Bull Thistle Control: Managing Bull Thistle Plants In Gardens

Bull Thistle Control: Managing Bull Thistle Plants In Gardens

By: Bonnie L. Grant, Certified Urban Agriculturist

Bull thistle (Cirsium vulgare) is a plant that is related to the sunflower family but has none of the charm and beauty of those sunny-nodding flower heads. It is a prickly biennial that grows freely in disturbed soils, pastures, ditches, roadsides and unmanaged spaces. The plant has colonized much of North America and is a pest plant in the garden and in agriculture. Bull thistle control can be manual or chemical with an emphasis on seed control. Learn how to get rid of bull thistle and prevent this prolific weed from taking over your garden.

What is Bull Thistle?

Bull thistle plants are native to Western Asia, North America and parts of Europe. What is bull thistle? It is a free-seeding weed with a prickly demeanor and rapid spread. The plant has the ability to produce around 5,000 seeds in a season. These bur-like seeds cling to animals, pant legs, machinery, etc. and get spread around with abandon. For this reason, bull thistle removal is a priority among farmers and meticulous gardeners.

Bull thistle starts life as a spiny leaved rosette. The hairy, prickly leaves overwinter to develop stems and branches of up to 2 feet in spring. It has a deep taproot, which makes manual pulling a challenge.

In summer the plant grows a scented flower that resembles a spiny globe topped with fringed pink petals. The flowers are produced at the ends of the tangled stem growth and last for several weeks before producing tiny striped seeds capped with white downy hairs. These attach themselves to any object that brushes against them.

How to Get Rid of Bull Thistle Manually

The stubborn plant can arise like Lazarus from the ashes if hand pulling leaves behind any of the root. Casual removal with this method is likely to leave behind the genesis of a plant in spite of the foliar amputation.

Digging the plant out with a spade or hori hori is the best approach to mechanical bull thistle control. Take care to remove the entire fleshy taproot for best results. In order to reduce the seed population, cut off the seed head and tuck it into a sack to keep the fluffy seeds from dispersing.

Other Types of Bull Thistle Removal

In agricultural situations, the introduction of a bull thistle seed head gall fly has been proposed as a biological agent. However, it has been shown to have limited effectiveness. There is also a weevil that is an effective control agent, but it can also affect desired thistle species.

Chemical treatment is most effective on the first year rosettes of bull thistle plants. The types of sprays used in agricultural scenarios are dicamba, glyphosate or 2,4D. Note: Chemical control should only be used as a last resort, as organic approaches are more environmentally friendly.

For widespread control, mowing twice per year has been effective in reducing the population by preventing seed heads. Of course, your battle with the plant will only be as effective as your neighbors because of the travel ability of the downy seeds.

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Bull Thistle Removal - How To Get Rid Of Bull Thistle Weeds - garden

How to control those ‘darn’ thistles in pastures was a topic at the recent Upper Midwest Grazing Conference in Dubuque, Iowa. The spiny nature of thistles prevents livestock from grazing near them and heavy thistle infestations may cause large areas of pastures to be left ungrazed. The overall result is that thistles lower the productivity of pastures.

Thistle management consists of proper identification, and then combining various cultural, mechanical, chemical, and perhaps biological measures. Perennial thistles, Canada thistle being the major one, can exist for many years and they reproduce from both seed and underground parent rootstocks. The creeping root system enables this weed to spread yearly.

Biennials, like musk, plumeless, and bull thistle, live for two years and reproduce only by seed. After germination, they form a prostrate rosette ranging from 4 to 18 inches in diameter before becoming dormant in the late fall. Exposure to cold winter temperatures is necessary to trigger these thistles to flower the second year after sending up a flower stalk (called bolting). Each plant can send up several stalks and produce numerous flower heads, each with viable seeds. After flowering or with the first frost, biennial thistles die in the second year.

Using good cultural practices (including rotational grazing, maintaining optimal soil fertility, and periodic mowing) that result in vigorous, dense, and uniform stands will help keep pastures competitive with weeds. Selective grazing, resulting from overgrazing, often leads to invasion by perennial weeds.

Crop rotation is a valuable weed management strategy for temporary pastures, especially since biennial thistles cannot tolerate tillage or crop competition.

