Fall Flame Weeding: Targeting weed seeds before they enter the seedbank

Flaming is commonly used on organic and low-external-input farms to control weed seedlings, especially annual dicots in slow-to-emerge crops such as carrot and beet. Flaming could, however, also be used to kill weed seeds before they enter the seedbank, a strategy used in recent years by Rob Johanson on the Goranson Farm in Dresden, Maine. In our field studies, pre-dispersal flaming did not affect viability of common lambsquarters or redroot pigweed seeds. Flaming, however, shows promise as a method to reduce density of weed seeds following dispersal. Greenhouse and field studies conducted on-farm demonstrated that flaming could kill weed seeds on the soil surface. Typical tractor speeds used for other flaming operations, e.g., 1.6 mph, killed about 50% of the most sensitive species (i.e., hairy galinsoga), however, with the flame dosage doubled, i.e., to 0.8 mph, flaming reliably killed 75% or more seeds of mustard, large crabgrass and hairy galinsoga. There was no advantage to further doubling the flaming dosage, as seed mortality was similar with both 0.8 and 0.36 mph treatments. With an estimated cost of $150 per acre for 0.8 mph treatment, fall flaming could prevent large weed seedbank credits, especially relatively sensitive species including hairy galinsoga.

Introduction:

Annual weeds are consistently ranked as a challenging production problem for organic and low-external-input farmers. While these growers often focus weed management efforts on controlling weed seedlings, often densities are very high requiring multiple cultivation events and, in high value crops, supplemental hand weeding. If, however, a broader perspective to weed management is used, and strategies are employed to reduce the weed seedbank, subsequent weed seedling densities are proportionally reduced and the outcome of physical weed control improves (Gallandt, 2006). Weed seedbanks vary widely across Northern New England organic farms, ranging from a low of approximately 2,500 germinable seeds m-2, to over 25,000 m-2 (Gallandt, unpublished, Figures 1 and 2). Practical strategies to reduce the germinable weed seedbank begin with effective cultivation. Targeted fallowing, timed to coincide with peak emergence potential of problem species is also foundational for managing the seedbank. Also, in diverse enterprises, intensive sequences of very short-season crops can incorporate frequent soil disturbance, which encourages weed germination, while completely preempting weed seed rain. The combined approach of encouraging germination losses or “debits” to the seedbank, while preventing seed rain “credits,” is described in a recent webinar by the PI: http://www.extension.org/pages/62445/cultivation-and-seedbank-management-for-improved-weed-control-webinar Despite a thoughtful and well-executed weed seedbank management plan, weeds escaping early season weeding may become large and produce abundant seed rain (e.g., Figure 3). In a previous, but related NE SARE research project (LNE06-237: “Managing weed seed rain: A new paradigm for organic and low-input farmers”) we examined contrasting fall weed management practices in an attempt to maximize post-dispersal seed mortality. In this study we demonstrated that complete preemption of weed seed rain resulted in rapid and large reductions in the weed seedbank. The central hypothesis motivating this work was related to fall cover cropping which we speculated would incorporate newly dispersed seed into the soil offering protection from seed predators (e.g., Figure 4). Using mesh exclosures post-dispersal through the following spring, we found that seed predation had a similar large effect on the seedbank, but this effect was inconsistent: in one year predation reduced the seedbank 40%, but in three other years predation losses were not detected. Rob Johanson, a diversified organic farmer in Maine, routinely flames potato to kill vines prior to harvest, typically travelling 1.5 mph, with 40 psi at the burners, but flaming speed depends on the condition of the vines and may range from 1.0 to 2.0 mph, occasionally two passes are made. Several years ago he started intentionally reducing his tractor speed to increase flaming dosage in fields with abundant fall seed rain. In short, he reasoned that post-dispersal seed flaming could be used to quickly and effectively kill weed seeds prior to their entering the seedbank. Standing weeds in potato fields are blackened and smoldering post-flaming, and it seemed logical that flaming would kill weed seeds on the soil surface. In 2010 we conducted several preliminary dose-response experiments examining effects of increasing flame duration on condiment mustard (Sinapas alba). Increasing exposure from 1 to 5 seconds in a propane flame of 900C reduced viability (Figures 5 and 6). The effective doses were, however, quite high indicating that practical tractor speeds and propane rates might be effective only on certain weed species. Our aim with this project was to evaluate flaming effects on weed seeds, both pre- and post-dispersal, and the effect on the subsequent weed seedbank, with a particular focus on species with average seed mass less than mustard that would likely be more susceptible to flaming. Flaming effects on weed seedlings (Ascard, Hatcher, Melander, & Upadhyaya, 2007; Ascard, 1994, 1998), and effects of field burning on seeds (Ensign, 1973; Young, Ogg, Dotray, & Press, 1990) have been studied previously. However, I found only a single report of flaming effects on weed seeds in the literature. Pali, Pomsar, & Reisinger (2007) examined flaming effects on seeds of several annual weeds and found mortality to ranged from a low of 80% for redroot pigweed (Amaranthus retroflexus) to 100% for maple-leaved goosefoot (Chenopodium hybridum) and barnyardgrass (Echinochloa crus-galli). These results, and the recent experience of Mr. Johanson at the Goranson Farm, indicates that seed flaming has promise as an additional tool for managing the weed seedbank.

• Figure 2. Greenhouse flats demonstrating variability in weed communities among New England organic farms.
• Figure 3. Examples of abundant weed seed rain in organic carrot and soybean.
• Figure 4. Diagram depicting seed fate post-dispersal, and following fall tillage.
• Figure 5. Dose-response showing condiment mustard mortality in response to increasing flaming exposure.
• Figure 6. Condiment mustard seeds following flaming treatments.
• Figure 1. Weed seedbanks on 23 New England organic farms.