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burning weed seeds

This project will include field experiments conducted at one farm in Maine, and another in Vermont. The on-farm studies will be conducted on a uniformly cropped weedy field following cash crop harvest. At each farm, in the fall of 2011, weeds will be flail mowed and four replicated strips will be subject to the following treatments: no flaming; flaming at 40 kg propane per ha; flaming at 80 kg per ha; and flaming at 160 kg per ha (note: a 20-40 kg rate would kill sensitive weed seedlings). In the spring of 2012, soil sampling and subsequent greenhouse germination assays will measure the weed seedbank in each treatment. Ten soil cores (6.5 cm diam by 10 cm deep) will be randomly chosen from each plot, bulked, and spread in greenhouse flats, and watered to encourage weeds to germinate. Seedlings will be removed monthly, followed by drying and crumbling of soil and re-watering (four cycles in total). To measure the contribution of fall flaming on weed management in the subsequent crop, in June of 2012 we will record weed density after the final cultivation event at each farm. Specifically, eight 0.5 sq. m quadrats will be censused in each replicate of each treatment. Weeds will be identified by species and counted.

In this project we aim to test the hypothesis that fall flaming can efficiently and effectively kill weed seeds on the soil surface. While flaming is widely used to kill small weed seedlings, and field stubble burning is known to kill weed seeds, we are not aware of any research in which flaming has been used to target seeds. This idea comes from the experience of Rob Johanson, a diversified organic farmer in Maine who, while flaming potato vines in particularly weedy fields observed smoldering weed residues and reasoned that he was likely killing weed seeds. He has further observed that flamed plots generally have lower weed pressure the following year. We aim to confirm this observation with a series of on-farm experiments evaluating three flame doses and their effect on the following season’s germinable weed seedbank and early-season weed seedling densities. Laboratory studies conducted with condiment mustard (Sinapis alba), indicated that a 900C propane flame required 3 sec. or more exposure to reach mortality levels of 80%; mortality was 100% at 5 sec. of exposure (Figures 1 and 2). Field studies will include higher temperatures to reduce the required exposure time, and will evaluate the combined stresses of flaming and overwinter mortality. Results will be shared with other growers during a Field Day and a session on “Managing the Seedbank for Improved Weed Management” at the 2012 Maine Organic Farmers and Gardeners Association Farmer to Farmer Conference. We will also produce a short video on fall flaming which will be posted to our YouTube Channel (zeroseedrain), and the national eXtension website.

Propane usage will be recorded by weight using portable field scales, and will be used to estimate per ha costs for the flaming treatments. Thermocouples and a portable temperature recorder will be installed in each treatment to record the thermal dose.

Project objectives from proposal:

Objective: To measure weed seed mortality in response to flaming in the field.

Performance Target: We expect that the germinable weed seedbank density will be inversely related to the propane flaming rate. Further, the spring seedling census data will proportionally reflect the effects of flaming on the germinable seedbank, with the lowest surviving weed density in the 160 kg per ha propane treatment, and the greatest weed density in the control treatment. Adoption of this promising management practice will depend on the expected tradeoff between faming cost and weed control benefits measured the following year.

If the idea of weeding using a flame thrower makes you uneasy, it’s time to find out more about using heat to kill weeds. Flame weeding is safe when you use the equipment properly. In fact, in many cases, it’s safer than using harsh chemicals that can contaminate groundwater and leave toxic residue on your garden vegetables. Read on to learn how to use flame weeders and when flame weeding is suitable.

Flame weeding works best on annual weeds that are 1 to 2 inches (2.5-5 cm.) high. Use flame weeders to kill weeds that grow around garden barriers and fences. They excel at killing weeds in sidewalk cracks, and you can even use them to kill stubborn, broadleaf weeds in lawns because mature lawn grass blades are protected by a sheath. Once you have a flame weeder, you’ll wonder how you ever got along without it.

What is Flame Weeding?

Once you have passed the flame over the weed, the leaf surface changes from glossy to dull. If you are concerned that the weeds aren’t dead, allow them to cool and then squeeze a leaf between your thumb and finger. If you can see a thumbprint in the leaf, the flaming was successful.

The problem with flame weeding in gardens is that it’s hard to expose the weeds to the flame without exposing your plants as well. In vegetable gardens, use a flame weeder to kill weeds that emerge after you sow seeds, but before the seedlings emerge. You can also use it to kill weeds between rows.

A flame weeder setup consists of a wand connected to a propane tank by a hose. You’ll also need a dolly to carry the propane tank, and a flint igniter to light the flame if the wand doesn’t have an electronic starter. Read the instruction manual completely before using a flame weeder.