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Reduced risk weed control strategies in carrot production in organic and mineral soils

Project Code: PRR06-700

Project Lead

Diane-Lyse Benoit - Agriculture and Agri-Food Canada


To investigate alternative cultural practices for weed control in carrots

Summary of Results

Carrots are susceptible to yield and quality reductions from competition with weeds. Among a wide range of weed species affecting carrot production, ragweed and redroot pigweed are identified as the dominant issues in both mineral and organic soils. Ragweed control has been particularly difficult in organic soils, where the majority of carrots are grown. Linuron is the only herbicide available for broadleaf weed control in organic soils and ragweed has shown resistance to this chemical. New, lower risk options and integrated approaches were needed to mitigate resistance issues and effectively manage problem broadleaf weeds species in carrot production.

This project evaluated a number of physical weed management techniques as alternatives to the industry standard, including propane flaming, shallow tillage/mechanical cultivation, stale seedbed and precision mowing, alone or combined, along with lower risk control products and application methods. Efficacy trials were conducted over three years, 2006-2008, in both organic and mineral soils at field sites in Prince Edward Island (PEI), Nova Scotia (NS) and Quebec (QC).

Trials in PEI and NS compared control efficacy and crop yield under a variety of weed management systems in carrot crops grown in mineral soils and on raised beds. Treatments included: no weed control; using linuron alone applied broadcast pre- and post-emergence (industry standard); banded linuron application on the bed tops followed by post-emergence cultivation on the sides of the beds (e.g. using side knives) as needed; propane flaming (pre-emergence or pre-planting or both); pre-emergence flaming combined with cultivation on bed tops and sides with tender plant hoes; and banded Eco-Clear (acetic acid) on either stale seedbed or beds prepared at planting. Weed biomass and carrot density, yield and quality were measured at harvest for each treatment.

Results indicated that generally, weed biomass was lower and carrot yield was higher in all treatments when compared to the no weed control. The broadcast linuron application was, however, superior to all other treatments providing the highest weed control and carrot yield at the lowest cost. The most economical and effective alternative weed control program was where raised bed formation is supplemented by banded application of linuron over the bed tops and combined with mechanical cultivation with side knifes between beds. Banding linuron treatments on top of the bed reduced herbicide use by 66% with no significant reduction in carrot yield, but it may require multiple passes with tillage equipment to control weeds between the rows.

Propane flaming was not as effective as banded linuron in weed biomass reduction on top of the bed primarily because it had no residual activity. Pre-emergence flaming followed by either cultivation or side knifes provided also economical weed control, but resulted in lower carrot yield. This technology will require refinements for more precise application with better crop safety measures and consistent coverage before it can be recommended. Eco-clear did not appear to be a valid option due to poor efficacy and high costs. The use of stale seedbed practice combined with raised bed formation gives no additional weed control advantage.

Trials in QC were carried out to devise a precision cutting tool that takes advantage of the morphological differences between carrots and weeds. Greenhouse and field experiments determined the re-growth potential and the most suitable cutting heights of carrots, common ragweed and redroot pigweed to achieve maximum damage to weeds with minimal impact to carrots. Monitoring of weed emergence patterns, confirmed that ragweed has a narrow period of peak emergence in the spring; therefore mowing has the potential for effective control as it can target the bulk of the population at the ideal cutting time. In contrast, the redroot pigweed emerges regularly throughout the season.

The precision cutting tool was designed to mow vegetation at a height of about 6 cm from the soil surface. A firm surface and levelled bed is a prerequisite for the precision cutter to work properly. Although being cost effective, the weed cutter only results in partial control of ragweed due to secondary branching and re-growth following the cutting. The weed cutter appeared slightly more effective for the redroot pigweed; however regrowth can occur and new seedling emergence of redroot pigweed is possible throughout the season. Therefore, precision cutting must be combined with another control method especially between carrot rows in order to give a competitive advantage to carrots. Further research is required to identify such a supplementary control technique that will improve weed control and carrot yield.

In conclusion, an integration of chemical treatments with cultural control techniques is the most promising management approach devised from this research as an alternative option to full reliance on few herbicides. For carrot production in mineral soils and on raised beds, the most economical, effective and environmentally responsible weed control approach is where the banded application of linuron is combined with mechanical cultivation using side knifes in between the beds. This approach can be recommended to growers for commercial adoption and also can be transferred to other regions or vegetable crops (e.g. parsnips and potatoes) using similar crop production systems.

For carrot production in organic soil and where ragweed is either linuron resistant or the dominant weed, the control program with the most potential is one where linuron is applied broadcast prior to carrot emergence, followed by a single cutting at 3 leaves stage of carrots. This approach has the potential to reduce pesticide use by 50%; however, it is still in the research stage, and therefore not ready to be transferred to producers.

For more information please contact Dr. Diane Lyse Benoit (, Agriculture and Agri-Food Canada.

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