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Watch Out for These Weeds in Drought

Jim Johnson

By Jim Johnson
Senior Soils and Crops Consultant

Posted Sep. 30, 2011

Last update: February 2018

When drought induces the import of vast quantities of hay into Oklahoma and Texas from neighboring states and beyond, there is a risk that invasive weeds will be brought in with that hay. Many of these plants could cause problems, but some weeds deserving particular attention are:

  • Diffuse knapweeds (Centaurea diffusa)
  • Spotted knapweeds(Centaurea stoebe)
  • Leafy spurge (Euphorbia esula).

They are listed as noxious weeds in many states and are very invasive.

What are Knapweeds and Spurge?

Knapweeds look similar to thistles. Spurge has milky sap and showy yellow flower bracts. Pictures and detailed descriptions of these weeds are readily available on the Internet.

All three of these plants are warm-season forbs, can survive as perennials and are not native to North America. Once established, these weeds can quickly crowd out desirable forages and native plants. They are vigorous competitors for water and exhibit allelopathy, which means that they exude chemicals in the soil that inhibit the growth of desirable forage plants.

All three reproduce by seed, but leafy spurge and diffuse knapweed also reproduce from root buds. All three prefer dry, disturbed sites that have bare soil and weak grass stands. However, leafy spurge and spotted knapweed are well adapted to a wide variety of environmental conditions.

The key to controlling these and other invasive weeds is to catch them before they get established and reproduce.

Check the hay you purchase, before or as you feed it, for weeds you do not recognize. This may alert you to potential future problems.

Consider feeding the imported hay in a confined area. This will limit the amount of your property where seeds are dispersed.

Monitor your property, especially hay feeding areas, for weed seedlings you do not recognize. A good time to do this is about one week after every rainfall.

Control infestations when they first appear and before they have a chance to spread. Almost all weeds are easier to control when they are small and actively growing, so do not allow them to escape treatment and produce seed. Once seed is produced, the problem has grown exponentially larger.

Seeds for all three of these weeds can lay dormant in the soil for several years, so continue to monitor for these weeds in the future.

Chemicals labeled for the control of these weeds include most of the 2,4-D type products such as picloram, dicamba, aminopyralid, etc. Always read and follow label directions.

Other control measures may include sheep and goats, prescribed fire and bio-control with insects that are specific pests to these weeds.

In addition to these three weeds, many more weed problems could also develop after drought and hay feeding. Be prepared to implement an aggressive weed control program if any unfamiliar, potentially invasive weeds are discovered.

Diffuse knapweeds

diffuse knapweeds
Richard Old diffuse knapweeds
Figure 1.
Joseph M. DiTomaso, University of California - Davis, bugwood.org
Richard Old, XID Services, Inc., bugwood.org
USDA APHIS PPQ Archive, USDA APHIS PPQ, bugwood.org

Spotted knapweeds

spotted knapweeds
spotted knapweeds
spotted knapweeds
Ohio State Weed Lab Archive, The Ohio State University, bugwood.org
Eric Coombs, Oregon Department of Agriculture, bugwood.org
Bonnie Million, National Park Service, bugwood.org

Leafy spurge

Figure 1.
Figure 1.
Figure 1.
Steve Dewey, Utah State University, bugwood.org
Leslie J. Mehrhoff, University of Connecticut, bugwood.org
William M. Ciesla, Forest Health Management International, bugwood.org

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