Improving Reduced Tillage in Northern Great Plains’ Onion Production through Early Season Soil Tarping

Allium cepa L., commonly known as onions, are highly susceptible to weed competition due to their minimal canopy cover. Weed management is critical for their production. One potential weed management tool, especially for organic growers, is soil tarping. This study evaluated the impact of two types of early season tarping (solarization and occultation) and duration of tarping (6, 4, and 2 weeks) on weed control in ‘Patterson’ and ‘Candy’ onion production. Field experiments were conducted during the 2023 and 2024 growing seasons at the South Dakota State University Specialty Crop Field in Brookings, SD, USA. Solarization was conducted using clear tarps secured with sandbags and buried edges. Occultation was evaluated using white side up and black side up silage tarps, applied at respective weeks before removal and onion planting. Tarp treatments and no tarp controls were arranged in a randomized complete block design with four blocks and ten 7.32 × 3.05 m treatment plots per block. Tarps were placed in April at respective weeks before removal in May. Immediately following tarp removal, clear and control plots were tilled to remove high weed pressure, and all 0.76 × 3.05 m planting beds were harrowed within each plot. Four rows of onion transplants were planted 1 to 2 days following tarp removal in mid or late May, depending on the year. Response variables for data collection included weed type, height, and biomass as well as time to cultivate and onion yield. At tarp removal in both years, all occultation treatments resulted in less weed pressure than the no tarp control. Solarization treatments had varied results based on year. There was no difference in onion yield due to tarping treatment either year, likely due to biweekly hand weeding that evened out treatment effect on weeds over the growing season. Both years, the Patterson onion cultivar yielded more marketable onions than ‘Candy’; 40% of ‘Candy’ onions and 75% of ‘Patterson’ onions were marketable in 2023; in 2024, 65% of ‘Candy’ onions and 73% of Patterson onions were marketable. Soil tarping may be an effective option for farmers to reduce early season weeds in onion production; however, it should be used alongside other management strategies to obtain a viable yield.