May 25, 2022
Controlling weeds through tillage
Sam Hitchcock TIlton, OGN Correspondent

I enjoy helping people kill weeds. Why? Not because of an ancient grudge or instinctual hate – there are times when weeds can be a protective skin that saves the land from the ravages from erosion – it’s just that weeds can stress farmers out, lower yields and make harvest more difficult. 

I deal a lot with cultivators and weeding machines that do the delicate work of killing weeds growing with the crop. But this is hard work for a machine to do and they’re not always successful, especially when weed densities are high. The best perspective is to be controlling weeds at all the times and in all the operations when your crop is not growing, so that weed populations are as low as possible when your crop is in the ground. 

If growing a corn or soybean crop – or an even faster-growing vegetable crop – there are many tillage operations done to that piece of land throughout the year, both before and after planting. I’d like to walk through some considerations for tillage that can help you make the most of each tillage operation, so that instead of turning under a crop or preparing a seedbed, you’re also controlling weeds at the same time. To that end, I wanted to take some time to talk about ways that you can use tillage to control weeds.

When I meet a farmer or give a talk, I often ask a question first: “What are your top three to five weeds?” This is crucial. Your goal as an intelligent grower (in my opinion), is to understand the personality of your main weeds so that you can design a cropping system that targets their weak spots. 

Weed guru Chuck Mohler divided weeds into three main categories – annuals, fixed perennials and creeping perennials (that spread through underground roots). 

Make a list of your top weeds and sort them into those three categories. To control specific weeds through tillage, you want to know the best time of year to target it and the most appropriate type of tillage tools to use. I thought the clearest way to share this would be to choose a particular type of weed and explore how we can use tillage to control it. 

Let’s start with creeping perennials. Examples include bindweed, nutsedge or quackgrass, which are some of the most frustrating weeds out there. We want to reduce the population of these tough guys before we put our crop in the ground. First, let’s find the weakest point in the weed’s lifecycle so we can use tillage at that time. Do you know that the entire D-Day invasion force was kept waiting in England for days as General Eisenhower waited for the perfect weather conditions for the assault? Similarly, hold your tillage until the time is right and your target weed is at its weakest. 

Creeping perennials spend all summer using solar panels (leaves) to create sugar from sunlight and then storing that sugar in their roots. Then the plant shuts down for the winter and draws on some of that sugar to stay alive. Come spring, the plant must spend its root-stored sugar to grow new shoots and leaves. At this point, there is a period when the plant has used most of its sugar to grow new stems and leaves but before those new leaves have recharged the root with sugars – this is the weakest point in the creeping perennial’s life cycle. 

Each creeping perennial has a slightly different time when it has reached its lowest energy reserve. For quackgrass, it is when the plant has three or four leaves. We can’t put a calendar date on this timeframe period because it depends on soil temperature and soil moisture – that’s what field scouting is for. 

You’ve identified your top weeds – you know some of them are creeping perennials – and you’ve researched those creeping perennials and found the point in their life cycle when they are weakest, the perfect time to till.  

Now, what type of tillage tool to use?

For this decision it is helpful to know a little more about thine enemy. For years, I heard people say not to use tillage tools that cut quackgrass into small pieces because each of those small pieces will sprout and turn into another plant. However, the smaller the piece of root is, the weaker the resulting plant will be (less sugar in the root to feed shoot growth). 

Because cutting plants into pieces makes them weaker, choose tillage tools that will cut your creeping perennials into small pieces. These are tools like discs and rotary tillers. Chisel plows and moldboard plows, on the other hand, don’t cut the roots into small pieces and they can also drag them to other parts of the field. 

The remaining root fragments will keep trying to sprout (depleting more of the plant’s energy), but we can send them to their demise using maybe one of two techniques:

  • Either drag them to the surface with a field cultivator or tine-weeder for a few weeks to dry them out and then plant our crop, 
  • Or moldboard plow them to bury the weakened fragments deep in the soil. 

When buried deep in the soil, these weakened root fragments will have to use a lot of energy to send their shoots all the way up to the surface. Hopefully, they will not have enough energy, or if they do, by the time the weakened shoots break the surface they will be shaded out by the canopy of our dense and thriving crop.

You can see from this example with creeping perennials that a knowledge of your weed’s personality is crucial to management. But where to find this information? I have a few ideas for you – a new book called “Manage Weeds on Your Farm” by Mohler, Teasdale and DiTommaso. It lays out a lot of good foundation knowledge in a simple way and includes a section in the back with personality information on all main weeds with how to target them – read this book! (Thank you to us for paying taxes and government funding in the form of SARE for making this book a reality). 

You’ll also want a book on weed identification for your region, something like “Weeds of the Northeast” from Cornell University Press. “Weed Management for Organic Farmers” by Davies, Turner and Bond is another great book to teach about how knowledge of weed personalities can influence tillage choices.

I had better leave it there for now, in future columns I hope to discuss more about how we can use tillage to reduce our weeds. 

Until then. Yours, Sam. 

Top photo — Quackgrass shoots with rhizomes ready to sprout into shoots. The dense layer of quackgrass roots are generally in the top 6 inches of the soil profile. So, consider that when adjusting your tillage implement, you’ll want a working depth of more than 6 inches. Photo: Sam Hitchcock Tilton


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