Effect of Season-Long Flaming Followed by a Grass Cover Crop on Canada Thistle Biomass in Organic Strawberry Production

Flaming reduces competitiveness of Canada thistle.

Perennial weeds are a chronic problem associated with organic strawberry production in Colorado. Crop rotation, mechanical cultivation, hand weeding and hoeing typically are used to manage these weeds. The use of thermal (flame) weed control systems addresses this problem and is an alternative to mechanical cultivation and herbicides for controlling weeds. The purpose of this study was to examine the effect of multiple flame applications on Canada thistle biomass in a fallow strawberry field. The 2.5 acre strawberry field was located at Berry Patch Farms, a certified organic farm near Brighton,
Colorado.

Weeds were flamed using the Flame Engineering TD-12 LPS Alfalfa Field Flamer. The Alfalfa Field Flamer utilizes liquid spray flaming that creates combustion at the base of the plant to produce 2000 degrees Fahrenheit. The cost was $1800, and the flamer consumed 35 gallons of propane per acre at a cost of $40 per acre. This machine is ten feet wide and produces a wall of flame. The propane tank holds 200 gallons of propane and can treat approximately 6 acres depending on travel speed and the ambient air temperature at time of application. The speed of application (tractor speed) was approximately 3 mph.

Five flame applications were used during the 2004 growing season. The aboveground portion of the plant was harvested and weighed within 3 to 5 days after each flame application. Good control was seen immediately after the flame treatment on Canada thistle; however, rapid re-growth continued throughout the summer, thus requiring multiple flame applications. Aboveground fresh weight of Canada thistle foliage treated with just one flame application averaged a 63.3% reduction when compared to the untreated check. Five days after the fifth and final flame application, the above ground fresh weight of Canada thistle foliage averaged 88.6% less than the untreated check. At this time root mass was harvested and weighed. Root biomass comparisons revealed a 55% reduction in fresh root weight following five flame applications.

In August 2004, the field was disked, beds were established using black plastic mulch, and strawberries were planted. Then the rows between the strawberry beds were planted with crested wheatgrass, intermediate wheatgrass, and pubescent wheatgrass cover crops. On June 1, 2005, a full year after the first flame application, the above ground fresh weight of Canada thistle foliage averaged 87.8% less than the untreated check. Root biomass comparisons revealed a 62.4% reduction in fresh root weight. Canada thistle plant population density fell from 37.3 plants per square meter to 2.05.

Based on results experienced in this study, flaming weeds using the Flame Engineering Alfalfa Flamer provided excellent activity on small annual weeds and good to fair activity on deep-rooted perennial weeds. Multiple applications are needed to provide season long management of weeds.

The reduction in the population and biomass of Canada thistle cannot be attributed to flaming alone, but rather the multiple cultural and mechanical weed management tactics used during the one-year period. Five flame applications reduced the competitiveness of the Canada thistle. This allowed the fall-seeded grasses to become adequately established to displace the Canada thistle populations in 2005. Mowing, hoeing, hand weeding and steaming at the plastic bed and grass row interface will be the only weed management tactics utilized during the remaining life of this strawberry field.

Thaddeus Gourd, Extension Agent (Agriculture), Adams County, and Tim Ferrell, Berry Patch Farms