Bull thistle – February 2021 Weed of the Month

bull thistle plant on trail
In winter and early spring, bull thistle clumps are easy to spot along trails and grassy areas. Photo by Sasha Shaw.

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.

single flowerhead of a bull thistle plant
The flowerhead of a bull thistle plant has bright pink-magenta flowers surrounded by narrow, sharp spines. This thistle is the National Emblem of Scotland. Photo by Jennifer Andreas.

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.

bull thistle plant in flower
Bull thistle is found worldwide often as a colonizer in disturbed or recently logged areas, providing nectar and seeds for wildlife, but also headaches and costly impacts for hay growers and pasture managers. Photo by Sciadopitys, Creative Commons license.

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!

Identification

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.

bull thistle plant
Bull thistle plants are spiny everywhere you look – flowers, stems, and leaves.
bull thistle flower and bud
Bull thistle flowerheads are large and rounded, single on stem ends, and surrounded by thin sharp spines. Photo by Jennifer Andreas.
stem of a bull thistle plant
The stems of bull thistles are hairy and have wings tipped with spines. Photo by Jennifer Andreas.
leaf of a bull thistle plant
Bull thistle leaves are dark green, with pointed lobes, and pale colored sharp spines on the tips of the lobes. Photo by Jennifer Andreas.
underside of a bull thistle leaf
The underside of a bull thistle leaf is densely covered with white hairs. Photo by Jennifer Andreas.

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.

young bull thistle rosettes
When bull thistle first appears, the young rosettes are often densely fuzzy with white hairs.
large rosette of a bull thistle plant with a person's foot standing on it for scale
Bull thistle rosettes can grow up to three feet wide, with broad leaves flattened against the ground effectively suppressing the growth of other plants.
bull thistle plant starting to grow taller
In spring, bull thistle plants start to bolt, meaning the stems elongate and the plant starts form buds. Photo by Sasha Shaw.

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.

seedhead and flowerhead of a bull thistle plant
Bull thistle spreads by seed carried on a feathery, wind-borne structure called a pappus. Seeds are also spread by attaching to animals, people, and equipment, Photo by Ivar Leidus, Creative Commons license.

Control

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 plant with buds
Thistles are tricky to tell apart so be sure to check with an expert before removing. This bull thistle photo was sent to us through the app King County Connect.

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.

Learn more

King County Noxious Weed Control Program

Washington State Noxious Weed Control Board

US Forest Service



Categories: Weed of the Month

Tags: , , , ,

2 replies

  1. Is Bull Thistle a biannual? when I’ve come across Bull Thistle in the lowlands, I assumed you had to get the whole root, but if I understand you correctly, there is something called the “root crown” and if I cut off the plant below that, it won’t grow back. But does that only work on second-year plants?

    • Yes, it is a biennial, thanks for asking! I meant to include that. First year plants (rosettes) you do need to remove the whole root, but bolting or flowering plants just need the top couple of inches removed. thanks for asking the good questions and for removing bull thistle when you see it in the wild areas!

%d bloggers like this: