also occur where the turfgrass or groundcover is
weakened by adverse environmental conditions, diseases,
or pests to the extent that it cannot compete for nutrients,
water, or light with weed species. Weeds are very common
where the grass or groundcover species being grown is
not well-adapted to its environment.
Life cycles
The life cycles of weeds can be grouped into the following
major types.
Summer annual weeds. These weeds grow each spring
or summer from seed. Examples include prostrate spurge,
ragweed, large crabgrass, and goosegrass. They mature,
produce seeds, and die in one growing season. Seeds generally
overwinter before germinating the following spring. The
majority of annual weeds are of this type. Some annuals,
such as crabgrass, can root from leaf-stem junctions,
forming dense colonies.
Winter annual weeds. These weeds (e.g., henbit,
shepherdspurse, annual bluegrass) germinate in the fall
or late winter from seed, mature and produce seed during
the following spring, and die in early summer. Seeds of
most of these species are dormant during the spring.
Indeterminate annual weeds. These weeds, such as
chickweed and annual bluegrass, can germinate and grow
during most seasons in certain regions.
Biennial weeds. These weeds may germinate at any
time during the growing season. Examples include wild
carrot, bull thistle, and mullein. They usually produce
a radial cluster (rosette) of leaves lying close to the
soil during the first season. In the second year they
produce flower stalks (using food stored from the first
season's growth), produce seeds, and die.
Perennial weeds. These plants live for three or
more years. Some species may not flower the first year,
while others may produce mostly nonviable seeds. Many
perennials (e.g., curly dock, dandelion, and common milkweed)
spread primarily by producing seeds, while others (e.g.,
field bindweed, white clover, silverleaf nightshade, bentgrass,
and quackgrass) spread both by seed and vegetatively.
The latter can occur by rhizomes, stolons, tubers, or
rooting of stem nodes that touch the soil.
The seasonal abundance of weeds is related to their specific
life cycles. Summer annuals grow from spring until fall,
then are killed by low fall or winter temperatures. Winter
annuals grow from fall to late spring, so they are usually
not found during the summer. Biennials grow during the
spring, summer, and fall of their first year, survive
over the following winter, and flower during the next
growing season. Therefore, some biennial stages are likely
to be present at any time of the year. Perennials grow
during each growing season. Their aboveground structures
may die over the winter (e.g., yellow nutsedge) or may
remain viable but dormant.
Knowledge of the life cycle of a particular weed species
is an important part of its management. For example, mowing
a patch of annual weeds to remove the flowers can prevent
seed set. Refrain from cultivating areas where there are
high populations of weeds that reproduce by rhizomes;
this cuts the rhizomes into pieces and each piece can
generate a new weed plant.
Impact of Weeds
The most obvious impact of weeds on turf areas is the
competition and replacement of desired plants by weed
species. In the case of weeds that overgrow an area and
then die, such as crabgrass and knotweed, unsightly dead
areas can be created. This often leads to the necessity
for increased expenditures for turf maintenance. In landscape
beds, weeds can grow among desirable plantings or among
groundcovers and create an unsightly nuisance. This can
lead to the need for hand weeding, which entails a high
labor cost.
Toxicity to humans and animals is also a consideration.
Some common weeds are poisonous if consumed (e.g., black
nightshade, pokeweed, poison hemlock, and Johnsongrass);
cause inflammation when touched (e.g., stinging nettle,
poison ivy, oak, and sumac); or cause allergic reactions
(e.g., common ragweed, goldenrod). Visitor injury or annoyance
can result from bees or wasps seeking nectar from some
weeds. Furthermore, many weed plants or their seeds have
spines, thorns, or burs which can have similar allergic
effects.
Weedy areas provide habitat for beneficial insects but
may also attract rodents and arthropods such as rats,
ticks, and fleas that might attack humans and domestic
animals or carry diseases which will affect humans and
domestic animals. Weeds can also serve as hosts for some
fungal pathogens and insects which might attack desirable
plants.
