CALEDONIA, Minn. — Controlling weeds – especially ones with invasive properties – is challenging. Part of my job is to help people make decisions related to weed control. Here are some things you can expect when calling the Extension office looking for advice.
- There is often no silver bullet to control the weeds we have today. Complete control takes time and work. And you will often need to use multiple approaches over several years.
- The Extension office will not provide a magic formula for mixing chemicals for weed control. The only spot to find legal mixing options is on the label of the product you are using.
- Suggestions of weed control methods from the Extension will include a range of options, for example cultivation/hand pulling, burning, crop rotations and chemical weed control. Extension is not biased for or against any specific method.
My job is to educate the public about the inevitable trade-offs that come with decisions on weed control. For example, how do we effectively control weeds while at the same time maintain and promote environmental stewardship? Here are some common suggestions I give – which often receive skeptical replies – and why I give them.
Sometimes pulling weeds is the best option
Manual removal of weeds – whether by mechanical cultivation, with a hoe or by hand – is time consuming, tough work. But certain circumstances make this approach preferable over others. Consider the following example.
Wild cucumber has exploded this year. This vine grows on fence lines and windbreaks and has a pretty, vertically presented white flower. Wild cucumber is considered a native to North America but for many it is less than desirable to have growing on your property. On a small scale – e.g. in the windbreak on your home property – my suggestion is to pull whatever you can reach off your trees and pull the shallow roots out of the ground.
People often laugh at me when I say this but consider the alternatives. Wild cucumber can be controlled using products like dicamba and glyphosate. But if these products touch vulnerable plants underneath the vine they have the potential to kill those plants as well. Some herbicides can even leach into the soil and be taken up by roots so it is a challenge to use herbicides underneath trees as well. Manual removal of wild cucumber might not be feasible at the large scale but it is certainly preferred when we want to save the vegetation underneath the vine.
Spray weeds when they are small
How small is small? Chemical weed control is typically most effective when weeds are 2-6 inches tall. Weeds become increasingly tolerant to herbicide applications the larger they get. This year was a prime example of how a wet spring can make spraying during that 2-6 inch window very difficult. Take giant ragweed for example.
Giant ragweed is a common weed that is present in rural and urban settings all across the United States. Take a look down waterways, along fence lines and in unmown areas and you will see this plant. Giant ragweed has large leaves – upwards of 12 inches long and 8 inches wide – with 3 to 5 lobes. It can be upwards of 12 feet tall and is a major source of highly allergenic pollen.
This plant is often a problem in soybean fields when producers are unable to make a timely herbicide application. Producers then have to come back in with something aggressive – which may also hurt their crop – to try to control the larger ragweed plants. And there is no guarantee in this approach. After a chemical application like this ragweed will often look brown and dead but within a week or two can green back up and still produce viable seed.
Just because you don’t see it, doesn’t mean it isn’t there
Why is my office receiving so many calls about wild cucumber this year? Because their white flowers are very noticeable while going 60 mph down the road. But what weed do we not see this year or at least not as much of as in previous years? Wild parsnip. Last year wild parsnip covered the countryside in its yellow flowers but this year the visible population was significantly reduced. So do we still need to be concerned with wild parsnip? Yes.
Wild parsnip is a particularly nasty weed. It is highly invasive and can cause significant skin damage to humans and livestock. It is rather easy to identify when in bloom due to its yellow flowers. But as a biennial, it actually has two growth forms: a rosette and a flowering stage. The rosette grows close to the ground and can be very difficult to see in fully grown ditches and pastures. So just because we do not see it does not mean wild parsnip is not there. And if it is there we should still plan to control it this fall.
We at the Extension office are more than happy to help you with your weed questions. But do not call us expecting easy, one time solutions. Be prepared to answer questions. Where is the weed growing? What other plants are in the area? Do you plan on harvesting any sort of crop or forage from that area? Answers to questions like this aid in the identification of the weed species and allow for better suggestions for removal and control.
— Michael Cruse, University of Minnesota Extension
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