Once biennials bolt or produce a seed stalk, they are less sensitive to herbicides. Mowing at this stage (before flowers open) will help reduce seed production. Some regrowth will occur, so a second or third mowing may be necessary. Even close mowing does little to control biennials the first year or during the rosette stage. Repeated cutting of the crown of biennial thistles 1 or 2 inches below the soil surface will eventually reduce the stand by preventing seed production. Mowing is generally less successful on deep-rooted perennial weeds and brush.

Weeds vary in their susceptibility to herbicides. Be sure to read and follow the label when selecting and using pesticides. A new broad-spectrum herbicide, Aminopyralid, manufactured by Dow AgroSciences, may be registered by the end of 2005. Once registered, it will provide another option for thistle control. Depending upon the livestock species, some herbicides have a legal interval between application and reentry for grazing/harvest. Certain herbicides may also have long-term residual effects on the site. Many herbicides will kill forage legumes that are present in the pasture.

Biennials are most easily controlled by herbicides while in the seedling or rosette stage in the year of germination and up to bolting stage in the second year. A fall or spring herbicide application will be effective. After bolting, they are much less susceptible to herbicides and mowing becomes the best option.

Perennials should be treated by a translocated herbicide while in the bud-to-early flower stage or in the fall regrowth stage. At this stage, the herbicide can move downward with food reserves to the roots, thus killing the entire plant.

No single practice will result in thistle-free pastures. A coordinated approach that combines various measures is needed. Details on pasture weed control are found in the 2005 Illinois Agricultural Pest Management Handbook, available at Extension offices.

Column adapted from material prepared by Dr. Jerry Doll, former Extension weed scientist, University of Wisconsin, Madison.


Thistles in a Nutshell

Here’s what you need to know about Thistles

Appearance

The treatment of Thistles in a lawn is the same for both species. That said, it’s handy to be able to identify one from the other.

Creeping Thistles

The most common species of Thistle found in lawns.

Creeping Thistle or Cirsium arvense grows erect, spineless stems from a creeping rootstock. These stems produce wavy leaves that look similar to the leaves of Dandelions, except they’re spiky and sometimes have hairs on the underside.

The flowers, which are rarely seen in lawns due to mowing are Lilac in colour and are borne in clusters. They bloom between June and September and give off a distinct smell of honey.

Dwarf Thistles

Dwarf Thistle or Cirsium acaule isn’t as common but it can make an appearance from time to time.

Like Creeping Thistles, the leaves are similar to Dandelions and spiky but they’re pretty much hairless. Instead of growing on stems, Dwarf Thistle grows a rosette of leaves.

Growing from the centre of this rosette of leaves is the reddish purple flower which blooms between June and September.

A Perennial Weed

Thistles are perennial weeds that grow back year after year forming deeper and deeper roots as it establishes itself.

They don’t tolerate mowing well though and this on its own can be enough to get rid of them.

Preferred Habitat

Like most weeds, Thistles can grow in most soil types but they do excel in certain conditions

  • Creeping Thistle grows best in newly sown lawns
  • Dwarf Thistle prefers chalky soils


How Can I Eliminate Thistle?

What can I do to eliminate thistle on our property? We are constantly pulling thistle out of our flowerbeds. And I must own stock in Roundup at this point. Nothing seems to keep it at bay. I am not opposed to clearing the beds and starting over if that's what it takes, since we moved into the house only a couple of years ago. How can I get rid of the thistle for good?

Thistle is a tough one because, as you've probably noticed, it has such an extensive root system. If there are a lot of thistles in your neighborhood, it may be very difficult to competely keep the weed at bay as new seeds will enter your yard every year. But to get rid of the ones you have, the key is persistence -- either spray, dig, or cut back and cover up the plants that are currently growing. Re-attack them each time you see regrowth if you keep on them, you will eventually exhaust the root systems.

Do you have a layer of mulch over your flowerbeds? It's not a miracle cure, but it will definitely help to have a 2-inch-deep covering of some sort of material (pine needles, shredded wood, pecan hulls, etc.) over your soil. That will prevent many of the thistle seeds from sprouting and help knock back the root systems that are in the soil.