Weeds can also grow large enough to cover signs, block
trails, or obstruct historic landscapes or vistas, interfering
with visitor use of the park. Weeds that grow on buildings
can cause structural damage if they grow into cracks in
mortar or bricks; sometimes they will stain buildings
as well.
Weed Habitats
Two habitats will be considered in this report; landscaped
areas (where natural vegetation has been replaced or augmented
with other plants, usually for aesthetic purposes) and
buildings. Weeds growing in landscaped areas are found
where soil has been exposed or disturbed by traffic or
weakened by adverse environmental conditions, diseases,
or pests to the extent that they cannot compete for nutrients,
water, or light with weed species; where the desired plantings
are not as well-adapted to their environment as are native
or exotic weed species; and where the growth of the desired
plants modifies their local environment so that natural
ecologic succession to weed species occurs (in the absence
of control).
Buildings, ruins, and other artificial sites can be considered
disturbed environments, which will become populated by
pioneer plant species if there is no intervention. Weeds
can become established anywhere that a suitable substrate
and water are found. Gutters, cracks in roofs, walls or
foundations, and chinks in masonry all can provide suitable
locations for germination of weed seeds. Woody plants
can take root in soil pockets or deep cracks and crevices.
MONITORING AND THRESHOLDS FOR WEEDS AT DEVELOPED
AND HISTORIC SITES
Regular monitoring for weeds is an essential part of a
weed integrated pest management program. Weeds are most
easily removed when they are small or present in low numbers;
in the case of weeds which grow into structures or walkways
it is important to remove them before serious structural
damage occurs. In addition to monitoring for population
density, identification of the species is important. The
biology of the weed will often determine when it is to
be removed or the most appropriate herbicide if chemical
control is necessary.
Conduct weekly inspections around buildings and in landscape
beds, recording weed species observed. Some estimate of
density, such as number per square foot or number along
a transect, should be recorded as well. If structural
damage is already occurring, this should be noted as well.
This type of information will help to correctly time weed
removal. It will also help to prioritize areas for weed
management if resources are limited and to evaluate the
success of weed removal strategies used.
Certain areas are more likely than others to have high
weed populations, and these should be the emphasis of
your monitoring program. For example, recent cultivation
will expose previously buried weed seeds to light. Heavy
pedestrian traffic can lead to bare, compacted soil, which
may be likely to support weed growth.
NON-CHEMICAL WEED CONTROL IN DEVELOPED AND HISTORIC
SITES
The types of intervention strategies employed for management
of weeds in the areas mentioned above will depend on where
the weeds are located (landscape or structural), the size
of the area in which the weeds are to be managed, the
biology of the weed species present, the resources available
for weed management, and the weed density that can be
tolerated. Minimizing the spread of existing weeds and
preventing the growth of new weeds should be the focus
of a weed management program. One weed plant can produce
hundreds of seeds which could potentially disperse over
a wide area.
The objective of the site must be considered before selecting
a weed control strategy and technique, especially in the
case of historic sites. Filling and sealing the chinks
in a stone wall might prevent weed growth, but that option
is not available if it is not historically correct. Careless
use of electric weed trimmers could damage fragile historic
structures. Certain herbicides are corrosive and should
not be sprayed near susceptible surfaces. The cultural
resources staff should be consulted before implementing
weed control in historic sites.
Weeds in the Landscape
Weeds in the landscape are generally considered to be
unsightly and thus have a very low aesthetic threshold.
In the case of new plantings, take time before establishment
to remove existing weeds. In the case of existing plantings,
emphasize the use of monitoring to detect weeds while
they are still small or present at low population densities.
Weeds Around Buildings and Structures
Early detection and removal of weeds around buildings
and structures such as benches and fences is especially
critical. Once weeds grow into foundation cracks they
become unsightly, difficult to manage and can do serious
structural damage. This leads to high maintenance and
repair costs. Weed growth into structures and on patios
and walks can be partly prevented by proper maintenance
of these structures. Filling of cracks in mortar and sidewalks
so that organic debris cannot accumulate inside them will
help to eliminate the entry and subsequent germination
of weed seeds.
