This time of year, you may notice low clumps of bull thistle growing along trails or in disturbed grassy areas. Bull thistle (Cirsium vulgare) is also called spear thistle, which perfectly describes the mean-looking, spear-shaped leaves armed with sharp spines on the tips. Native to much of Europe and large parts of Asia and North Africa, bull thistle is now found throughout the world, including all 50 states in the U.S. Although it is a Class C noxious weed in Washington and can be a serious pest in pastures, hay fields, and newly planted forests, it is fortunately not difficult to control compared with other thistles. But even with its intimidating name and appearance, bull thistle also has many positive traits.
Bull thistle flowers provide abundant nectar for pollinators of all sorts. Birds and small animals eat the seeds and use the fluffy thistledown as nesting material. It is even edible when young if you know how to harvest it. However, if you live near pastures or hay fields, you would be doing your neighbors a huge favor if you kept bull thistle from spreading. Bull thistle seeds are infamous for moving far and wide in the wind and with the help of people and animals. A dense infestation of bull thistle can significantly reduce the forage quality of a pasture, lower hay value, and slow down the growth of young tree seedlings.
In King County bull thistle control is up to the individual landowner. Deciding what to do with your bull thistle depends a lot on what you are doing with your land (and what your neighbors are doing). Just remember, like most weeds, the amount of bull thistle will likely only increase if left alone!
If you find plant identification difficult, bull thistle is a good starter plant. It is very common so you won’t have any trouble finding it. It is conspicuous any time of year so you can study it whenever you want. And it is closely related to several other thistle species, some native to Washington, some not. But you can learn to tell it apart by noticing a few key characteristics and feel like an expert botanist in no time.
First, bull thistle plants are spiny all over. Flowers, leaves, and stems all have very stout, sharp spines. Second, there is only one flowerhead at the end of each stem. These flowerheads are big and rounded, with narrow spines densely crowded around the base. Third, the stems have wings with spines, making it painful to grab hold of (unlike our native thistle species). Fourth, bull thistle plants are usually branched, bushy, almost ball-shaped, and usually about 4 to 5 feet tall. Oh, and the flowers are quite pretty and bright pink-magenta in color.
In winter and early spring, look for rosettes of dark green, spiny leaves that are often covered with white hairs. These rosettes vary in size from fairly small to truly giant (up to three feet across). Because bull thistle is a biennial (meaning it usually takes two years to complete its life cycle), rosettes usually grow for about a year before bolting. After over-wintering, second year plants will mature and form flowering stems in the late spring and summer, set seed in the late summer to fall, and then die after the seeds disperse.
Bull thistle spreads entirely by seeds that are carried by wind, water, animals, equipment, hay, and people. This plant is determined to go to seed! If flowering plants are cut before they go to seed, they usually re-sprout and flower again on side shoots. And if the cut flower stems are left on the ground, they will almost always form seeds anyway.
If you decide to remove your bull thistle, the best timing is before you can see any pink in the buds. At that time, you can either dig up the plant completely or simply sever the root a couple of inches below the ground to remove the root crown. If you wait until the flowers turn pink, they will likely go to seed after being cut. In that case, you should gather up the flowering stems and dispose of them in yard waste or pile them up somewhere to decompose where you don’t mind having thistle seeds (and covered so your neighbors don’t get them either).
If your crop of bull thistle is too daunting to dig up, another approach is to mow the thistles close to the ground at bud stage on a regular basis to prevent them from flowering. This will weaken the roots over time and allow the other vegetation to move in. Bull thistle thrives on disturbance of any kind and especially over-grazing. Reduced grazing and reduced soil disturbance, combined with seeding or planting competitive vegetation, will go a long way to reducing bull thistle problems. And if you are lucky enough to have a maturing forest to help you, shade from trees is the best long-term control.
If you are hiking in the mountains, make sure you know how to identify native thistles before pulling any plants. Sadly, people have been known to pull native thistles thinking they were doing a good deed. It’s best not to remove thistles in the mountains or natural areas unless you are certain. There are two native thistles common to our area that look like bull thistle: edible thistle (Cirsium edule) and clustered thistle (Cirsium brevistylum). When in doubt, take photos and share them with us or report them on iNaturalist or EDDMapS, or through the King County Connect smartphone app.
Bull thistle is a non-regulated noxious weed in King County, so we aren’t tracking locations and control isn’t required. However, we can provide advice on control options for different areas and situations. Contact us or visit our web site at the link below for more information.