Controlling Canada Thistle – Canada Thistle Identification And Control

Canada thistles are common weeds in many areas of North America. They have been known to grow up to 2 feet tall and spread over 3 feet wide. The leaves are opposite, oblong-oval shaped, dark green and hairy at the base. They have four leaflets which vary from 1/8 inch long to nearly 5 inches long (3 cm).

The flowers are white or pinkish red and range in size from 1/4 inch across to almost 1 inch across. There are two types of seeds: white to black, oval shaped, round and slightly flattened, and black to reddish brown.

The name “canadahouse” comes from the fact that they were first found growing near the shores of Lake Ontario in Ontario, Canada. They were called canadian thistles because of their resemblance to the thistle plant.

They are not native to North America; however, they have been introduced into some areas where they are now established. Their natural enemies include birds, insects and other plants. They do well in poor soils and in disturbed soil conditions. They prefer moist but well drained soil with good drainage.

The way that the Canada thistle grows makes it very difficult to get rid of completely. The deep taproot can make it difficult to control, especially in warm season grasses. The roots also have a system of branching roots that can extend up to 18 inches from the main root system. The vertical stems and horizontal stems means that the plant can spread out to find more sunshine and allow the leaves to get more sunlight.

The horizontal stems also produce buds that allow the plants to reproduce. The buds can form roots while attached to the mother plant or can break off when touched the ground and grow as a new plant.

They are considered a noxious weed in most places and are generally controlled by herbicides, hand pulling or mowing. An important thing to remember is that control of Canada thistle is more effective before it goes to seed. Once it starts to flower, it is more difficult to control and the seeds can remain viable in the soil for up to seven years.

Once established they are very hard to get rid of because the tap root can extend down 6 feet or more. If you do manage to get rid of them, they can also reappear from the root pieces left in the soil. A good garden hoe (weedwhacker works too) and some persistence can allow you to get rid of them.

They are grouped with the normal (or smooth) thistles and the musk thistles. The normal (or smooth) thistles have a more papery texture to their leaves and the musk thistles have fuzzy leaves and flower heads.

The life cycle of the Canada thistle is similar to other types of thistle. Each plant can produce as many as 2000 seeds and each plant can grow from 1 to 3 feet tall. The root system of the plant is very deep (5 to 6 feet) and has numerous side roots and it takes 6 to 12 inches of soil to cover the main root.

Controlling Canada Thistle – Canada Thistle Identification And Control at

The new leaves start out green in color, but mature to a silvery blue-green in color. Once they are mature, they tend to droop downward and form a nearly straight line with the stem. The leaves have a very rough, sandpapery texture and are 2 to 6 inches in length. They can grow as long as 3 to 4 inches wide and ¾ of an inch thick.

The top of the leaf is rounded and the bottom tapers to a point.

The stems and petioles (stems that connect the leaf to the main stem) are green to reddish green in color and are hairless. The stems are round and hollow (except at the bottom where the root is).

The flower heads are green and start out as a tight bud. The head is surrounded by two large, bright yellow rays (sometimes there can be just one large ray) that can be up to 6 inches long. These surround a smaller disc of flowers that are also bright yellow in color. The center of the flower is filled with small purplish black florets.

This thistle seed head can be up to 4 inches across and is very noticeable, but unlike the sow thistle , it does not have spines. The stem that holds the flower head is also smooth.

The seed head is also surrounded by leaves that are deeply lobed into 3 sections (ternately lobed) that are 2 to 5 inches in length and two shades of green in color.

The stem for this plant is also green, but can have tinges of red or purple and are hairless. They grow to be 3 to 8 feet tall and the upper leaves are generally smaller than the lower leaves. The first two leaves that grow near the base of the plant are very large, almost circular and have smooth margins.

Canada thistle is in the Sunflower (Asteraceae) family and grows as a biennial or short-lived perennial . It is a very hardy plant that grows in poor soil (especially heavy clay) and can grow in a variety of different soil types including sand, loam and gravel. It generally grows in fields, waste places, open woods, along roads and other open areas.

While most think of Canada thistle as being a pest of cultivated land, such as crop fields and vacant lots, it is also a pest in uncultivated land such as pastures and meadows. It was originally brought to the United States in the late 1800’s as an ornamental plant. It was also used as a forage crop, but this soon turned out to be a bad idea.

Controlling Canada Thistle – Canada Thistle Identification And Control -

It is most often confused with the musk thistle and the normal or smooth thistle (Cirsium discolor). The difference between these three plants can best be seen in their seed heads.

Sources & references used in this article:

Identifying acetolactate synthase inhibitors for potential control of quackgrass (Elytrigia repens) and Canada thistle (Cirsium arvense) in corn (Zea mays) by CL Sprague, AL Frasier, D Penner – Weed technology, 1999 – JSTOR

Canada Thistle (Cirsium arvense Scop.) Control with Cultivation, Cropping, and Chemical Sprays by JM Hodgson – Weeds, 1958 –

A meta-analysis of field bindweed (Convolvulus arvensis L.) and Canada thistle (Cirsium arvense L.) management in organic agricultural systems by N Orloff, J Mangold, Z Miller, F Menalled – Agriculture, Ecosystems & …, 2018 – Elsevier



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