Biosolarization and Cover Crop Impact on Weeds and Soilborne Pathogens

Strawberry beds undergoing solarization during August. (all photos courtesy A.M. Tubeileh.)

Soils contain a lot of good things, but they are also reservoirs for weeds, pathogens and nematodes, which, if left uncontrolled, can devastate crop yields. If soilborne pests rise to economically damaging levels, it becomes necessary for growers to use soil disinfestation techniques to kill soilborne organisms.

Most conventional growers use fumigants for soil disinfestation. Fumigation is unavailable to organic growers leaving organic growers few options for controlling soilborne pests. However, over the past couple of decades, there has been substantial research into organic soil disinfestation techniques due to increasing regulations for conventional fumigants. Researchers at California Polytechnic State University, San Luis Obispo recently conducted research on soil solarization and biosolarization, two organic soil disinfestation techniques, on organic strawberry production at the Cal Poly Organic Farm in San Luis Obispo, California.

Strawberries in May (peak production period for San Luis Obispo) of non-solarized (left) and solarized plots (right) (all photos courtesy A.M. Tubeileh.)

Soil Solarization

Solarization involves placing clear, thin (25 to 50μm), low-density polyethylene tarps over irrigated soil to increase soil temperatures to lethal levels for pathogens, pests and weeds. In general, temperatures generated during soil solarization range from 104 to 158 degrees F. The tarp is left on the soil for four to eight weeks, depending on the soil temperatures generated during solarization. The efficacy of solarization is primarily based on ambient air/soil temperatures and intensity of solar radiation. In most cases, solarization should raise the ambient soil temperature between 10 and 20 degrees C (18 to 36 degrees F) in the top six inches of the soil. Solarization is most effective when used during the summer when solar radiation is high in sunny, warm climates. Costs of solarization vary depending on plastic prices, but in general, the plastic costs in between $150 to $300 per acre. The best plastics are clear/transparent, one to three millimeters thick and UV-inhibited to prevent breakdown in sunlight.

Biosolarization, which combines the use of organic soil amendments and soil solarization, has been proven to enhance the results of solarization in numerous field experiments. Organic amendments commonly used are plant residues, animal manure, compost and other high-nitrogen organic materials such as blood meal. Biosolarization is a relatively new area of research and can reduce the time needed for solarization as well as increase solarization’s effectiveness in areas with marginal conditions for solarization. For example, in San Luis Obispo, we are about five miles from the coast, experience frequent, foggy mornings and rarely have temperatures above 90 degrees F. We included biosolarization of cover crop residues in our experiment to see if it would increase the efficacy of solarization in our climate.

The rest of this article will focus on the results from our research. For more information on solarization and biosolarization, including application techniques, please see our previous Organic Farmer article from April/May 2019 or UC extension’s webpage on solarization


Solarization Research

As part of our research, we conducted lab experiments under simulated solarization conditions assessing the time needed to kill weed seeds at five different temperatures (104, 113, 122, 131 and 140 degrees F). Seeds tested included little mallow, redstem filaree, bristly oxtongue, annual sow thistle, common purslane, common lambsquarters and redroot pigweed. Efficacy of solarization temperatures differed between different species. In general, cool-season annuals annual sow thistle and bristly oxtongue were more susceptible to heat treatments than warm-season annuals such as redroot pigweed. Additionally, hard seeded (thick seed coats) seed were relatively unaffected by heat treatments taking long duration to kill. Time and percent mortality of weed seeds were used to create thermal death models for weed seeds at each temperature (Figure 1). Additionally, models were used to estimate the amount of time needed to kill 90% of seeds for all species tested (Figure 2). Redstem filaree germination rates were unaffected by heat treatments. Additionally, common purslane was unaffected by heat treatments below 113 degrees F or lower and redroot pigweed was unaffected by temperatures of 104 degrees F or below. Results indicated daily temperatures above 122 degrees F are needed for four to eight weeks to achieve adequate weed management via solarization.

Figure 1. Logistic regression models showing how germination rate of different weed species is affected by the duration of exposure to 122 degrees F under laboratory conditions simulating solarization.

We also conducted field experiments on solarization of sudangrass residues in organic strawberry production.

