Buzz
Field thistles are among the most challenging weeds to eliminate. They are invasive and toxic plants, so much so that many regions have laws prohibiting their growth. While field thistles are notoriously difficult to remove, persistence can lead to success. It may take 2 or 3 seasons to completely eradicate them, but don’t lose hope.
Here are 10 effective methods to eliminate even the most stubborn field thistles.
Steps
Systemic Herbicides
If you don’t mind harming surrounding plants, this is a viable solution. Choose a systemic herbicide containing glyphosate, the most effective ingredient for permanently eliminating field thistles. Follow the instructions on the product label. Apply during the plant’s growing season for optimal results.
- Note: The World Health Organization (WHO) classifies glyphosate as potentially carcinogenic. It is banned in some countries and U.S. states. Check local regulations and always wear gloves, a mask, and protective goggles when spraying. Keep pets and livestock away from treated areas.
- Multiple applications may be necessary. If the thistles regrow, reapply the herbicide. It may take several seasons to fully eradicate them.
- Unfortunately, glyphosate is a potent chemical. This herbicide will kill all plants it contacts, so you might unintentionally harm nearby harmless vegetation. However, if you’re treating an isolated area, this may not be a concern.
Selective Weed Killers
If you want to eliminate field thistles in a garden with existing plants, opt for selective herbicides. Selective herbicides containing clopyralid or triclopyr will target field thistles without harming surrounding vegetation. Choose organic weed killers designed for this purpose and carefully read the label to ensure they won’t damage your plants. Mix and apply the herbicide as directed. While the risk to other plants is low, aim to spray only the thistles for precision.
- Combining selective herbicides with competitive planting is one of the most effective strategies for eradicating field thistles.
- Don’t assume the herbicide is ineffective if thistles reappear after treatment. Continued use will prevent seed spread and eventually kill the roots as new growth is suppressed.
Competitive Plants
Perennial plants like alfalfa can outcompete field thistles for nutrients. Planting alfalfa is an excellent choice, as it absorbs soil nutrients before thistles can establish. However, any grass or plant grown around thistles can have a similar effect. Use a roller or harrow to press seeds into the soil. Water the seeded area thoroughly and maintain frequent watering 3-4 times daily. Reduce watering once the grass sprouts and allow it to grow densely.
- Among competitive plants, alta fescue is the most effective against field thistles, while Troy bluegrass is the least effective.
- This method may fully resolve the issue over several years, but it’s a long-term solution. Field thistles may still occasionally reappear.
Hand Weeding
If field thistles grow in patches, you can remove them manually. Wear gardening gloves for protection. Use garden shears to cut visible stems and immediately dispose of the clippings in a trash bag to make root removal easier. Use a shovel or spade to dig up the roots, ensuring you remove them entirely from the soil and discard them in the trash bag. Clean up all debris, shoots, leaves, and roots to completely eliminate field thistles, then dispose of the trash bag properly.
- Wash tools with soap and hot water after use. Even a single seed left on gardening tools can lead to regrowth when used again.
- Field thistles have deep roots, so this task requires effort. Ensure you remove every part of the plant, as thistles can regrow from any remaining fragment.
Mowing
Mow the thistles before they flower to prevent further spread. Cut field thistles just before their flowering period to reduce seed production. Start in early spring and continue mowing whenever new shoots appear. If possible, mow before heavy rain; cutting the thistles close to the ground and allowing the stumps to soak in water encourages fungal growth and disease in the roots.
- The best time for this is after rapid growth and tuber formation but before flowering. Starting at this stage prevents seed dispersal.
- You can combine this method with herbicide application. Multiple mowing sessions over several growing seasons may be necessary for complete eradication.
Tilling the Soil
Till the soil in autumn to weaken the roots and starve the thistles of nutrients. Use a tiller if available, or a shovel to turn the soil. Remove all visible thistles and their roots. This process disrupts the root system and kills the thistles before the first winter frost. You may need to repeat this multiple times during the season.
- Avoid tilling during the growing season if you can’t remove all plant fragments, as tilling can spread seeds. If you can thoroughly clean the area, tilling during the growing season is possible, but repetition may still be required.
Covering
You can use a heavy, flat object to smother field thistles. Find a wide plywood board or metal sheet and place it over the thistles. Weigh it down to press the edges into the ground. This method blocks sunlight and water, causing the thistles to die within 2-3 months. Afterward, remove the cover, dig up the roots, clear away dead plant debris, and dispose of it.
- Field thistles may regrow if the roots aren’t completely dead during the covering period, but this is a relatively easy option. If it doesn’t work, you won’t have invested much time or effort!
Livestock
Some animals, like goats and cattle, enjoy eating field thistles. If you have a farm with livestock, remember that certain animals can help control thistles by grazing on them. Goats are the best candidates—let them graze in areas infested with thistles. Cattle may also eat thistles occasionally, though some cows aren’t fond of them.
- Livestock won’t completely eradicate field thistles, but they can help manage the problem while you use other methods. Just ensure they don’t graze in areas treated with herbicides.
- Chickens might also eat thistles, though poultry farmers note that thistles aren’t ideal for chickens. Unfortunately, there’s limited research on this topic.
- Field thistles can be toxic to some livestock, though it’s rare for them to eat enough to cause harm. To be safe, provide other food sources so they don’t rely solely on thistles.
Vinegar
Vinegar is perhaps the only natural substance effective in eradicating field thistles. Pour white vinegar into a spray bottle and apply it directly to the thistles until they are thoroughly soaked. Typically, only one or two sprays per plant are necessary. Allow a few days to a week for the vinegar to take effect. Once the thistles have died, wear gloves to pull them out and dispose of them in a trash bag.
Insects
Certain beetles are known to feed on field thistles, weakening them. While many of these beetles are not easily found or attracted, you can purchase stem-mining weevils. Acquire a few hundred stem-mining weevils (Hadroplontus litura) and release them near the thistle patches. The adult beetles will consume the thistles and lay larvae within the stems. The emerging larvae will continue the reproductive cycle as long as thistles remain in the area.
Advice
- Field thistles propagate through seeds, but their roots spread so extensively that a single root system can support multiple plants. They can also regenerate from small plant fragments, even if the roots are severed. This is why multiple control methods are necessary to effectively manage and remove all traces of thistles.
Warning
- Some believe tilling the soil can eliminate field thistles, but this method often backfires. Most thistle varieties thrive in aerated soil, and if the roots aren't completely dead, they can regrow even faster.
- Many assume fertilizing the soil will eradicate thistles, but unfortunately, this usually encourages their growth. While fertilizing after herbicide application may benefit crops in the long run, fertilizing alone will only make thistles flourish more vigorously.
