Removing Weed Habitat and Improving Crop Health

Measuring the Success of Permanent Weed-Competitive Plant Species to Remove Weed Habitat and Improve Crop Health
Author: Kris Nichols, Chief Scientist

Rodale Institute’s Plants Protecting Plants: Removing Weed Habitat and Improving Crop Health Through the Use of Permanent Weed-Competitive Plant Species and Maintenance of Beneficial Soil Organisms demonstration project was implemented in four different organic management systems: 1. grain crops, 2. vegetable crops, 3. apple orchard, and 4. cattle pasture. Five perennial understory plants were selected as companion crops to assist in managing weeds and improve soil health in each of these systems. The selection criteria for the particular perennial understory plants to use is explain in another article titled ‘Selection Criteria for Successful Implementation of Perennial Understory Plants for Weed Management’ while the method for spreading the seeds after inoculating them with compost extract is detailed in the article titled ‘Seeding Perennial Understory Plants Amended with Compost Extract’. The impacts of these plants on the weed management, crop yield, livestock grazing, and soil health were measured to identify the best perennial, understory plants to use in each system.

Plot Establishment: In late summer 2015, plots were established at six field sites – two soybean, one corn, one butternut squash, an apple orchard, and a cattle pasture – in a randomized complete block design with three replicate plots for each individual perennial understory species – avens, chamomile, self-heal, thyme, and yarrow – and one mixed plot with all five species combined. The plots were 5’ by 10’ with either a 1’ or 5’ buffer between the plots. All buffers were planted with a rye cover crop in fall 2015 as the typical annual, organic no-till cover crop. In the corn, one of the soybean (Soybean 1) and the butternut squash (Squash) fields, the three blocks were side-by-side with 2-3 rows (~5’) of corn or soybean plants between each block. In the other soybean field (Soybean 2), plots were lined up one after the other going from east to west. The blocks at the Pasture site were established around shady areas and close enough to each other to be grazed simultaneously while the plots in the Orchard (Figure 1) were between different rows of trees in two different tree blocks.

The Corn and Soybean 1 sites were moldboard plowed prior to planting the cash crop (i.e. corn or soybeans respectively) in June 2015. The sod in the Orchard and Pasture was broken up by a rotovator immediately prior to planting the understory seeds. At the Squash and Soybean 2 sites, the fields were coming out of spring crops and going into winter cover crops. The late planting was done to get the perennial plants established in the late summer and fall so they would overwinter and the impacts on weed competition, crop yield, and soil health would be measured the next year during the growth of the cash crop in the Corn, Soybean 1 and 2, and Squash sites.

2015 Perennial Understory Planting
Seeds of five different perennial, understory plants species – avens, chamomile, self-heal, thyme, and yarrow – were inoculated with microorganisms which may improve germination and survival of the plants by soaking them in compost extract for 24 hours. The seed-compost extract mixture was mixed with dried compost in a bucket and then shaken over the Corn, Soybean 1, and Squash plots on August 7, 2015 and the Pasture and Orchard (Figure 2) plots on August 13, 2015. Plots were lightly raked after planting to more evenly distribute the seeds and provide good seed-to-soil contact (Figure 3).

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Figure 1. The Orchard plots were established between different rows of trees in two different tree blocks.
Figure 2. Research Intern Kari Bender shaking the perennial seed-compost mix was shaken over the Orchard plots to spread the seeds across the plot.
Figure 3. Research Intern Erin Ball lightly raking the perennial seed-compost mix in the Orchard plots more evenly distribute the seeds and provide good seed-to-soil contact.

2015 Weeding and Replanting
Two weeks after seeding, the plots were weeded to remove any competition to the seedlings (Figure 4). The late summer and fall of 2015 was abnormally dry and the slow-growing plants struggled to germinate and grow. Therefore, plots were hand-watered and then as precipitation increased in September, the decision was made to reseed all the plots (Figure 5) as well as to add an additional annual crop field site (Soybean 2 – Figure 6). This additional site was added to look at the efficacy of perennial herbaceous understory plants for weed management when soybeans are planted in 15” rows rather than the 30” rows used at the other location. In addition, these plots will be directly compared to plots containing annual cover crops. Additional hand-watering and hand-weeding occurred right after replanting (or planting at the new site) to stimulate germination and growth and reduce weed competition. However, by late fall, there still wasn’t enough plant growth to make adequate biomass measurements to determine soil coverage (Figure 7), and it was anticipated that the plots would need to be reseeded in spring 2016.