5 Answers 5

Now that the question has been updated to include "Thistles" I've revised my below answer a little.

Credit for the "dandelion" part of this answer goes to my mum.

When we (wife and I) moved into our current home 4 years ago, the front and back lawns were covered in dandelions (and other broadleaf weeds). I didn't want to go the herbicide route, and after speaking with my mum, she said the only way to truly get rid of (control) dandelions is to hand remove them (important: you need to remove root n' all) and recommended using a small hand garden trowel for doing so.

I didn't have a "small hand garden trowel", instead I bought a small (and cheapish) builders trowel and used that for 3 years:

Then last year on clearance (paid $1) I picked up a Fiskars Softouch Weeder:

Both of the above hand-tools have worked well in removing dandelions (roots included).

The first three lawn cutting seasons I pulled a lot! of dandelions, at least once a week I would walk the lawns and hand remove all the unwanted plants I saw (all of them went into a plastic bag for offsite disposal).

During last year's lawn cutting season I noticed a big drop-off in the number of unwanted plants I was removing, except for crabgrass (but that's a whole other story).

This years lawn cutting season I've only had to remove a handful of "small" (young) dandelions, this is to be expected as I can't control dandelion seed heads blowing into my garden (though I wish I could).

Yes, getting any unwanted plant under control via an "organic" approach is going to take patience and time, but if you stick with it, you will see (excellent) results.

  • And bare in mind, even if you go a "non-organic" route, you're pretty much "forced" to continually use the chosen method if you don't want to see the unwanted plant(s) return.

If you wish to read more about how I approach "organic" lawn care, go here on SE:

Personally I haven't had to deal with thistles in my lawn, that said I do occasionally have to deal with that unwanted plant (weed), along with dandelions and spurge in our street's common ground (flowering) areas that I maintain (I don't mow the grass, a lawn care company takes care of the mowing):

I remove "smaller" thistles using the above tools, basically I treat its removal the same as dandelions ie Remove the unwanted plant, root n' all.

For "large" thistles I use a spade to dig them out (root n' all). After digging them out I back fill the holes with some suitable material and make good the area.

Whilst I share your preference for the non-chemical route, I think your best course, given that this is a serious infestation, is to spray the leaves with a systemic weedkiller, such as Roundup, which will spread into the roots and kill them. Make sure that the dandelions are growing actively - mid to late-spring is probably the best time - and that there is enough leaf area to spray the weeds fully. You will probably need to spray them at least twice.

Short of hand-weeding, an alternative natural strategy (although less effective than the chemical one) would be to spread corn gluten meal over the whole area, cover it with black plastic sheets and weight them down. This will deprive the weeds of water, and of light which will prevent photosynthesis, and will eventually kill most of them - but it will take some time.

I hadn't realized that the weeds were in your lawn, and I can understand why you wish to avoid using chemicals, given that you have young children. In the circumstances, hand-weeding would seem to be the only way forward this tool has received excellent reviews and would take most of the backache out of the operation.

The weed&feed weedkillers are going to be insufficient for thistles, but should work on the dandelions. I think the thistle should be the priority though - it is spiky and looks bad.

Your options are physical removal or weedkiller: no free lunch, here! I would use a systemic "kill everything" weedkiller. You can use it as a spot treatment and if you get the thistles when they are still small, it is easy to apply to the crown without catching anything else nearby.

Yes this can pose a problem with kids, but you only need to ban them from the lawn for a week or so (read the instructions on the weedkiller). This should not be a problem if they are already refusing to walk on the lawn due to the spines. You could also try fencing off sections as you treat it?

Remove the dead thistles with a spade as the spines will still hurt. They should compost okay, but remove all seed heads and trash them (do this before the weedkiller as the seeds will only spread).

Nobody touched on a few points to help control dandelion and thistle so I thought I'd add my 2cents even though this is an old thread. Also I'm curious how you made out since this is an old thread.

Dandelions grow well in alkaline soil which most grasses do not. If your soil is alkaline your grass could be thinning leaving room for dandelion to grow. Do a soil test with your local university cooperative extension and make any changes to pH and fertility. It will do a lot to help thicken your grass to help prevent weeds.

