Every summer, we are asked about controlling big weeds in pastures. But, by the time we get the question, most of the damage is done and options are limited. Usually, these are warm-season weeds that were not controlled in a timely manner. Sometimes, though, the weeds over-wintered in a rosette stage and were bolting by the time scouting began.
Many species over-winter in a rosette stage and later bolt to produce tall stems and fruiting structures. Curly dock (Rumex crispus) is a perennial that grows as a rosette from September to March and bolts from May to June. Others, such as musk thistle (Carduus nutans), can be biennial if germinated in the spring or annual if germinated in the fall. Still others, like tansy mustard (Descurainia pinnata), are annuals that germinate in fall, over-winter in a rosette stage and then bolt in March or April.
Knowing the types of weeds, density and locations is necessary to develop a management plan; thorough scouting is the only way to discover this. Look for these cool-season plants in November and again in February or early March. Concentrate scouting in those areas where these weeds have been in the past. Determine if the population density reaches the economic threshold across the field or if spot treatments will be adequate.
Musk, Scotch (Onopordum acanthium) and Canada thistles (Cirsium arvense) are noxious weeds in Oklahoma. The Noxious Weed Law states, "It is the duty of every landowner in each county to treat, control, or eradicate all Canada, musk, or Scotch thistles growing on the landowner's land every year as shall be sufficient to prevent these thistles from going to seed. Failure of the landowner to treat, eradicate, or control all musk, Canada, or Scotch thistle may result in a fine not to exceed One Thousand Dollars ($1,000.00) for each violation per day."
Depending on weed species, biological, chemical or cultural control methods may work the best. A biological control example is using the thistle head weevil (Rhinocyllus conicus) and the rosette weevil (Trichosirocalus horridus) to manage musk thistles they can be effective, but it may take several years to obtain acceptable control. Plowing, hoeing or digging plants out below the crown level can control annuals or biennials. Use caution to avoid spreading perennial weeds vegetatively on tillage equipment. Mowing can prevent seed production if it is done when the terminal heads first start to bloom. It must be close to the ground, or secondary buds may still be able to flower and produce seed. Chemical treatments are usually most effective in November or March when applied to actively growing plants in the rosette stage. Timing depends on the species present and herbicide used. In general, fall application can use lower herbicide rates since the plants are small and tender. Always read and follow the herbicide label.
If you have any questions about winter weed management, contact a Noble soil and crops specialist at (580) 224-6500.