Integrated Pest Management Manual
Weeds of Developed and Historic Sites
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
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.
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.
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 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.
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.
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.
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.