Soil solarization for stellar weed suppression
Our sun has some incredible powers, not least of which is feeding the plants which feed us. But we can also harness its energy to help eliminate the plants we don’t want to grow – aka weeds.
Soil solarization uses the passive solar heating of moist soil under transparent plastic tarping to reach high soil temperatures detrimental to weeds. The target temperature is more than 104º F. Done right, the practice has the potential to increase crop yield.
A lot of research has been done to test the efficacy of solarization in open fields; Luize Lessmann, a graduate student in the University of Kentucky’s Department of Horticulture, wanted to observe the effects of soil solarization on weed management and lettuce in Kentucky and Tennessee high tunnels. She presented her findings at the American Society for Horticultural Sciences annual conference.
Lessmann noted that high tunnel production is increasingly important on small, diversified farms. Kentucky is home to approximately 1,700 high tunnels; Tennessee has over 700. As few herbicides are appropriate for use in high tunnels, over time, weeds can become problematic because they compete with crops for water, light, nutrients and space, causing issue with crop quality and yield.
Lessmann wanted to evaluate the effects of solarization on soil temperatures, weed biomass and emergence and lettuce yield. The study took place in Lexington, KY, and Knoxville, TN, in both open and closed high tunnels. The soil underwent solarization in April, July and September 2024, for either two or four weeks (and were compared to tunnels with no solarization). Data loggers recorded soil temperatures hourly, and temperatures were checked at 5, 10 and 15 cm deep.
Lettuce was transplanted in the tunnels four weeks post-solarization treatments. Marketable vs. unmarketable yields were compared at harvest time.
The study found maximum soil temperatures were 4º to 9º C higher in Kentucky and up to 14º C higher in Tennessee in solarized vs. non-solarized plots. Overall, soil temperatures were higher in Tennessee compared to Kentucky too.
The highest temperatures were recorded in July in the closed high tunnels, especially in Tennessee. Soil in the closed high tunnels accumulated more hours at more than 104º F vs. the open high tunnels – the temperature considered necessary to stop weed growth.
In Kentucky, commonly identified weeds in the tunnels included common purslane and crabgrass. In Tennessee, they were morning glory, johnsongrass, crabgrass and henbit. In addition to higher temperatures in July, weed pressure was higher in July as well.
As far as weed biomass went, there were promising results, especially in summer, in both locations, Lessmann reported. Even at harvest (summer and autumn), Tennessee had significantly less weed biomass in solarized treatments compared to non-solarized. She also reported less weed emergence in solarized treatments, especially in closed high tunnels in Tennessee. In general, however, solarization didn’t work quite as well in open tunnels as it did in the closed ones. Yes, the soil solarization kept the weed pressure down. But what about the cash crop?
The plots that underwent four weeks of solarization usually had the highest lettuce yield among all the treatments across the seasons and the states – although the differences were not always significant.
However, lettuce yield was higher in two-week and four-week solarized treatments compared to non-solarized, regardless of the plot being closed or open high tunnel.
To see if there is true efficacy to these treatments, Lessmann is currently repeating the experiment.
by Courtney Llewellyn