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Figure 4. Research technician, Linda Zaoudeh, weeding the Pasture plots to remove any competition to the perennial seedlings two weeks after planting.
Figure 5. Kate Harms, Research Technician, reseeding the Pasture plots.
Figure 6. An additional crop field (Soybean 2) was added to the study in September 2016 after the harvest of a wheat crop and before planting a rye cover crop. This field is being seeded to the perennial understory plants by Research Intern, Guillaume Tant.
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Figure 7. As seen in the Squash (a)  and Soybean 2 (b) fields, the slow-growing perennials did not have adequate growth to allow for biomass sampling in fall 2015.

2016 Cash Crops
The Corn and Soybean 1 and 2 Sites were planted to the appropriate cash crops (either corn or soybeans) in late spring using the roller-crimper, no-till drill in a one pass operation while butternut squash was not planted at the Squash Site until early summer. These planting dates coordinated with the typical planting dates for these crops in our area. However, the butternut squash seedlings were not transplanted into black plastic, but rather into either mowed rye in the buffer strips or into the perennial biomass in the plots. Irrigation was also not used since the perennial understory plants were being observed for their abilities to help the cash crop manage water more efficiently.

2016 Observations and Measurements
All the sites were observed for perennial understory growth in April-August 2016 with numerous pictures taken to track the data. Observations in April were used to conclude that it was unnecessary to replant seeds in 2016 (Figure 8). In June 2016, weeds growing in the plots and the buffers between the plots were sampled using a 20- inch square at the corn, soybean and squash sites.  This created a weeded and a non-weeded area within each plot. By July 2016, the weeded and non-weeded areas within each plot were indistinguishable from the other areas, and weed biomass was again collected from each plot and the buffers. In late August, cash crop biomass was collected from each plot.

Figure 8. In April 2016, observations of the plots, like this Soybean 2 plot, indicated that replanting with perennial seeds was unnecessary.

Soil Sampling: Baseline surface soil samples (0-15 cm) were collected from each site at the time of perennial planting in 2015. In late August 2016, surface soil samples were again collected to identify any differences in soil health with the use of these perennial understory plots. All soil samples were air-dried, passed through a 2 mm screen, and then sent Pennsylvania State University Agricultural Analytical Services Laboratory for analysis of organic matter, carbon, nitrogen, phosphorus and other key nutrient analyses.

Results and Discussion
In spring 2016, observations of the plots showed good growth of the perennial plants particularly the chamomile and yarrow. Even though not all the perennials were growing well, the plots were not reseeded because the main objective of this demonstration project was to identify perennial understory plants that would grow in the sub-humid environment of Pennsylvania with its diverse climate and would compete effectively with weed species while improving soil health. Observations made in May and June continued to show good chamomile and yarrow growth, but plants like avens, thyme and self-heal were too short in stature to compete with weeds in the plots. At the Orchard (Figure 9a) and Pasture (Figure 9b) sites, the sod mix present in these plots in for more than 10 years made it impossible for the perennial understory plants to adequately establish and compete. Due to the lack of understory plant growth in these plots, biomass and soil collections were not made but observations continued for the remainder of the project.

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Figure 9. By late Spring 2016, the Orchard (a) and Pasture (b) sites had extensive regrowth of the old sod mix making it impossible for the perennial understory plants to adequately establish and compete.

