By: Casey Matney, Regional Extension Specialist
Range Management, Colorado State University Extension

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Following a drought, spring moisture can catapult annual weeds into action. Annual weeds grow rapidly and make haste in using what soil moisture is available in order to produce seeds. During a drought, native and desirable perennial plants face hardship due to dry conditions and high temperatures. Several months of these conditions can cause some plants to die and many others to be injured causing partial dieback of their growing points. Seedlings, very old plants, and all ready stressed plants suffer the highest rates of mortality. Going into the second year of a drought or the year following a drought, perennial herbaceous plants that were once large and vigorous have diminished in height, width, and root zone. On the landscape, this causes the ground cover occupied by these preferred plants to shrink, which leaves vacancies in the environment for annual and biennial weeds to grow. If you combine this phenomenon with fire or soil erosion during the same period, then the potential area open for colonization by weeds can be quite immense.

Fast growth rates allow annual and biennial weeds to overtake many desirable perennial grasses. The rapid onset of green growth can make the plains and hillsides look green in a hurry, but there are concerns and dangers.

Here are some tips for landowners, ranchers, and livestock producers regarding weed problems that can develop following a drought.

Watch for weeds returning to former problem sites

• Sites that used to be troubled with weeds may return to weedy vegetation following a drought.
  • Weeds are able to do this because long dormant seed in the soil may germinate following the drought and overtake drought affected plants.
• Control weeds as they appear and choose methods that do not harm the desirable existing vegetation.

Be vigilant for weeds at cattle feeding sites

• Due to shortages in vegetation during a large-scale drought, hay is collected and transported all over the country to feed hungry livestock.
  • Transported forages may contain weeds and weed seed.
• Avoid allowing new weeds to establish by identifying them early and controlling them before they set seed.
• Recheck feeding sites throughout the growing season to spot any plants that may have delayed germination.

Do not turn your livestock out into pasture without inspecting the vegetation

• While the pasture may look green and lush, there may be toxic annual or biennial plants in abundance this year.
  • Inspect pasture corners, feeding sites, areas around stock tanks and ponds, as well as the central pasture itself.
• If you do not know what a plant is, identify it, do not assume it is safe for animals to eat, especially if it is occurring in abundance in your pasture.

Manage weeds sooner rather than later

• Allowing annual and biennial weeds to grow unmanaged for even a week or two can mean the loss of a large amount of soil moisture as well as the potential for the weeds to reach seed maturity.
  • Stop weeds from producing seeds, if at all possible.
• If weeds are beginning to produce seeds, choose methods of control that reduce the spreading of seed to new areas.
• When allowing animals to graze weeds, do so before seed production to maximize use and forage quality.
• If weeds have already produced seeds, develop a control plan that can be used to limit weed production next year.

Not all annuals and biennials are bad. Some of the plants are native, and some are excellent forage for livestock

• Make sure you know what plants you have before you begin action to remove them.
  • By knowing what plants you have, not only will you be better able to determine if you need to control them, but you will have a much better idea of what methods will work best.
• Consider soil stability when controlling weeds. Choose your methods of control carefully so that you limit the possibility of soil erosion.
• Here is a good website with resource links to help you identify your plant.
  • Plant Identification
• Consider contacting your local Extension office for help if you are having difficulty identifying the plant on your own.

Choose an appropriate method of control

• There are four main methods of weed control: mechanical, cultural, biological, and chemical.
• Here are some links to help you understand what these methods are and how to use them.
  • Weed Management for Small Rural Acreages
  • Range, Pasture and Natural Area Weed Management

Graze lightly to moderately following a drought

• Removing too much of the desirable vegetation while the plants are still stressed from the drought can give weedy species a competitive advantage.
• Adjust stocking rates so that the existing desirable vegetation can rebuild their roots and increase in vigor during the growing season.
• If the weeds you are seeing are nontoxic and a potential source of livestock forage, make efficient grazing-use of them during the growing season to offset potential losses in desirable pasture grass production following the drought.

Keep close watch on your livestock

• If you move livestock to a new area or pasture that could contain toxic plants and the livestock begin to show signs of illness, move them immediately to a pasture or area that is known to be free of toxic plants.
• Consult a veterinarian if you are concerned your livestock have ingested poisonous plants.

Four toxic situations to watch out for

• Avoid turning your livestock into areas early in the growing season where weedy toxic plants are lush and green but grasses have not yet begun to grow.
• Avoid pastures where weedy toxic plants are very abundant or more abundant than usual.
• Avoid leaving livestock in a pasture when grasses have gone dry and dormant for the growing season but weedy toxic plants remain green and lush.
• Avoid turning livestock into a pasture where the only familiar forage plant to them is a toxic plant, especially when that plant is common or occurs in abundance.