Thistle Weed: Identification and Control

Thistle weeds may look pretty when they are in bloom, but usually they are unwanted invasive plants. How do you get rid of them?

There are approximately 200 different species of these annual, biennial, and perennial plants that grow all over the world. The majority of them are members of the Asteraceae (sunflower) family. It is important to remember that there are both native and invasive thistles.

Native species are advantageous to the ecosystem. They provide nectar and food to pollinators such as birds and bees (pun intended). The goldfinch makes use of plant down to line their nests. Furthermore, the likelihood of native thistle species invading and taking over an area is significantly reduced.

There are numerous noxious and invasive exotic species. These invasive species will appear in an area and threaten to annihilate the native plants. As a result, the food sources for the wildlife in the area that feed on the plants will be reduced. This, in turn, reduces the amount of prey available to predators that feed on the animals eating the plants. It’s similar to the butterfly effect in terms of ecosystems.

Another interesting aspect of thistle is that it is a paradoxical plant.

What exactly do I mean by that?

On the one hand, they are beautiful plants that are often so aesthetically pleasing that you may wonder whether you want to remove them from your perfectly manicured lawn.

Their leaves and stems (and sometimes even their flowers) are, on the other hand, prickly to thorny. If you touch one of these textures unprepared, you may have an uncomfortable time.

Not to mention that their rapid growth and invasive spread can quickly overwhelm your lawn, and they are classified as noxious weeds in nine different states.

When it comes to thistle, don’t judge a book by its cover. Because, in this case, beauty is only superficial.

First, consider the general characteristics shared by all members of the Asteraceae family.

Blooming pink flowers of a milk thistle plantt

General Thistle Characteristics

Thistle leaves are distinguished by leaf margins covered in fine, sharp prickles. Some species have spike-like prickles that cover the entire plant, primarily the stem but also the surfaces of the leaves and, on rare occasions, the flowers. The prickles are an evolutionary defense mechanism that prevents herbivores from eating them.

It’s worth noting that the prickles differ greatly between plants. Some are extremely fine and pliable to the touch. Others are quite heavy and stiff. As a general rule, species found in dry habitats have more rigid prickles.

Another feature shared by the majority of these plants is the formation of bracts in whorls that support the inflorescence. This is known as the involucre. The involucre structure of the plant is similar to that of a cup or a bowl. This clasping shape encompasses each flower head on the plant. The bracts, which surround the flower head, are usually spiny to the touch. Many have a single spine at the tip of the bract.

Another feature they all have in common is that their flowers are all pretty consistent in terms of color. Thistle flowers are typically white, purple, or pink. Some people mix these colors to make unique shades, but nearly everyone sticks to these colors. There are some outliers with solid blue flowers, but they are uncommon.

Commonly Found Varieties

Bull Thistle (Cirsium vulgare)

Bull thistle is a biennial type of thistle. It has leaves that are up to 8 inches longer than those of its relatives. Each of the elongated leaves has a spine on the tip and tapers at the end.

Bull thistle flowers can be white, pink, or purple. Spines are found in and around the base of the inflorescence’s head. The surfaces and edges of the flower petals are covered in very fine hairs.

Cirsium vulgare is common in areas that are not mowed frequently. Bull thistle thrives in areas such as roadside embankments, medians, and pasture land.

A flowering and yet to flower bull thistle

Canada Thistle (Cirsium arvense)

Canada thistle flower heads can grow to be between 34 and 1 inch long. They are pink to lavender in color. White flowers can also be found on occasion. A single Canada thistle can produce several flower heads. They appear singly or in small clusters at the tips of branching stems.

The involucre bracts are flat except for the tips. The bract tips fold back away from the inflorescence. The bracts lack spines but do come to a point. The bracts are frequently purplish in color.

Cirsium arvense leaves have no stalks and can grow up to 6 inches long. The edges are wavy and covered in yellow spines that are strewn about. Fine hairs may cover the underside of the leaves.

The lower leaves are heart-shaped and can grow to be up to 3 inches wide.

Upper leaves are typically narrower and less deeply lobed than lower leaves. They are also significantly more dentate or toothed than the lower leaves.

The stem is ridged and light green. Hairs can be found on the stems, but they are usually fine and softer rather than spiny.

Flowering cirsium in the meadow

Milk Thistle (Silybum marianum)

One of the more easily identified species of this weed is milk thistle.

The large flower head ranges in color from a typical purple to a bright magenta. Milk thistle is an annual or biennial plant that is considered a noxious weed that is difficult to control. The distinctive flowers appear singly at the stem’s tip.

The involucre bracts are fleshy and thick. They form an elongated triangle and come to an acute angle. A rigid spike protrudes from the bract’s tip.

The bright green leaves of the Silybum marianum are one of its most distinguishing features because they are covered in a distinctive white marbling. Early in their lives, young milk thistle plants appear in rosettes. Spines cover the edges of the gleaming leaves. The hairless surfaces of the leaves grow to be 2 inches wide and 5 inches long. The uppermost leaves of the milk thistle have no petioles and clasp the spine-covered stem.

Marianum pink flowers blooming beautifully

Thistle Control Methods

Once you’ve determined that you’re dealing with an invasive species or a native species that has overrun your lawn, here are 10 quick ways to deal with them.

1. Selective Herbicide

Selective herbicides containing triclopyr or clopyralid in the formulation will kill the weeds without harming the surrounding vegetation. While a selective herbicide reduces the risk of harming desirable plants and grass, try to avoid spraying any plant that you are not trying to kill. Better to be safe than sorry.