Weed Biology
Weed management must be based on a knowledge of the biology
of the weed species. This is in turn dependant on correct
identification of the weeds at a site. For instance, there
is no point in applying a pre-emergent herbicide for crabgrass
control if there is no crabgrass. It would also be fruitless
to apply a pre-emergent herbicide that acts by preventing
weed seed germination for control of established perennial
weeds. Likewise, it could do more harm than good to cultivate
a landscape bed for yellow nutsedge control in July after
nutlets have formed; the cultivation will break the nutlets
into small pieces and produce more weed plants. However,
cultivation early in the season could remove the young
plants before nutlet formation and might be an effective
form of nutsedge management.
Physical Methods of Weed Management
Barriers and mulches are often used to eliminate a substrate
in which weed seeds can germinate. While this is often
a good, long-term solution to a weed problem, it is usually
expensive to install. The elimination of the need for
weed management may pay for the installation of the barrier
over the long term, however.
One type of barrier would be the installation of paved
walkways rather than soil, or the use of pavement or bricks
under benches and around fences. This may not represent
a permanent solution if cracks (and subsequent weed growth)
are allowed to develop in the pavement. Depending on the
site, it may be objectionable for aesthetic reasons as
well.
Bare drainage ditches or pond banks can be lined with
stones or desirable vegetation to help eliminate bare
soil areas which are favorable for weed growth. This may
not work in high-use areas where children could play with
the stones, but might be a good solution to a weed problem
in low-use areas of a park.
Weed mats are frequently used in landscape beds as a barrier
to weed seedlings. These are made of materials which permit
passage of air and water to plant roots but serve as a
physical barrier so that weed seedlings cannot develop.
While they are often effective, initially they are expensive
to purchase and install. Also, weeds which grow through
them cannot be pulled because the barrier will tear. For
a complete discussion of the pros and cons of these materials,
as well as a list of suppliers, see Billeaud and Zajicek
(1989) and Lytton (1990).
Use of mulch in a landscaped areas is another common practice
to reduce weed populations. This will not eliminate a
problem, since weeds can grow through a mulch or germinate
in it as it starts to decompose. A wide variety of material
is available for use as mulch; the most appropriate mulch
for a given situation depends on expense, effectiveness,
aesthetics, availability, and types of plants growing
in the mulched area. For example, plastic sheeting can
be an effective mulch but it is unsightly and may pose
disposal problems. Some stones or cinders may drastically
alter soil Ph, while decomposition of sawdust or non-composted
bark mulches can rapidly deplete soil nitrogen. For more
information on the advantages and disadvantages of different
mulch materials, as well as information on specialty materials
which may be locally available, contact the Cooperative
Extension Service at your land grant university.
Another type of mulch to consider is a living mulch. This
involves the use of a groundcover to cover the soil around
larger landscape plants. Sometimes this is supplemented
with the use of a fast-growing annual to fill in bare
areas between groundcover plants before they become large
enough to cover the soil. Care must be taken not to use
an invasive groundcover which may itself become a weed.
Mechanical Weed Management
Cultivation and hand-removal of weeds will be most cost-effective
in small areas, eliminating small, newly established weed
plants during seasons (usually the spring and fall) when
the soil is moist and weeds are most easily removed. Keep
in mind that there are certain times when cultivation
will do more harm than good. Cultivation of annual weeds
when mature seeds are on the plants is probably not a
good idea, nor is hoeing of perennial weeds that regenerate
by rhizomes or tubers after these structures have formed.
Regular mowing is often sufficient to control weeds over
large areas. In small areas, electric weed trimmers or
propane burners are often used for weed control.
Biological Control of Weeds
Biological control of weeds in rangelands and waterways
has been extensively investigated and seems to have a
great deal of potential. This is not so for weeds in landscape
settings, however. The only weed that would be found in
a landscape that is currently under investigation as a
biological control candidate is Canada thistle, Cirsium
arvense. It is doubtful whether weed densities required
for a biological control agent to be effective would be
tolerated in a landscape. For more information on biological
control of weeds, see Grossman (1989a) and Grossman (1989b).
