May 15, 2011
Study evaluates solarization for weed control
The polyethylene was put in place and the sod strips were placed over the edges of the poly and anchored in place using sod staples.
By Pam Charbonneau,
OMAFRA turfgrass specialist

This experiment on solarization was dreamed up at the lunch table at the Guelph Turfgrass Institute. We were considering non-selective vegetation control treatment options for home lawn renovation, following the Ontario pesticide ban. What were the viable options for home lawn turf? There was research underway (conducted by Cynthia Siva, supervised by Dr. Katerina Jordan) that was comparing propane flaming, acetic acid and sodding to glyphosate as a home lawn renovation technique. This study was intended to add to the alternatives.

Solarization (burning off) is often talked about as an alternative for non-selective control vegetation. Preliminary research revealed that solarization has been used successfully in climates where high temperatures and many hours of sunshine are the norm.

What is soil solarization?

Soil solarization is an environmentally friendly method of using solar power to control disease agents and weeds in the soil, usually in the form of a transparent polyethylene cover. References to the process go back as far as the mid-1800s, when glass frames were used.  

The modern take on solarization was described in 1976 by Katan et al., for controlling soil-borne pathogens and weeds, mostly as a pre-plant soil treatment. It was achieved by covering soil with transparent polyethylene during the hot season. In the short term, clear plastic is thought to work better than black. However, black plastic may work to kill vegetation in the long term (more than one growing season) by effectively starving perennial plants of light as opposed to superheating the soil and killing plants with high temperatures.  


Our research objectives were:
  • Determine the duration of solarization needed for total vegetation control of a mixed stand of turf and weeds (two weeks, four weeks and six weeks).
  • Determine the best cover: Clear polyethylene, which rises the temperature of the plants and soil above optimum, or black polyethylene, which starves plants of light and hence photosynthates.
  • Compare solarization to steam (will be covered in future Horticulture Review article by Rodger Tschanz).
  • Evaluate how effective solarization and steaming were at killing weed/grass seeds in the soil seed bank (will be covered in a future article in Horticulture Review by Kathleen Dodson)

Materials and methods

This experiment was conducted at the Guelph Turfgrass Institute in 2010 on a low maintenance mixed stand of turf, containing mainly Kentucky bluegrass, perennial ryegrass, dandelion, black medick, clover, narrow-leaved plantain, chickweed, birds’ foot trefoil, thistle and field bindweed growing on native soil, maintained at 8 cm mowing height and unfertilized.  

All polyethylene covers were removed on Aug. 26. The steam treatment was applied on Sept. 14 (steam results will not be reported here).

Eight randomized point quadrants, measured 60 cm x 60 cm with 25 points in each quadrant (points 10 cm apart) for a total of 200 points in each plot were used to quantify all plant species in the plots before the polyethylene was put in place. The same technique was used on Aug. 30, Sept. 13 and Oct. 7, to quantify the vegetation three days after poly removal, 18 and 42 days after poly removal.   

Even though there was a wide range of weed and grass species, only those present in high percentages are reported here, namely dandelions, total weeds, Kentucky bluegrass and perennial ryegrass.  In addition, only the final rating date after the polyethylene was removed will be reported (Oct. 7, 2010).  


After removing various polyethylene covers on Aug. 26, it appeared that many of the dandelions were dead. By the time the final rating was conducted on Oct. 7, most of the dandelion tops were killed, but the tap roots survived and roughly two months after the polyethylene was removed, the dandelions had grown back. None of the solarization treatments were successful in controlling dandelions.

This is not surprising, given the fact that dandelions are a perennial weed with a deep tap root. None of the treatments were significantly lower than the untreated control. One surprise finding was that clear poly that was on for four weeks, seemed to increase the number of dandelions. It is known that if the tap root has been cut or damaged, additional plants will develop. It is possible that the four-week poly treatment damaged the dandelion root, but stimulated the development of additional plants. If a site has a high number of dandelion plants growing, solarization, whether a four- or six-week duration, or black or clear poly, will not control dandelions. The clear four-week treatment actually significantly increased the population.  

Looking at the effect of the solarization on the total weeds, the black two weeks, clear two weeks, black four weeks and black six weeks all gave the same level of weed control. The clear six weeks was significantly better than the clear two weeks and the black six weeks. There is something happening with the clear four-week treatment. It might be a reflection of the increase in dandelions for the clear four-week, because they constitute a large part of the weeds found in the plots.

Perennial ryegrass

All of the solarization treatments significantly reduced the percentage of perennial ryegrass cover. The black two weeks was significantly better than the untreated control, but the remaining treatments were significantly better than the black two week. This is not surprising. Perennial ryegrass is a bunch-type grass with no below-ground structure to help it survive adverse conditions.

Kentucky bluegrass

Kentucky bluegrass is well suited to survive adverse conditions, because of its underground rhizomes. In fact, many of the solarization treatments increased the percentage of Kentucky bluegrass cover.   This was the case for all of the treatment combinations, except for the clear six-week and black six-week treatment. If Kentucky bluegrass composes a large part of the vegetation that you wish to control through solarization, then a minimum of six weeks is needed and clear and black polyethylene work equally well.  


Looking at all of the data, the clear six-week treatment significantly reduced dandelions, total weed, perennial ryegrass and Kentucky bluegrass. Combining the percentage coverage for total weeds and the main grass species, the clear six-week treatment resulted in 11.5 per cent of live plants, with the remaining vegetation dead. All other treatments fell short in the ability to control one or more of the species of vegetation in the plots, given the environmental conditions that existed throughout the study.  

Research will be repeated in the 2011 season. The timing of poly cover placement will be different. In 2010, the timing of the of the poly covers placement was staggered. The six-week cover was put in place on July 11, the four week cover on July 26 and the two-week on Aug. 10. All covers were removed on Aug. 26. In 2011, all covers will go on the beginning of July and removal will be staggered. This will take advantage of the higher day and night temperatures during July. In addition, thermocouples will be added to one replication of each treatment to track the soil temperature at the 2.5 cm depth to determine what temperatures are achieved under the poly covers.  Thirdly, the plots will be broadcast seeded after treatment to determine the success of a total renovation using solarization.
Pam Charbonneau may be contacted at 519-824-4120, ext. 52597, or by email at