Using a selective herbicide in conjunction with the planting of desirable vegetation in the area to crowd out any Asteraceae family members that may appear. Don’t be disheartened if the plant does not die.

2. Systemic Herbicide

If the area you’re treating is overgrown with thistle and other plants you don’t mind killing, use a glyphosate-containing non-selective systemic herbicide. Glyphosate is the most effective and efficient ingredient for permanently killing them. It will also kill everything else it comes into contact with.

Spray the glyphosate early in the growing season for the best results.

A person spraying some weedicide

3. Sow Plants to Compete With Thistle

Perennial grass plants will prevent thistles from gaining nutrients. Without a doubt, the best plant for this is alfalfa. This is because alfalfa will begin absorbing nutrients from the soil before the thistle has even sprouted.

Growing grass plants of any variety will benefit your lawn. Spread the seeds around the area and gently rake them into the soil. If you have a lawn roller, get it out and thoroughly roll the area. This ensures the best soil-to-seed contact and allows the seeds to absorb nutrients as soon as possible.

4. Removing Them Manually

If your Cirsium infestation does not cover a large area of your lawn, you can manually pull the weeds up. If you go this route, use a spade, a hoe, or a hand trowel to pry up the root system. Make sure to move every root, leaf, stem, flower, or other plant scrap that you find in the soil.

Make sure to thoroughly clean your tools to remove any seeds that may have gotten stuck on them. If you start using that tool again and that seed makes it to the soil, the thistle will return for a second act.

5. Mowing the ThistleĀ 

If you can mow thistle back before it blooms, you have a good chance of winning the battle before it even begins. If you catch the plant before it blooms, it hasn’t had time to mature and develop seeds for reproduction. Remember that many of these are annual plants that must go to seed and produce entirely new plants to complete the cycle of life.

Begin mowing the area as soon as possible in the spring. Continue mowing as needed whenever new growth appears.

A pro tip is to time your mowings just before a heavy downpour. This harms the plant’s root system. This, in turn, exposes the base to excessive amounts of water, allowing fungal infections and other diseases to develop in the roots.

A red lawnmower ready to trim some grass

6. Tilling the Soil

Cultivating the earth during the autumn months weakens the roots and eventually starves them to death.

This task is best accomplished with a tiller. If not, get yourself some work gloves and a rake or shovel. Dig into the soil and remove any parts of the root that you find in any area where you find this weed (or the remains of one). This will destroy any existing root systems in the soil and aid in the death of the majority of the plants before the winter months.

7. Solarization or Covering Up the Plants

You can suffocate the plant by placing a flat, heavy object over it, such as a piece of plywood. Weigh down the plywood with heavy objects so that the sides dig into the ground, allowing no light and very little oxygen to reach anything beneath the plywood.

Solarization works on the same principle, except you cover the problem area with black plastic sheeting and stake it into the ground. The black color of the plastic will attract sunlight while keeping it out of the picture.

Both of these methods will eliminate Asteraceae plants from your lawn.

8. Grazing Livestock

Because thistle is frequently found growing in pastures, it only makes sense to let loose cattle, goats, or horses on the problem. All three of these animals are fond of thistle and would do an excellent job of keeping it at bay.

This is especially true of goats.

Although the animals will not completely eradicate thistle, they will consume enough of the plant to keep invasions at bay.

I would not recommend bringing livestock to your charming little subdivision to graze on your lawn. However, if you have a pasture and some animals, why not let them do the work?

A dark brown goat wandering in the field

9. Spray Vinegar on the Thistle

To be honest, vinegar is probably the only natural weed killer in your thistle control arsenal. The standard 5 percent white vinegar will not be effective against these tenacious weeds. Instead, use 20 percent concentrated industrial-strength white vinegar.

Simply pour the vinegar into a spray bottle and spray the thistle once or twice. Using vinegar to combat these noxious weeds will take some time and effort on your part. The thistle will regrow relatively quickly at first. When it regenerates, return with the vinegar and repeat the process. Continue returning with the vinegar, and the root system will eventually dry out and the plant will die.

Patience. It may take several growing seasons to completely destroy the thistle’s root system.

Remember that vinegar is a non-selective herbicide that will kill any plant life that comes into contact with it. If you have desirable ornamentals or turfgrass in the same area as the thistle, make sure to only spray the thistle with vinegar. Otherwise, you’ll unintentionally create a new issue for yourself.

10. Use Some Beneficial Bugs

There are some insects in mother nature that enjoy eating thistle. As these bugs consume the plant, its structure weakens and its surface area disappears.

The problem is that most of these bugs are not commonly found in your lawn and are difficult to attract. So, if you want to use insects to combat the thistle invasion, you’ll most likely have to buy them.

Shop around for 100 stem-mining weevils (Hadroplontus litura). Once you have them, release them near the thistle patch. The plant will be eaten by adult stem-mining weevils.

After that, the weevils will lay their eggs inside the stem. The larvae will hatch here and feed on the inner stem until they reach maturity. They will remain as long as there is enough thistle actively growing in the area to feed all of the weevils.

This method is most likely the least dependable. These insects are alive and moving all over the place. As a result, there is no guarantee that they will persist long enough to completely eradicate the thistle. When the thistle numbers fall low enough, the bugs will see the writing on the wall. They recognize that their food supply is nearly depleted and that they cannot support all of the insects in the area. So they’ll migrate to greener pastures in search of a more reliable food source.

A gray beetle sitting on the leaves of a plant
Jeffrey Douglas
Jeffrey Douglas own a landscaping company and has been in the business for over 20 years. He loves all things related to lawns or gardens and believes that proper maintenance is the key to preventing problems in the first place.
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