Vegetable (Squash) Crop Site
In 2015, performance of the understory plants was best at the Squash site (Figure 10). This site also exhibited the best weed suppression by the perennial understory plants in the spring and early summer 2016 (Figure 11), but by the time the squash was planted the weeds had taken over most of the plots, except for the chamomile plots (Figure 12).  Because of the competition between weeds and perennial understory plants (Figure 13), there was only one out of five butternut squash plant remaining in two of the plots by August. Without black plastic and irrigation, only one squash plant out of six survived in the buffers between the plot blocks. Therefore, it is impossible to determine the impact of the perennial understory plants in this demonstration plot. Using a different vegetable cash crop or at the very least including irrigation may make a substantial difference in the future.

 figure-10-squash-2015 figure-11-squash-2016
Figure 10. The perennial understory plants appeared to be performing the best at the Squash site in 2015.
Figure 11. In early 2016, the perennial understory plants continued to perform well at the Squash site.
Figure 12. Butternut squash seedlings (a) were transplanted into the plots on June 13, 2016, but by this time, the weeds had taken over most of the plots (b), except for the chamomile plots (c).
Figure 13. Competition between the perennial plants and weeds reduced the number of butternut squash plants from five to one out in only two of the plots by August.

Grain (Corn and Soybean) Crop Sites
The chamomile and yarrow exhibited the best growth at the Corn (Figure 14a) and Soybean 1 (Figure 14b) and 2 sites. While self-heal and thyme were present in these plots, their growth was at such a slow rate that it was difficult to find them during the July and August sampling. After rolling down the rye and perennial understory plants in the late spring for planting (Figure 15), the chamomile and yarrow did regrow but the impact on weed management was minimal (Figures 16). At the Soybean 2 site, volunteer vetch from a winter cover crop in previous years germinated and grew aggressively outcompeting many of the understory plants as well as impacting the soybean crop plant even though these fields had a rye cover crop planted in them in fall 2015 while vetch was the 2014 cover crop.

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Figure 14. Both chamomile and yarrow performed well in early 2016 in the Corn (a) and Soybean 1 (b) plots.
Figure 15. The rye cover crop in the buffers between the plots as well as the perennial understory plants were rolled down prior to planting either corn or soybeans in the Corn (a) and Soybean 1 (b) and 2 (c) fields. The Soybean 2 picture was taken six days after rolling and shows how effective the rolling was at terminating the rye cover crop.
Figure 16. After rolling the Corn (a) site, which had corn in 2015 2016, the Soybean 1 (b) site, which had soybeans in 2015, and the Soybean 2 (c) site, which had oats in 2015, were planted to soybeans, corn, and soybeans, respectively, in 2016. These pictures illustrate that although the chamomile and yarrow did regrow after rolling, the impact on weed management was minimal particularly in the Soybean 1 fields.

Pasture Site
Although some of the perennial understory plants were present in the plots and the plots were grazed in early May and mid-June, these plants did not compete well with weeds or the grasses present in the sod (Figure 17). By the time the plots were going to be sampled in June, it was determined that the plots would not be sampled because of the lack of understory plant growth. The plots continued to be observed and in mid-August when the steers were put back on the plots, the steers grazed around the plots but avoided grazing in the plots (Figure 18). It is possible that either the scent from these perennials or their palatability made the steers reject them as forage along with the forage near them. This is important to keep in mind if using these plants in a pasture setting.

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Figure 17. In the Pasture plots, the perennial understory plants were did not compete well with weeds or the grasses present in the sod.
Figure 18. In mid-August when the steers were allowed to graze the plots, grazing occurred more around the plots then in them.

Orchard Site
By May 2016, it was impossible to distinguish where the plots were located at the Orchard site (Figure 19). The growth of perennial understory plants observed in April was rapidly lost to the grass mix in the sod. Observations continued to be made at this site until the end of August but no changes were seen and the sod continued to fill in.

Figure 19. The perennial understory plants could not be seen by early 2016 at the Orchard site.

Weed and Crop Plant Biomass
Overall, the perennial sod mix at the Pasture and Orchard sites dominated over the perennial understory plants in 2015 and 2016 so biomass samples were not collected at these sites. In 2016, weed biomass was measured at the Corn, Soybean 1 and 2, and Squash sites in June (Figure 20a) and July (Figure 20b) to determine which perennial understory plants better managed weeds particularly when compared to the typical annual rye cover crop. Both the perennial understory plants and the rye plants were crimped and rolled when planting the corn and soybeans while the rye cover crop was mowed about two weeks prior to planting the squash. Weed biomass was lowest in the rye plots at all of these four sites except the Soybean 2 site where there was a lot of volunteer vetch growth during the June sampling. Overall, weed biomass was the lowest in the plots at the Soybean 1 site in June while in July this site had the highest overall biomass levels. In July, weed biomass in the rye plots at the grain cropping sites (Corn and Soybean 1 and 2) was equal to or higher than all the other plots while at the Squash site weed biomass was lowest in the rye plot. Cash crop biomass was highest in August in the rye plots at all the sites, except in the Soybean 2 site (Figure 21).  Other than the relationship between low weed biomass and high cash crop biomass in the rye plots, regression analyses showed no consistent relationships between weed and cash crop biomass for the perennial understory plants.

