Canada Thistle: The Good the Bad and The Ugly

Flower

Canada thistle (Cirsium arvense) is an invasive weed found in almost every state, including Idaho. Everywhere it occurs it is problematic and in many areas is on the invasive-noxious weed list. The University of Idaho Extension has published a handbook: Idaho’s Noxious Weeds. See the link below for the full printable 9th edition. Canada thistle is listed on page 22. So how can this plant be considered “good,” “bad,” and “ugly” all at the same time? The following discussion will answer this question as well as offer methods of control.

Quick Identification

Canada thistle, also known as Creeping thistle, is an herbaceous perennial in the Asteraceae family that grows to 5 feet tall. It grows in roadsides, cultivated fields, pastures and non-croplands landscapes.

Rosette Stage

Leaf

Stem

This thistle is most known for the following characteristics: 

·    -toothed and spiny oblong leaves

·    -purplish-pink flower heads, which grow in numerous clusters and vertical and lateral to 20 feet deep and 15 feet across

Here's a video that may aid in identification.

Canada thistle – What’s Good About It?

There are some benefits to nature and for the environment.     

This thistle is beneficial to pollinating insects, butterflies, predatory insects and some moths and birds.The caterpillar of the painted lady butterfly (Vanessa cadui) feeds on the leaves. 

Canada thistle is a good food source to many songbirds which feed on the nectar and seeds including the American Goldfinch and Indigo Bunting. An extensive root system can also help break up compacted and heavy-clay soil.

                                                       

Beautiful Weed with Bad Habits

Canada thistle is hard to control and kill because of its extensive root system. It likes to form dense colonies that often crowd out and replace native grasses and other desirable plants. In pastures and range lands cattle generally will not graze near infestations of Canada thistle. It produces nitrate, which can be toxic to horses if ingested in large quantities. In cropland, Canada thistle causes extensive yield losses.

Noxious weeds are non-native, invasive plants. At first glance they may appear pretty, but invasive plants can pose serious environmental threats such as the displacement of native species, and the destruction of crop land.

Canada thistle Infestation/Colony 

Canada thistle can be said to be ugly because it is on the noxious weed list in 43 states. In fact, it is one of Idaho’s most damaging noxious weeds. The Wikipedia definition is as follows: “A noxious weed is a weed which is considered to be harmful to the environment or animals, especially one which may be subject of regulations governing attempts to control it.”

Management and Control of Canada Thistle

Management and control of Canada thistle can be very difficult. It takes repeated efforts using several methods done over several years.  A task that requires consistency!  An “Integrated Management Plan”  that combines different techniques and actions may work. 

One important principle is to stress the plant and force it into exhausting nutrients stored in the roots. The second important principle is that Canada thistle resists any stress or attack by trying to produce new shoots from any little piece of root left in the ground. Consistent efforts can bring success!

Integrated Methods

Manual, Hands-On: If you have a small infestation, digging with a garden fork or hoeing should be done when the plant is young and not flowering. But this may just encourage new shoots because most of the root(s) go deeper than the fork can reach. You can also use scissors or a hoe to cut the whole plant off at the ground level. Doing these actions repeatedly and before Canada thistle blooms will keep it from seeding and stress it enough to discourage some new plant growth.

Cultural: Canada thistle generally likes poor quality soil. Improving fertility of the soil may discourage this thistle and make it more advantageous for desired plants. The first thing to do is to have the soil tested. The results will indicate what your soil needs to improve fertility.  

Mechanical: This involves mostly mowing in lawns, gardens, croplands, and non-croplands. This should be done once a week every 3-6 weeks from mid-June to September. Mowing in the spring should be done at early bud stage and before the thistle flowers and sets seed. Mowing can be done in the fall when there is a second flush of shoots. This may be the best time to mow because plant growth is slowing down and energy is going to the roots.

Chemical: Herbicides have been used to control and eradicate Canada thistle in gardens, croplands and in non-croplands. If you are using herbicides be sure to read and follow the product label instructions including precautions and warnings. Additionally, make sure Canada thistle is listed on the product’s label.

Organic: Also known as mechanical Many people want to avoid chemicals and use more natural methods in their garden such as weed management and solarization. Solarization is a great option for home landscapes

Here's the link for the complete .pdf of Idaho's Noxious Weeds.

Now is the time to or mow, mow, mow to eradicate this non-native invasive plant!