Recently a customer called asking about extra-strength vinegar to use a weed killer on Canada thistle. While extra-strength vinegar can work as a weed killer, we can’t make any promises as to its effectiveness on Canada thistle. We’ve fought it ourselves in the past and know how tough it is!
With some work, we were able to eliminate large patches over a period of several years so that we were just dealing with single plants popping up here and there.
Initially, we did spray some of the thistle patches with actual weed killer and while that did work to kill existing plants, it doesn’t work to prevent new seeds from germination, and thistle plants are experts at creating lots of seeds, so we saw little long-term benefit from spraying (and obviously, lots of long-term drawbacks!).
What ultimately did the trick was mowing the sections frequently enough to keep the plants from forming flowers and ultimately, seeds.
Canada thistle has an extensive root system and will regrow from its roots, but by mowing it multiple times during the growing season, over a few years we saw drastic improvement. After five or so years of staying on top of it, we were mostly dealing with single plants here and there on our property. Those occasional plants were easy enough to manage by either digging up, or cutting back to the ground repeatedly until they didn’t come back anymore.
As much as possible, it’s important to mow before flower buds form, because if the flower buds are formed when you mow, they can continue to mature and produce seed even after cut. (Similar to what a yellow dandelion flower does after you pick it.)
Additional Options to Consider
Here’s a couple of other suggestions that could be used in conjunction with the elimination techniques:
- Overseed the area with a native grass mix to give the thistle some competition.
- Water the area frequently to get more of the thistle seeds to sprout so that you can eliminate them faster than allowing them to sprout over several years. (These first two options could work well together.)
- Get goats or sheep – they can accomplish the same effect as mowing and you can get milk, meat, or wool as a bonus!
Our current property doesn’t have thistle, but we are now dealing with bindweed and applying many of these same principles. Overall, it’s just a lot of work and patience, but isn’t that the way to be successful in a lot of things?