Figure 20. Dry weed biomass was measured at the Corn, Soybean 1 and 2, and Squash sites in June (a) and July (b) 2016 - select photo to enlarge.
Figure 21. Cash crop biomass was measured Corn, Soybean 1 and 2, and Squa sh sites in August 2016 - select photo to enlarge.

Soil Health
The Pasture and Orchard sites had the highest percentages of soil organic matter (8.0 and 7.4%, respectively) compared to 3.9, 4.5, and 3.4% at the Corn, Soybean 1, and Squash sites, respectively. These old sod sites also had the highest nitrogen, potassium, sulfur, calcium, magnesium, and zinc levels while the Corn and Soybean 1 sites had the highest phosphorus concentrations. The Squash site had the lowest concentrations of all the nutrients measured. However, all the fields had nutrient concentrations that were optimum or above optimum. These trends were the same for the vegetable and grain crop fields at the end of the demonstration project as well but the Soybean 2 site had the highest magnesium levels and organic matter percentages were all in the 4.0 to 4.5% range. Overall, the higher levels of organic matter at all the sites in 2015 and 2016 as well as the optimal or above nutrient levels probably reduced some of the need for the perennial understory companion plants and made it more difficult for the microbial inoculum from the compost extract to provide them with an advantage over the weeds.

Key Conclusions
1. Prior to using a perennial understory plant to manage weed pests and improve soil health, it is important to screen a variety of plants to make sure that they are suitable for your climatic conditions and overall agricultural management practices. See the article titled ‘Selection Criteria for Successful Implementation of Perennial Understory Plants for Weed Management’.
2. Perennial plants are usually slow growing and need to be planted at optimum times. It is important to weed around seedlings in the first few weeks after germination and to water the plants if necessary. Extra work initially to get perennial plants well established will be beneficial to weed management over time.
3. The efficacy of using perennial understory plants may not be observed in the short duration or where soil health is high, but measurable results are expected over time and will be more visible in semi-arid or arid environments, under climatic uncertainty, or in highly disturbed soils.
4. Greater success will be achieved if seedlings are used rather than planting seeds, but this might not be feasible for large farms so plant selection should include testing species that establish quickly.
5. Some herbaceous, perennial understory plants are not palatable for livestock or may reduce overall grazing of neighboring forage plants. To maintain diversity within a pasture or rangeland, it may be desirable to select perennial understory plants that will not be overgrazed and therefore will remain present in a location. However, if the plants are not palatable or cause an animal health issue, these plants should not be used or their seeding density should be very low. In addition, if the scent or taste restricts grazing to the point that animals are not grazing other forages a particular area, such as what occurred in our project, then other plant species should be considered.
6. Some herbaceous, perennial understory plants will attract pollinators as well as predatory insects which might provide additional benefits particularly for insect pest management. It is important to select plants based on the objectives that you wish to achieve.

Author Acknowledgments
The authors would like to thank Dr. Gladis Zinati, Associate Research Scientist; former Research Technicians - Rae Moore and Linda Zaoudeh; and Research Technicians -  Marisa Wagner and Kate Harms, as well as the following research interns: Aprile Doubt, Guillaume Tant, Kari Bender, Erin Ball, Emily Lesher, Britta Schumacher, Scott Vondy, Miranda Lachman, and Lily Draklellis – for their invaluable assistance in this project.

Acknowledgment Clause
This material is based upon work supported by the Natural Resources Conservation Service, U.S. Department of Agriculture, under Grant Agreement Number 69-2D37-13-670. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.


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