Mowing and clipping are methods that can reduce weed problems. “You need to mow in a timely manner, before the weeds get too big, and definitely before they go to seed and spread,” said J.D. Green, Extension Weed Scientist, University of Kentucky.
This means knowing the weeds, and their life cycle. “Mowing is most effective for upright weeds, especially for minimizing new seed production,” explained Green. That means you can whack them down before they bloom and go to seed, thus halt their spread.
“Thistles, biennials and perennial weeds are more challenging," he continued. "Mowing them will help suppress top growth and keep them from going to seed, but it doesn’t affect their root system and they will regrow again next year. This is why herbicides are a good tool to use in conjunction with mowing, especially for weeds like Canada thistle that regrows from the roots." (For more on herbicide use on horse pastures see "Weed Control: Safe and Unsafe Spraying Practices.")
“Mowing periodically—at the right time—to keep them from going to seed and spreading may be all you need to do with some weeds,” said Green.
With grazing and timely mowing, you might be able to reduce the population of some weed patches if they are never allowed to go to seed, but other weeds are very long-lasting and durable, coming up again from the roots, and you will never completely eradicate them.
“Some, like Canada thistle, I recommend mowing during summer and spraying the regrowth in the fall," suggested Green. "Do it before a frost, however, or you lose your opportunity to kill them with the spray.”
Mowing is effective for many weeds, such as cocklebur and ragweed, to reduce new seed production. “But if you see these kinds of weeds in an open pasture area, they are an indication that you have a lot of bare ground,” noted Green. "You’ve probably overgrazed it or had too many horses in a small space and beat out the grass, and these opportunistic weeds are moving in to try to cover the bare spots."
Mowing is helpful for controlling upright weeds, but won’t affect low-growing, creeping, ground-covering weeds.
“On some of the frequently mowed Thoroughbred farms here in Central Kentucky, the biggest weed problems are low-growing plants like plantain, curly dock and a grass called nimblewill," said Green. "These are some of the same weeds we have to deal with in lawns--weeds that thrive in spite of mowing. Many pastures on Thoroughbred farms get mowed frequently, and these weeds are low enough they get missed, and are taking up space that should be growing forage. Nimblewill, for instance, is a perennial grass that horses won’t graze. So even with frequent mowing, you could still have a significant weed problem.”
In a small pasture, you might pull or dig out weeds if there are only a few showing up. This works for burdock or biennial thistles such as musk thistles and bull thistles. If a few plants are starting to move in and you don’t want to spray, walk your pasture and dig up those weeds. If you choose that route, destroy those plants before they reproduce, which means getting rid of them before they bloom.