Selection Criteria for Successful Implementation of Perennial Understory Plants for Weed Management
By Dr. Kris Nichols, Chief Scientist and Marisa Wagner, Research Technician
Prior to initiating 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 in the field, potential perennial understory plants were screened to determine if they meet the selection criteria for success. The selection criteria were that plants will:
1. Survive Pennsylvania’s yearly climatic changes,
2. Cover the soil surface adequately to compete against weeds,
3. Remain low-growing enough to not negatively impact cash crop yield or interfere with harvest operations,
4. Survive disturbance from mowing, grazing, wheel traffic, or being walked on during plant management such as weeding or harvesting, and
5. Be inexpensive to purchase and available in large enough quantities.
Cover crops or green manure crops are frequently used to protect the soil and add fertility, but unlike the perennial plants being used in this project, these crops are annuals and new seeds need to be purchased every year. Successful establishment was defined as 85% of the soil covered, but previous research studies found that the most effective species needed to be low-growing and stolon-building in orchards (Korte et al., 2010). In pastures, plants should be clump forming and shade tolerant with delayed green-up in the spring (Flynn et al., 2013), while in corn fields, quick and reliable establishment but slower-growing mixtures are important criteria (Wortman et al., 2012).
Overall, when looking for a perennial understory plant to act as a weed barrier, it is important to take into consideration how these companion plants will impact crops plants and both height and groundcover or spread are important for making this determination as well as responses to mechanical operations and flowering. The data collected in this phase of the study was important for determining which plants to use in the next phase where the perennial understory plants will be growing with grain, vegetable, or forage crops as well as between rows of apple trees in our orchard.
Plant species native to Pennsylvania or commonly found in Pennsylvania were initially selected for testing including Sedum spurium (Voodoo), Thymus serpyllum (Elfin Thyme), Thymus citrodorus (Lemon Thyme), Veronica prostrates (Goldwell Speedwell), Isotoma fluviatillus (Blue Star Creeper), Sedum hybridium (Mongolian Stonecrop, Czar’s Gold), Hyperium perforatum (St. John’s Wort), Trilforium (White Clover), Chrysognum (Eco-Laquered Spider Plant), Frageria virgineanum (Wild Strawberry), Carex Pennsylvania, Anternaria (Pussy Toes), Juncus tenuis (Path Rush), Oxalis violacea (Sorrel), Pachysandra percumbens, Canada Tick Trefoil, and Perennial Lupine. Although these species met the first criterion, it was difficult to find enough inexpensive seeds for all these plants to plant on a field scale.
Therefore, plant species were re-evaluated, and a new group of plants were selected for culturing in the greenhouse to evaluate germination and produce seedlings for in-field evaluation of growth height and spread or soil surface coverage and testing the response to mechanical stress, i.e. being driven on or mowed. Of the 20 species cultured in the greenhouse, only seedlings from ten species – Geum urbanum (Avens), Oenothera biennis (Evening Primrose), Achillea millefolium (Yarrow), Sedum spurium (Voodoo), Dianthus (Cheddar pink), Prunella vulgaris (Self-Heal), Lupinus (Blue Lupine), Thymus vulfaris (English Thyme), Chamaemelum nobile (Roman Chamomile), and Talinum paniculatum (Fame flower) - were planted in the field. These ten were selected based on the amount of fine root development in the pots. The ten species were planted in single and mixed species (Figure 1) groupings in beds created by spading a sod buffer site.
After planting, all of the beds were weeded to give the small seedlings a chance to compete with the heavy weed seed bank of broadleaf plantain, dandelion and thistle and mulched to prevent rapid water evaporation from bare soil. The heights and spreads of the plants were measured weekly along with counting the number of plants for each species that had died (Figure 2). Weed growth in the beds was determined by collecting all the weeds within a quarter meter square every two weeks and measuring dry weight. The roots of these weeds were clipped below the soil surface to try to prevent regrowth, but only the shoot was dried and weighed. In mid-August, the plants were driven over by a tractor and then flail mowed a week later (Figure 3). Data on the percentage of plants still living after this stress as well as the height and spread was collected right after mowing and then five and eight weeks later.
The data in Table 1 was used to determine which species best met all the criteria required of an appropriate perennial, understory plant for testing in the different management systems. Due to being in full sun and exposed to high summer temperatures, Blue Lupine withered and died prior to any driving or mowing stress. Evening Primrose in the mixed species bed was severely infested with Japanese Beetles, but none of the other plants were damaged by these insect pests. In addition, the rigid stems of Evening Primrose and Fame Flower snapped when they were driven over, and the plants did not regrow. Initially, Self Heal and Thyme survived being driven over the best while the Sedum plants had several leaves turning black and Chamomile was flowering during the time of stress which initially was thought to have a negative impact. By the last sampling period, Chamomile saw no major decline and was not removed from selection because of these stressors.
Of the four species that didn’t have a 50% or greater loss due to the environmental or mechanical stresses described above, yarrow and chamomile did grow the tallest. The height and spread data showed that yarrow and chamomile grew to 7.4 and 9.0 inches tall, respectively, with spreads of over 16 inches. The height may prevent these plants from being appropriate for the demonstration plots, but the spread is good as long as it does not smother the cash crop. Self Heal struggled to compete in the unweeded areas, but the plants were short in stature (~5.5 inches) with a large spread (~15.2 inches) which could make them an ideal weed suppressing understory plant. Avens also survived the stressors well and was short in stature (~4.8 inches), but these plants did not seem to struggle with competition as much as Self Heal and only had a spread of ~12.7 inches. Given all of the data and the observations during the growing season only five plant species – Avens, Chamomile, Self Heal, Thyme, and Yarrow – were selected to be tested in the next phase. Although Thyme did not survive the mechanical stressors as well as some of the other species, it did meet the other selection criteria and was included in the next phase. In particular, Chamomile, Yarrow, and Thyme best fit this criteria for low seed cost and high availability, particularly certified organic seed.
Table 1: Seedlings transplanted into field plots were evaluated for the percentage of plants still alive, the spread across the soil surface, and the height weekly for six weeks and then again immediately after being driven over and mowed and five and eight weeks later.
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Korte, N., and S. Porembski. 2010. Suitability of different cover crop mixtures and seedlings for a new tree row management in an organic orchard. Gesunde Pflanzen 62(2): 45-52.
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The authors would like to thank Dr. Elaine Ingham for her contributions to developing this project and Dr. Gladis Zinati, Associate Research Scientist, Rae Moore, former Research Technician, Marisa Wagner, Research Technician, and Aprile Doubt, Research Intern, for their invaluable assistance in this project.
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.