Weeds love it when it’s dry; weed control measures


Extension staff
SUN Columnists

During times of drought, native and desirable perennial plants often face hardship due to dry conditions and high temperatures.

Annual weeds, on the other hand, grow rapidly and make haste in using what soil moisture is available in order to produce seed.

The following information will help you understand the four methods of weed control; mechanical, cultural, biological and chemical.


Prevention is the first line of defense and keeps weeds from occurring or increasing in an area. Preventive techniques include planting high quality, weed-seed-free crops or grass seed. Legislative items, such as clean-seed acts and weed-management laws, also can help stop weed problems before they occur or may deter weed spread.

An important preventive measure related to control is to keep weeds from going to seed. This is important for annuals and biennials, because that is the only way they reproduce. Perennials reproduce from seed, as well as vegetatively from their root systems. Annual weeds live for one growing season, biennials for two and perennials more than two. However, preventing seed set is extremely important to keep perennials from starting new infestations some distance from existing ones.


Eradication is the removal of weeds from an area so they will not recur unless reintroduced. If eradication creates an open area, one weed problem may be cured simply to create another one. If eradication is necessary, revegetate the ground to prevent another weed infestation. Eradication is desirable for small patches, 10 to 100 feet in diameter, but not always for larger ones.


Control, the most common management strategy, reduces a weed population to a level where you can make a living off of or enjoy using the land. Adequate control also may prevent future infestations. There are four control methods: cultural, mechanical, biological and chemical.

Cultural control methods promote growth of desirable plants. Seeding is the most commonly used cultural control method and must be combined with control methods that decrease the target weed population and gives the seeded species an opening in the environment to successfully germinate and establish (colonize). The USDA-Natural Resource Conservation Service is an outstanding source of information as to what species to plant in a particular area and at what rate. Fertilization, irrigation and planting at optimum densities let crops compete with weeds and not with each other. While nitrogen fertilization increases yields in grass hay meadows, it also fosters weed establishment and growth. Fertilize cautiously, especially with nitrogen, and only when necessary as determined by soil testing.

Mechanical control methods physically disrupt weed growth. This is the oldest control method and is used most often worldwide. Tillage, hoeing, hand-pulling, mowing and burning are examples. To mulch or smother weeds often is considered mechanical, even though it simply excludes light rather than physically disrupting weed growth.

Biological control methods use an organism to disrupt weed growth. Often the organism is an insect or disease and a natural enemy of the weed. This is called classical biological control. Classical is not the only form of biological control. Livestock can be effective weed-management tools if used correctly. However, improper livestock management (overgrazing) can be extremely damaging to the environment and exacerbate weed problems.

Chemical control methods use herbicides to disrupt weed growth. The first rule of any pesticide use is to read the label before using the product and follow all directions and precautions. (NOTE: Avoid using soil-active herbicides, such as Tordon, Banvel/Vanquish/Clarity, or Telar, near windbreak plantings and other desirable woody vegetation. Plant injury or death can occur. Do not allow any herbicide to drift onto woody or other desirable vegetation for the same reason.)

Weed management systems

A weed management system uses two or more control methods. The key is to encourage desirable plant growth with optimum fertilization, when necessary, and/or irrigation (cultural control). Plant competition is an often overlooked tool and should be used first, but not exclusively. When enhancement of the desirable plant community is necessary, make sure you seed at optimum rates to ensure establishment and subsequent competition with weeds. Generally, perennial, sod-forming grasses compete best with weeds.

Till, hoe, hand-pull, mow or mulch (mechanical control) if desired. Herbicides (chemical control) are powerful tools that should be used judiciously, not exclusively. Unfortunately, too often herbicides are used to make up for poor cultural or mechanical management decisions. Herbicides may be a component of the weed-management system. Biological controls can also be part of a system. Several natural enemies currently are available from the Colorado Department of Agriculture. Livestock grazing can be effective, depending on the weed species, if the livestock are properly managed for weed control.

For more information on control of specific weeds on small acreage sites, visit the website at www.ext.colostate.edu and download Fact Sheet No. 3.106 Weed Management for Small Rural Acreages.


June 20 — 4-H Sewing Project meeting, 3 p.m.

June 21 — 4-H Wolf Creek Wonders Club meeting, 2 p.m.

June 25 — Extension Advisory Committee meeting, noon.

June 25 — 4-H float planning meeting, 4 p.m.

June 26 — 4-H Sports Fishing, 6 p.m.

June 26 — Archuleta County Fair Board meeting, 6 p.m.

June 27 — 4-H Lamb, Goat, Swine Project meeting, 4 p.m.

June 29 — 4-H Dog Agility meeting, 10 a.m.