CHEMICAL CONTROL OF WEEDS IN DEVELOPED AND HISTORIC
SITES
When selecting a herbicide for use against a weed it is
essential to identify the weed species, since many herbicides
are specific in the types of weeds they kill (e.g., only
grasses prior to germination, only broadleaf plants, most
effective against poison ivy). Some herbicides are non-selective
and will kill all vegetation whose leaves they contact;
others are selective but are absorbed by roots of non-target
plants and may injure or kill them as well. Mulgrew (1990)
is a good resource for information concerning the use
of herbicides in landscape beds. You should also contact
your regional Integrated Pest Management coordinator or
state Cooperative Extension Service for herbicide recommendations
for your area, as well as for information on new herbicide
formulations, since these change frequently.
Also consider that use of a non-selective herbicide for
weed control may lead to an increase in weed problems
in the future. The bare ground created in this situation
could serve as a site for invasion by new weed species.
REFERENCES
Literature cited
1. Billeaud, L.A. and J.M. Zajicek. 1989. Mulching for
weed control. Grounds Maintenance (February), pp. 16-18+.
2. Grossman, J. 1989a. Update: biological control of weeds
-- what's happening, what's needed. IPM Practitioner 11(6/7):1-11.
3. Grossman, J. 1989b. Update: biological control of weeds
-- what's happening, what's needed (part 2). IPM Practitioner
11(8):1-8.
4. Lytton, P. 1990. Miraculous textile mulch: a fabric-ation?
American Horticulturist (March), pp. 8-9.
5. Mulgrew, S.M. 1990. 1990 Herbicide Guide for Controlling
Weeds in Nurseries and Landscape. University of Massachusetts
Cooperative Extension Service, Amherst, MA.
Weed Identification
1. Barkley, T.M. 1983. Field Guide to the Common Weeds
of Kansas. University of Kansas Press, Lawrence.
2. Baldwin, F.L. and E.B. Smith. Weeds of Arkansas Lawns,
Turf, Roadsides, and Recreation Areas: A guide to Identification.
MP 169 of the Cooperative Extension Service of Arkansas,
Little Rock, AR 72203.
3. Dennis, L.J. 1980. Gilkey's Weeds of the Northwest.
Oregon State University Press, Corvallis.
4. Muenscher, W.C. 1980. Weeds. Cornell Univ. Press, Ithaca,
NY.
5. Nelson, E.W. 1979. Nebraska Weeds. Nebraska Dept. of
Agriculture, Lincoln.
6. Regional Technical Committee of Project NC-10. 1960.
Weeds of the North Central States. University of Illinois
Agric. Exp. Sta., Urbana.
7. Stucky, J.M., T.J. Monaco, and A.D. Worsham. 1983.
Identifying Seedling and Mature Weeds Common in the Southeastern
United States. Agriculture Res. Service. North Carolina
State University, Raleigh, NC.
8. Weed Identification Guide. Southern Weed Science Soc.
309 W. Clark St., Champaign, IL, 61820.
Weed Management
1. Anderson, W.P. 1983. Weed Science: Principles. 2nd
ed. West Publishing Co., St. Paul, MN.
2. Aldrich, R.J. 1984. Weed-Crop Ecology Principles in
Weed Management. Breton Publishers, North Scituate, MA.
3. Bohmont, B.L. 1990. The Standard Pesticide User's Guide.
Prentice-Hall, Inc., Englewood Cliffs, NJ.
4. Herbicide Handbook. 1989. Weed Science Soc. of America,
309 West Clark St., Champaign, IL, 61820.
5. Shurtleff, M.C., T.W. Fermanian, and R. Randell. 1987.
Controlling Turfgrass Pests. Prentice-Hall, Inc., Englewood
Cliffs, NJ.
6. Weed Control Manual and Herbicide Guide. 1991. Meister
Publishing Co., Willoughby, OH. (Updated each year. Lists
currently available herbicides by common and trade names.)
7. Weed Control and Plant Growth Regulation. 1989. Air
Force Manual, 91-19. Headquarters of the Air Force, Environmental
Directorate, Bolling AFB, Washington, D.C., 20332-5000.