The objectives of our field experiment were:

  • To determine if soil solarization can reduce weed and pathogen pressures and improve plant health and strawberry yields in San Luis Obispo County,
  • To determine if the effect of sudangrass cover crop residues will increase the effects of soil solarization, and
  • To compare the effects of sudangrass residue mulching vs. incorporation on weed populations, pathogen populations and strawberry health and yields.

The experiment was designed so that we had three cover crop treatments: A control, one where sudangrass was left as a surface mulch after mowing, and the other where sudangrass was incorporated into the soil after mowing. Within each cover crop treatment, we solarized half and left the other half non-solarized.

‘Piper’ sudangrass was planted in mid-May using a seed drill. In mid-July, sudangrass was chopped and shredded with a tractor-drawn flail mower and incorporated into the soil in our incorporated treatment. After incorporation, beds were created in all plots except mulched plots. In mulched treatments, sudangrass residue was left on the soil surface and no beds were created. On July 26, solarization plastic was hand-applied onto solarized plots. After applying plastic, fields were irrigated for 72 hours using one line of drip tape till fields reached field capacity. Tarps were left on for five weeks and removed on August 31, 2018. Strawberries were planted in October after doing weed population assessments in our various treatments.

Table 1. The yield in grams per 30 plants from solarized treatments (n=12) and sudangrass treatments (n=8) from March through June.


Maximum soil temperatures in solarized plots were 118 degrees F at a soil depth of two inches and 42 degrees C at a soil depth of 6 inches. On average, temperatures in cover crop mulched plots were 4 to 6 degrees F lower than other solarized plots. Temperatures in all solarized plots were 18 to 30 degrees F higher than non-solarized plots. In initial weed biomass assessments taken six weeks after tarp removal, non-solarized incorporated plots reduced weed biomass by 24.4% compared to the control. Non-solarized mulched plots reduced weed biomass by 95.6% compared to the control. All solarized plots resulted in similar reduction in weed biomass compared to the control with an average reduction of 97.1% ± 0.6%. Efficacy of solarization treatments decreased with time. In final weed biomass assessments taken 15 weeks after tarp removal, the only solarization treatment providing a significant reduction in weed biomass compared to the control was incorporated plots with solarization resulting in 67% lower biomass than the control. Mulched plots without solarization also provided significant control, reducing weed biomass to 84.1% of the control. However, in non-solarized mulched treatments, the sudangrass re-grew after mowing and did not die until the winter.

This led to poor strawberry establishment, although the strawberries later recovered.

Solarization reduced verticillium wilt populations by 80.7% compared to non-solarized plots. Solarized plots had much lower disease incidence throughout the growing season. Non-solarized plots started to experience disease symptoms in April and were not producing fruit by May. Solarized plots experienced almost no disease pressure till late May/June when temperatures warmed. Solarized plants experienced disease pressure from Charcoal Rot, Macrophomina phaesolina, a warm-season pathogen which we theorize was not reduced to the same degree as verticillium wilt populations were. Total plant mortality was significantly higher in non-solarized plots with 35.5% mortality compared to 16.0 % mortality in solarized plots. Additionally, solarized plots had much higher yield than non-solarized plots. The different cover crop treatments did not have a clear effect on reducing verticillium wilt population, disease severity or increasing yields.

Non-solarized (left) and solarized (right) beds of strawberries during June.


Key Takeaways

Solarization provided effective weed management for 3.5 months after tarp removal, reduced verticillium wilt populations, reduced disease severity and increased yield compared to non-solarized plots.

Cover crop treatments did not enhance the effect of solarization. Cover crop treatments did not have a significant effect on verticillium populations or yields. Mulched treatments did reduce weed population and had lower disease severity than other treatments.

Solarization effectively killed mowed sudangrass, preventing it from regrowing. Solarization of mowed cover crops provides a potential mechanism for killing cover crops for organic growers wishing to perform no-till production. However, more research is needed into this topic.

The effectiveness of solarization depends not only on the temperatures you can achieve, but on the disease and weed species present in a field as well. Particularly in areas with cooler climates or frequent foggy/cloudy days during the summer, knowing the temperature thresholds required to kill the pests in your field can be important in determining whether solarization is a viable solution. This is another area where more research can be done. First, develop models to help growers estimate the temperatures they can achieve during solarization. Secondly, models can be used to determine the susceptibility of different pest species to solarization.