Set your lawn mower to the highest setting to help shade out new seedlings. It's also good for the grass. Water and fertilize appropriately. There are other things that might be keeping your lawn from growing thick such as dull mower blade or disease. Try to figure out what the problem is and correct it. A healthy, thick lawn is a big factor in controlling weeds.

One popular weeding tool that works well for dandelions and other weeds that have a long tap root is the Weed Hound Elite (check the link for my thoughts on it on my site). It's nice because you don't have to get on the ground to weed and it works very well.

You can spot spray the weeds in the fall using an organic herbicide such as vinegar. Spray them, wait a week, if they start coming back spray them again a week later. Do this for 2-3 weeks and you'll drain a lot of the reserves from the roots. A combo of the weed hound and spraying works well. Then reseed those areas of the lawn.

Check out Organic Lawn Care for the Cheap and Lazy. There's more info on proper mowing, fertilizing and watering and specific info on dandelions and thistle.

I know it seems like I'm carrying the "calcium" flag on this site as it seems calcium is the answer to almost any problem, but once again, if you want to be rid of dandelions, check your calcium levels:

"The lawn on the left received applications of high-calcium limestone and gypsum the lawn on the right did not."

Another testimonial from a credible source can be found here: Calcium Works! Not A Single Dandelion.

I'm a believer that each plant has its perfect soil conditions where it competes well with other plants. Clover competes well with other plants in a low-N soil. When the N levels are raised, clover doesn't compete as well as grass and the clover goes away over time. Likewise, dandelion competes in low-calcium soils. Or maybe it's better to say dandelion competes in high-potassium soils. which is really saying the same thing.

What this image is saying is simply when rainfall has leached calcium to the point that it is no longer the dominant mineral, then potassium becomes the dominant mineral in the soil.

Why am I bothering to tell you this?

Weedy plants are often controlled by the application of herbicides. Here we explore an alternative method of control. We suggest that the abundance of an undesired plant species (here dandelions: Taraxacum officinale) may be controlled by modifying interspecific competition via changes in resource supply rates. This hypothesis is supported by several lines of evidence. First, analyses of effects of different patterns of fertilization on plant-species abundances in the 140-yr-old Park Grass Experiment at Rothamsted, England, show that Taraxacum abundances were highly dependent on potassium fertilization and on liming, but not on addition of other nutrients. Potassium fertilization led to a 17- to 20-fold increase in Taraxacum abundances in the classical Park Grass data, and to a 4- to 7-fold increase in the modern data. Liming led to a 2- to 3-fold increase for classical data and to a 3- to 4-fold increase for modern data.

Second, in a greenhouse study in Minnesota, Taraxacum had a higher requirement for potassium and had its biomass more limited by potassium than any of five common grass species of Park Grass. This suggests that Taraxacum may be a poorer competitor for potassium than these grasses, but this mechanism has not yet been tested. Third, in a series of Minnesota lawns that had not received fertilizer or herbicides, both Taraxacum density and abundance were significantly positively correlated with its tissue potassium levels.

This demonstration that desired and weedy plant species can differ in their resource requirements suggests that adjustments in resource supply rates may determine the outcome of interspecific competition, allowing desired species to competitively control weedy species. In particular, for soils with low potassium levels, the use of potassium-free lawn fertilizer is predicted to decrease Taraxacum because of competition from grasses like Festuca rubra.

The best way to lower potassium levels in the soil is by raising calcium levels.

If you raise calcium, it will dilute potassium in the soil and make it very uncomfortable for the dandelions, but the grass will love it.


Four ways to control Thistles:

