Thistle Control
By Peder Hoyum
Identification
Thistles are a group of flowering plants in the aster family, which includes the artichoke. The most remarkable and identifiable thing about this group would have to be its incredible propensity for growing sharp spines on its dark green leaves, stems, and purple flower buds.
Thistle
Thistle Leaves
Thistle Flowers
Thistle is one of those plants that seems to embody pure vegetal evil. They are kind of like a cactus, in that evolution has given them the ability to defend themselves against the attack of browsing animals. If you've ever run across a grass lawn barefoot, you know that one run-in with their flesh-piercing spines is enough. They are also a noxious weed, which can reduce productivity in crop fields and ruin a perfectly good garden. All thistles breed through passing seeds, but some are able to overwinter and propagate perennially via rhizomes and roots; this means that control is mostly going to be centered upon preventing reproduction. However, killing the plant is always an option.
Controlling Thistle
Controlling the thistle before it blooms is the most important thing. Each plant can produce up to twenty thousand seeds, and they can easily drift one hundred yards or farther on a windy day. You can see how this could be a problem. Within only a few years, the progeny of one thistle could easily infest an acre or two of pasture. That's why the proactive approach is going to be the best approach: kill it and prevent it from reproducing. Thistle seed will still blow in from surrounding properties, but at least you know you aren't damning yourself to a lawn and garden full of green, stabbing hell plants.
Cultivation can prevent thistle seedling growth in spring. In a garden or yard setting, a lot of cultivation can be done with hand tools that dig up soil containing seedlings and turn them over. Over a larger area, you may want to use a garden tiller or a plow. You can plow under a patch of thistles any time of year, but it is better to do it before they have been allowed to form flower heads, which might mature into seeds. Once you have turned over the soil, consider spreading a desirable cover crop like a pasture grass mix or clover.
Manual removal by digging up the plant is the sure-fire method. This won't be practical if you are facing a field of thistle, but if you just have a couple of the spiny-leaved bastards poking up in your yard, get out your spade and go to town. Get them when they are young, and be sure to remove the root; some thistles can grow back from even the smallest little bit of root left in the ground. If the thistle is bigger, you may have to dig out a large root clump, shake out the dirt and dispose of the whole plant. I suggest you invest in some thick, leather gloves for this task.
Thistle in lawn should be mowed short and prevented from flowering. This is certainly not the best option for those who like running around barefoot. Even with mowing, there will be some low-lying leaves and a clump of spiny nastiness left to continue growing. The thing with biennial thistles is, if you are able to prevent it from flowering, you will essentially kill the plant in two years after fall comes and they freeze out—assuming you have frosts where you live. Also continually mowing the thistle short will weaken the plant substantially and make it more susceptible to other control methods.
Killing a bigger patch of thistle is possible with sheet mulches. Thistle is tough stuff, and it can grow through or on top of a lot of organic mulches. That's why you need thick newspapers, plastic sheet mulches, or the permeable garden fabric. Plastic comes in larger sheets, so it may be cheaper and more practical for this application. If you can mow the patch down with a brush cutter or a very hearty lawn mower so that the area is flat, it will make laying the plastic down much easier and make it less likely to become torn or punctured. As you lay it out, weigh it down, especially around the edges. After a month or two of summer heat, everything under that mulch will be dead—even seeds.
Chemical Thistle Control
The fact of the matter is that most people won't have the patience or time needed to control thistle using manual labor or natural methods; they are going to use chemicals. In lawns, where the point is to promote grass growth, you can use broad leaf herbicides like 2,4-D and MCPP. These will target anything that isn't a grass type of plant. These are often applied broadly across an entire lawn to promote a grass monoculture by weed-n-feed companies. If you are looking for spot treatments, a glyphosate, non-selective herbicide like Roundup will do the job. It's quick and easy. Just be careful with it because it will kill everything in the area.
Natural Thistle Control
Organic herbicides. If you aren't into the usual commercial herbicides, there are some homemade options. It is said that strong white vinegar kills thistle leaves, but the plant might grow back from the roots. Another option is cutting the thistle off at ground level and pouring salt on the cut stem; this is supposed to kill the root as well.
Ruminant power. If you have an acreage surrounded by a fence and the facilities to house them, herd animals are very effective weed mowers. Cows and horses will nibble flowers off of thistles, and goats will eat the whole plant, but the animals need to be contained in one area for a while or they will overlook thistle for easier plants.
Biological control. If you can afford them, there are a number of beneficial insects that can be used to target thistles. Along with the thistle stem gall fly (Urophora cardui), there are many weevils that eat thistle seeds such as: Larinus planusstem, Ceutorhynchus litura, Rhinocyllus conicus, Bangasternus orientalis, and Trichosirocalus horridus. Thistle seeds are also a favorite of Goldfinches.