Oct 25, 2024
Soil tarping effects on weed pressure and onion production 

Onions are particularly susceptible to competition with weeds during production. This is due to their slow growth and minimal leaf canopy that reduces their ability to shade out weeds. It is important for growers to have effective weed management strategies to produce optimum onion yields.

‘Candy’ onions growing in SDSU Specialty Crop Field. Photo courtesy of Hannah Voye, Rhoda Burrow and Kristine Lang.
‘Candy’ onions growing in SDSU Specialty Crop Field. Photo by Hannah Voye.

Soil tarping is a practice used by small-scale vegetable growers to reduce weed competition with a cash crop without the use of herbicides and extensive tillage. Two types of soil tarping are used: solarization and occultation. 

Solarization

This process uses clear greenhouse plastic laid over a growing area to harness energy from the sun and warm the soil. The plastic is commonly secured with sandbags or by burying the edges of the plastic with soil to better trap heat and moisture. During hot summer months, or in a warm climate, temperatures beneath the plastic can become high enough to kill weed seeds and seedlings.

Clear greenhouse plastic secured with buried edges and sandbags for solarization. Photo by Hannah Voye.
Clear greenhouse plastic secured with buried edges and sandbags for solarization. Photo by Hannah Voye.

If used early in the growing season, or in cooler climates, temperatures beneath the plastic warm soil enough to germinate a high amount of weed seedlings before the growing season begins.

White side up silage tarp secured with sandbags for occultation. Photo by Hannah Voye.
White side up silage tarp secured with sandbags for occultation. Photo by Hannah Voye.

The weed seed bank is exhausted so less weeds compete with the crop during the growing season. In this method, light tillage may be needed to clear out the germinated weed seedlings before a crop is planted. 

Occultation

This type of soil tarping utilizes an opaque tarp laid over the soil and secured with sandbags to block light from reaching weed seeds and seedlings, therefore preventing weed growth beneath the tarp. Upon tarp removal, a weed-free planting bed is available.

During the 2023 growing season, researchers at South Dakota State University in Brookings  measured the effects of early season soil tarping on weed pressure and onion yield. Three tarp methods were tested: double-sided black/white silage tarps with black side up, double-sided black/white silage tarps with white side up and clear greenhouse plastic. 

Black side up silage tarp secured with sandbags for occultation. Photo by Hannah Voye.
Black side up silage tarp secured with sandbags for occultation. Photo by Hannah Voye.

In addition to tarp material, the time the tarps covered the soil was studied. All tarps were placed over the soil for six, four and two weeks before they were removed. Six-week tarps were applied April 18, four-week tarps were applied May 4 and two-week tarps were applied May 15. 

Plots with no tarp were included as a control treatment. All tarps were removed on May 30. After tarp removal, clear tarp and control treatments were lightly tilled to eliminate germinated weeds. All treatments were lightly harrowed in planting rows to loosen soil for easy planting of ‘Candy’ and ‘Patterson’ onion transplants.

Area at tarp removal on May 30 that was tarped for occultation with black side up tarp for two weeks. Photo by Hannah Voye.
Area at tarp removal on May 30 that was tarped for occultation with black side up tarp for two weeks. Photo by Hannah Voye.

Onions were drip irrigated and total rainfall was 5.6 inches from June to August. Broadleaf and grass weed height, count and weight data were collected biweekly throughout the growing season. Data for marketable and cull onion yield was collected after harvest and curing.

Area at tarp removal on May 30 that was solarized for six weeks with clear greenhouse plastic. Photo by Hannah Voye.
Area at tarp removal on May 30 that was solarized for six weeks with clear greenhouse plastic. Photo by Hannah Voye.

At tarp removal, all black side up and white side up silage tarps showed low broadleaf and grass weed counts, heights and total biomass. The control and six-week clear tarp plots had the highest grass weight averages, followed by two-week clear and four-week clear. 

Broadleaf weed counts were highest in the control treatment followed by the clear tarp treatments. Grass counts were highest in the two-week clear treatment, closely followed by the six-week clear treatment and the control treatment.

Freshly weeded onions in SDSU Specialty Crop Field. Photo by Hannah Voye.
Freshly weeded onions in SDSU Specialty Crop Field. Photo by Hannah Voye.

Data collected on June 12, two weeks after tarp removal and planting, showed an average broadleaf weed count per acre in the 6-week clear treatment to be 67% less than the control treatment, supporting solarization to have reduced the weed seed bank. 

High weed pressure in control treatment area on May 30 where no tarp was applied. Photo by Hannah Voye.
High weed pressure in control treatment area on May 30 where no tarp was applied. Photo by Hannah Voye.

There were no differences in onion yield due to tarp treatment. This was most likely due to biweekly hand weeding after each data collection event.

Some broadleaf and grass weeds growing in SDSU Specialty Crop Field. Photo by Hannah Voye.
Some broadleaf and grass weeds growing in SDSU Specialty Crop Field. Photo by Hannah Voye.

From one year of data, there is evidence to support occultation can reduce weed pressure at tarp removal, and solarization may reduce weed pressure in growing season when applied for 6 weeks. Tarping, however, should be used along with other weed management to produce optimum onion yield. To learn more about ongoing soil tarping research, visit extension.sdstate.edu/tags/horticulture-research

Written by Hannah Voye, Rhoda Burrows and Kristine Lang

Hannah Voye is a graduate research assistant who is completing her second year as a graduate student studying Plant Science at South Dakota State University. She can be reached at [email protected].

Rhoda Burrows is a professor at South Dakota State University and can be reached at [email protected].

Kristine Lang is an assistant professor at South Dakota State University and can be reached at [email protected].




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