Make a management plan now for perennial weeds in 2020

Author: Natalie Hoidal, UMN Extension

Do you have perennial weeds in your fields?  Perennial weeds are extremely difficult to manage, especially in organic systems. Effective strategies require persistence, often over the course of several years. In this article, we'll look at one of the most difficult perennial weeds of all - Canada Thistle - and what strategies can be used for management.

What makes Canada Thistle so hard to manage?

• Roots can grow up to 18 feet deep & a single root can spread 20 feet in diameter!
• Root fragments can survive up to 100 days in soil - while frequent tilling is a common management strategy for Canada Thistle, tillage creates more root fragments, which can sprout into new plants. 
• It can survive 2-3 years under intense management - even if substantially reduced, one root fragment can quickly become 100.

Canada thistle root system after 2 years from original one foot of root area - Purdue (Merrill A. Ross)

What are its weaknesses?

• Canada Thistle requires a lot of light to grow. At 60-70% light interception (or, when 30-40
  % of ambient light is blocked), shoot growth will be reduced. At 20% interception (when 80% of light is blocked), you can actually kill the shoots. This means that strategies like occultation and well-established cover crops can play a major role in reducing the vigor of Canada Thistle!

• All perennials experience distinct periods of carbohydrate movement. During vegetative growth, plants push their stored resources above ground to maximize above-ground biomass. This is because above ground biomass is able to photosynthesize, which ultimately allows the plant to accrue more energy. Early in the spring, thistle plants use the stored energy in their roots to grow new above ground shoots. Once the shoots become large enough to start producing their own energy, they continue to use that energy to grow above ground, but they also start to send some of it back to the root systems, which spread quickly through the soil. What does this mean for management? When the plant is pushing its stored energy above ground, you can remove biomass by mowing, or sometimes with frequent tillage. By frequently mowing, you slowly deplete the plant's energy, and force it to utilize more stored, underground resources to re-sprout. 

Lifecycle of Canada Thistle

The following diagram from Stasica, 2009 provides a nice overview of the Canada Thistle lifecycle. Plants emerge in the spring (usually early to mid May in MN) and continue to grow until they flower in midsummer. During this period of vegetative growth, the plant is developing above ground shoots, but it's also sending carbohydrates back down into the roots, which spread vegetatively. Seed production occurs in July and August. After flowering, the aboveground biomass dies back. In the fall, plants will produce a rosette. 

Lifecycle of Canada Thistle from Stasica, 2009. I've overlaid pink arrows showing carbohydrate movement at each stage of the lifecycle, and maroon boxes suggesting management strategies that match each life stage. 

Let's discuss some management strategies for Canada Thistle along with pros and cons of each approach. 

Repeated mowing

• Pros: Repeated mowing is not destructive to soil. It's an effective way of slowly depleting the plant's stored energy. This approach works very well when you have Canada Thistle (or any persistent perennial) growing along field margins or in walkways. 
• Cons: Mowing must be timed correctly. If you're not mowing frequently, you're allowing above ground biomass to photosynthesize and produce more energy for the plant. Some research shows that this approach is less effective than frequent tillage. This approach is also less practical when you have Canada Thistle growing in the middle of a field vs. on the margins. 

Smothering with cover crops

• Pros: Cover crops help to build soil organic matter. They may also help to improve infiltration in your fields. This is a great non-chemical approach to smothering Canada Thistle if you can get a dense enough stand in place to reach 20% light infiltration or less.
• Cons: It may be difficult to get a dense enough canopy to shade out the thistle. Monitor your cover crop stand regularly to check for weed escape. 
• Tips and resources: multiple research studies have shown that cover crop mixes are more effective at smothering thistle than just using one species. This is especially true for high biomass producing crops like sorghum-sudangrass, buckwheat, and  For some more in-depth information about which cover crops to choose for this purpose, see: 
• Cornercopia Student Farm's experiences
• Report of cover cropping trials for thistle management in Illinois, 2007

Tillage:

• Pros: Numerous studies show that frequent (every 2-3 weeks) tillage for up to three years can effectively control Canada Thistle. 
• Cons: Intensive tillage takes a toll on your soil and can result in compaction issues.

Solarization and occultation: 

• Pros: These are great ways to smother emerging thistle in the spring, as well as emerging rosettes in the fall. Minnesota growers who have used occultation for 4-6 weeks in the spring have been able to plant a crop in the same field in the same year without Canada Thistle issues
• Cons: The results of these practices can be deceiving. If you use solariazation and occultation in the spring and don't see Canada Thistle again during the summer, don't assume it's gone. You'll likely have to repeat the practice again next year. These practices help to disrupt the lifecycles of perennial weeds, but they likely have quite a bit of below ground stored energy in their root systems. 

Flaming

• Pros: Flaming is often cited as a tool for controlling Canada Thistles, and it can indeed be effective at reducing aboveground biomass. 
• Cons: Flaming is most effective when plants are quite small, and it only kills aboveground biomasss. Frequent flaming is an option, but you'll likely find that mowing is a more affordable option. 

Herbicides

• Pros: Herbicides work quite quickly compared with the other management strategies listed above. A systemic product will be able to reach the root system, whereas most of the aforementioned strategies only address aboveground biomass.
• Cons: There are no organically approved herbicides that will effectively control Canada Thistle. Organically approved products tend to burn aboveground biomass (usually with an acid like vinegar or citrus oils) or strip the plant cuticle away (usually with detergent-like products), but they are not systemic. These products can also have serious human health effects (e.g. being corrosive to skin and eyes), so make sure to read the label closely before using them. 
• Resources: See the Midwest Vegetable Production Guide for Commercial Growers for an up-to-date list of recommended products. 

Key takeaways

• Managing perennial weeds is a long-term game. Focus on reducing the plant's stored energy over time.
• Make a plan now for how you'll manage perennial weeds in your field. Prevention (such as occulatation or using smothering cover crops) will make management easier throughout the season. 
• If you've got Canada Thistle or other perennial weeds on your farm, create a management plan now. These plants can spread extremely quickly, so prevention is key.