The following is an excerpt from Helen Atthowe’s new book The Ecological Farm (Chelsea Green Publishing June 2023) and is printed with permission from the publisher. 
My On-Farm Weed Suppression Experiments
I was interested not only in examining the effects of tillage on soil and microorganism health, but also in evaluating the effect of tillage on weed suppression and crop yields.
Effects of Tillage & Applying Compost
In my first experiment, my goal was to evaluate which weed species would be encouraged over two growing seasons by different tillage and fertilizer treatments, in minimally tilled crop rows, in no-till crop rows, and in my farm-made compost. An area of original 50-year-old, untilled pasture on my farm served as the basis for comparison (the control).
The Compost Results
The compost I made at Biodesign Farm was a mix of manure, straw, and red clover, which was heated to USDA National Organic Program standards during the composting process. To evaluate the weed seed reservoir in my finished compost, I filled 1-gallon (4 L) pots with the compost, set up the pots in my greenhouse, watered them well, and then observed what sprouted and grew. I discovered that the greatest diversity and abundance of annual weed species was present in the pots when compared with numbers and species of weeds counted in my crop fields or the untilled pasture area. In fact, there were some species that I did not observe at all in my crop fields or in the pasture area. This suggested to me that the one compost ingredient I brought in from off the farm—the sheep manure from my neighbor’s farm—was the source of the unique weeds. When I stopped using manure, annual weeds also decreased in my vegetable fields.
The Crop Row Results
In 2006 I tilled some experiment rows shallowly in the spring and then did no further weed cultivation after that. I also left other rows untilled altogether to measure the effects of no-tillage. I identified the weeds that sprouted and grew in the rows, and I found higher numbers of annual weed species in the minimally tilled plots compared with the no-till plots and the control area. The untilled pasture control had the most perennial weeds and no annual weeds at all (had the most perennial weeds and no annual weeds at all (for full results for the “Weed Species Found in Biodesign Farm Weed Ecology Study” see page 170, table 8.6 of The Ecological Farm).
The lesson to be learned from this experiment is that tillage results in more annual weeds and that not tilling encourages perennial weeds over the long term. Another lesson is that applying compost can bring in new weed problems—even compost heated to 149 degrees F (65 degrees C) can be a source of weed seeds!
Read more: Compost like a pro with this simple technique.
Exploring Complex Interactions
In 2007, I set up another experiment as part of my quest to understand the interactions among soil health and nutrient cycling, crop yield, and weed competition. I evaluated how these interactions were affected by several soil/weed management treatments.
The five treatments were: no-tillage, minimal tillage, conventional tillage, tillage plus vinegar spray, and tillage plus paper mulch (EcoCover). The experimental area consisted of one 600-foot-long, 4-foot-wide (183 × 1.2 m) crop row split into replicated plots, three per each of the five treatments. At the start of the experiment, a 2-year-old red clover cover crop was growing in the row. I strip-tilled the treatment plots, except for the no-till plots, making three passes with the tiller in the tillage plots, but only one shallow pass in the minimal-tillage plots. Tillage plots were kept weed-free with cultivation to keep the soil surface bare all season. I applied a weed mat paper mulch over another set of tillage treatment plots and sprayed a vinegar herbicide on weeds twice during the growing season in still another set. In the minimal-tillage plots, I tilled only once and did not cultivate weeds that grew back during the season. In the no-till treatment plots, I closely mowed the red clover in the spring and then burned the remaining growth with an infrared flamer. I left the perennial red clover intact as a living mulch in the row middles on either side of all treatment plots. I planted all of the prepared plots with Brussels sprouts.
Here are the highlights of the results of this experiment:
Yield
Brussels sprouts plants in tilled plots covered with paper mulch had more biomass and the highest average yield per plant. Next best were the plants in the conventional tillage rows and the rows that were tilled and sprayed with vinegar before planting. I concluded that tillage increased yields by minimizing crop competition with other plants.
The minimal-tillage rows took fourth place, and the no-till plots had the lowest yield and smallest plants. Using the flamer was not my best vegetation suppression idea. Although these plots had the highest level of mycorrhizae, the red clover grew back from its strong roots within several weeks and competed heavily with young Brussels sprouts.
Soil Temperatures
Paper mulch and conventional tillage plots had the highest soil temperatures from April through the end of August. Thus tillage resulted in increased soil temperatures, especially early in the season. Soil temperatures were lowest in the no-tillage and minimal-tillage plots all season.
Mycorrhizae Presence
The greatest mycorrhizal density was observed in no-tillage plots; the second greatest density occurred in minimal-tillage plots. The tillage/vinegar treatment resulted in the lowest density, followed by conventional-tillage and tillage/paper mulch plots. It was clear that tillage led to a decreased presence of mycorrhizae.
Soil Aggregate Stability
Tillage plots tended toward lower soil particle aggregate stability, but the lower levels were not statistically significant in this 1-year experiment.
Soil Organic Matter
SOM levels in the plots in April, one week after initial tillage, were not statistically different. But, by September levels were slightly higher in minimal-tillage and no-tillage plots and lower in tillage plots.
In summary, my experiment showed there is a trade-off between tillage and no-tillage. The weed suppression due to tillage resulted in better yields, but the tillage led to decreased soil/microbial health. There is a possible sweet spot with a shallow, minimal-tillage approach. Yields were not quite as high as in plots that are tilled more frequently and to greater depths, but soil and mycorrhizae health were much better.
The results of this on-farm experiment, as well as my yield and pest records from 1993 through 2010, were a great help to me in designing the farm system that I use today employing permanent living mulch in row middles and very limited strip-tilling to prepare crop beds in the spring. I like the balance this provides for an economically sustainable yield without sacrificing long-term soil health.
I continue to try to understand how much tillage and/or reduction in weed/living mulch crop competition is necessary for economical yields with different crops in different climate conditions and soil fertility levels. It is an ongoing process: I learn from my dynamic agroecosystem even as I add and subtract management methods and strategies! (See page 172, Figure 8.10 of The Ecological Farm for my current methods and strategies.)
The Ecological Farm author Helen Atthowe has worked for 35 years to connect farming, food systems, land stewardship, and conservation. She farms and conducts research at Woodleaf Farm in Eastern Oregon and serves as a consultant with farmers across the United States and internationally. Helen and her late husband, Carl Rosato, co-owned and operated a certified organic orchard in California where they pioneered methods for raising apples, peaches, and other tree fruits without the use of any type of pesticides.
Her on-farm research includes ecological weed and insect management, organic minimum soil disturbance systems for vegetable and orchard crops, and managing living mulches for soil and habitat building. She is a contributing writer to The Organic Gardener’s Handbook of Natural Pest and Disease Control and other books. She has served as a board member for the Organic Farming Research Foundation and advisor for the Wild Farm Alliance. Atthowe has a master’s degree in horticulture from Rutgers University and has worked in education and research at the University of Arkansas, Rutgers University, and Oregon State University, and served as a horticulture extension agent in Montana, where she annually taught an organic master gardener course.
