Perennial Sow-Thistle Biology and Management Strategies

By Shannon Chant, PAg., Crops Extension Specialist, Regional Services Branch, Swift Current

Perennial sow-thistle (Sonchus arvensis) plants have been easy to find lately. Like other weeds, they can compete with crops for water, nutrients and sunlight. Perennial sow thistle can also be a host for aster yellows disease, a viral disease spread by the aster leafhopper that can impact canola and other field crops.

The best start to a weed control plan is to know the enemy! Let’s get started.

As the name tells us, perennial sow-thistle is a plant that can survive for many growing seasons. It’s also deep-rooted and reproduces by seed and vertical and horizontal roots.

Reproduction by seed:

• Optimal germination temperature is 25 to 30 C. Below 20 C and above 30 C, germination is poorer.
• Germination does not start until the soil warms up. In Saskatchewan, most seedlings do not emerge until mid to late May and germination can continue until late July.
• The optimal soil depth for emergence is up to 0.5 centimeters, but seedlings can emerge from up to three centimeters deep.
• Seeds are viable in the soil seedbank for three to six years, but most seeds germinate within a year.
• Flowering to seed maturation takes about 10 days.
• About 30 seeds are formed per head; plants usually have multiple heads and there can be up to 50,000 seeds per square meter. The number of seeds depends on the competition with other plants and its surrounding environment.
• Seeds can also continue to mature on stems that have been mowed if the tissue remains moist.
• Seed production may be poor in isolated patches because cross pollination between different plants is required. Plants in an isolated patch will not pollinate effectively as they all originated from the same root system.

Spreading by roots:

• Plants can colonize new areas rapidly and survive because of a spreading root system. A single plant can form a large patch of stems using this type of spread.
• Horizontal roots start to develop at the four-leaf stage. After a month, plants usually have seven to eight leaves and roots have spread sideways 10 to 15 centimeters. At four months, the roots are 60 to 100 centimeters long and up to 50 centimeters deep.
• Vertical roots can penetrate two meters deep and can produce vegetative buds that can reach a depth of 50 centimeters. New shoots that emerge during the spring and early summer are usually from buds that overwinter on vertical or horizontal roots while the shoots developed from mid to late summer are usually from the top 10 centimeters of soil but emergence below 10 centimeters is possible.

Methods for control:

• The best approach to control perennial sow thistle is to combine chemical and cultural methods to reduce the weed’s competitiveness, prevent flowering and reduce the ability for roots to spread.
• Herbicides:
  • Many products have top growth control or top growth suppression of perennial sow-thistle. While the roots are not necessarily killed using these products, above ground growth reduction will limit the ability to provide carbohydrates to roots in the fall to survive the winter.
  • Carbohydrate movement to the roots from the leaves occurs during the rosette stage and after flowering. Control activities should take place during these times if possible.
  • If a plant only has one shoot, carbohydrates move readily to the root system. If more than one shoot is present on a root branch, carbohydrate movement was primarily in the root tissue between the shoots.
  • Several products can be used in barley, wheat and oats but options are limited for pulses and canola.
  • Post-emergent herbicide application timing may miss the seedlings that germinated well into the growing season. Make sure to check re-cropping restrictions, especially for fall applied products and in drier growing seasons.

• Cultural control:
  • Tillage usually reduces perennial sow-thistle stands but the level of impact varies depending on the stage of weed growth and the type of tillage. To be more effective, tillage should occur in spring at the five to seven leaf stage. If root sections are buried 30 cm deep or more, only 10 percent of them will be able to produce shoots that make it to the soil surface. Small root pieces will likely use their stored carbohydrates to make shoots, which are less viable. This is only successful practice when shoots cannot make it to the soil surface.
  • Repeated tillage may be required to exhaust carbohydrate reserve in roots. Newly germinated seedlings will use carbohydrate reserves in the root for two to four weeks until they have 5-7 leaves and then will be able to accumulate new reserves in the roots.
  • If you are using tillage to control perennial sow-thistle in the fall, the initial operation may spread small pieces of root out. These pieces can produce entirely new shoots that need to be controlled. Root sections less than one centimetre can result in plants that flower within a year. Multiple tillage operations may be needed to kill these new shoots and prevent them from providing carbohydrates to the roots.
  • Mowing is less effective than tillage to reduce populations but removal of stems before flowering may be useful to reduce seed production.
  • Any practice used to optimize plant stand and only provide nutrients to the crop will help crops be competitive with this and other weeds.
  • Perennial sow-thistle is palatable to cattle and sheep and pasturing land can be used to reduce stands.
  • Spring cultivation followed by the establishment of perennial grasses or alfalfa can reduce populations up to 90 per cent.

For more information, read the 2024 Guide to Crop Protection and contact your local crops extension specialist or agronomist.