  1. Pull up as many of the existing thistle plants as you can. Make sure to get the whole root network. Since mature plants have root systems that can extend for 10 feet, you will have better luck pulling up immature plants. Remember: If even a tiny piece of the root survives, the whole cycle can start again. Don’t let thistle get to the stage in which the stems elongate and begin to produce flowers.
  2. Apply herbicides to kill thistle, especially in spring and fall, before thistles can flower and seed. Use glyphosate for your garden, and use a broad-leaf herbicide containing 2,4-D or MCPP for your lawn. Since glyphosate kills all plants, you must keep application specific. Use a sponge to apply, or cut thistle plants and then use an eye-dropper to put a drop or two into the stem. If done early enough, you can use chemicals in Hosta beds as Hosta typically sprout late so a chemical treatment will not affect it. Pre Emergent treatment of Preen granules can be used. MVG also offer a product called Burn Out which is environmentally safe. Put down mulch liberally. It conserves moisture, enriches the soil and prevents germination of new thistle.
  3. Apply vinegar. Fill an empty spray bottle with vinegar. No need to dilute as this makes it less efficient at killing weeds. Cut off at the base of the stem. Spray one to two sprays of vinegar directly on the cut of each thistle plant. The cut allows the vinegar to spread to the roots more quickly, which in turn kills the plant more quickly. Saturating the unwanted plants once a week with this mixture helps control the problem. Do not spray the soil with vinegar. Sprinkle a pinch of salt at the base of each plant. Salt dehydrates the plants which makes them die even faster. Too much salt will lower the quality of your soil, so use no more than a pinch of salt for each plant. After the plant withers and browns, removing it from the ground, roots and all, is a piece of cake.
  4. Lemon juice acts similarly to vinegar when it comes to killing weeds. The high acidic level breaks down the composition of the plant, drying it out and preventing it from retaining moisture. This drying process works even faster on a hot, sunny day, so timing your weed control regimen to accommodate this will work to your advantage. Pour the two ingredients into a large empty spray bottle. If you don’t have a spray bottle large enough, mix the vinegar and lemon juice in a blender and pour the contents into your spray bottle as needed.
  • 1/2 cup lemon juice
  • 1 quart white distilled vinegar
  • Spray bottle

Prevent new thistle invasion by keeping lawns thick re-seed disturbed areas. Keep gardens healthy and pest-free, and pull any new thistle plants as soon as they appear. Timing is everything, starting early is a good thing!

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Growth and reproduction

Prevention: Bull thistle only reproduces by seed so prevention of seeding and takign care not to spread seeds are key to preventing new infestations. Contaminated hay is a primary means of spread of this species so be careful to purchase weed free hay or watch closely for new plants in the areas hay is kept or spread. Do not leave cut stems of flowering bull thistle on the ground because they are likely to form viable seed after they are cut.

Manual control: Bull thistle can be dug up with a shovel. Usually removing the top couple of inches of root is sufficient to kill the plant, especially after it has bolted (produced stems). A shovel or other tool can used to chop off leaves from one side of the plant to gain easier access to the roots, which can then be dug up. Flowering stems should be collected and destroyed to keep them from forming viable seed.

Mechanical control: Mowed thistles will produce new branches from basal buds but close cutting or cutting twice per season will usually prevent seed production and reduce the population over time. More more effective control, cut plants with a sharp shovel at 1-2 inches below the soil surface prior to flowering. If only one cutting a year is possible, cut when plants are in bud for best results. Cultivation and tilling can also be effective to control bull thistle.

Chemical control: For information on herbicides and recommendations for control, please see the Pacific Northwest Weed Management Handbook. There are several herbicides that are effective on bull thistle, but it is important to carefully match the product with your site, local conditions and regulations, and other weed and land management issues, and always follow the label directions for the product you are using. If your site has animals grazing, please read instructions carefully on grazing restrictions. For grassy areas, it is best to use a selective broadleaf herbicide to keep the competitive grasses intact. After spraying, wait two weeks or more to give the herbicide time to work.

Biological control: The bull thistle seed head gall fly (Urophora stylata) lays eggs on closed flower buds in June and July. After hatching, the larvae burrow into the seed-prodcuing tissues to feed, forming galls and reducing seed production. If the bull thistle population is large enough to support a good sized population of this insect, it can be an effective way to reduce seed production of the bull thistle. This insect will not get rid of the bull thistle, however, just reduce its impact. See the biological control page for more information.

Cultural/grazing: Bull thistle is not as competitive in a well-managed pasture and regular cutting combined with good pasture management can reduce bull thistle to a manageable level. Also, goats and sheep have both been used for grazing management of bull thistle. Even horses will help by picking out many of the nectar-rish flowers and eating them before they go to seed.


Watch the video: Wild edible Bull thistle