Organic Farm Calls: @#*%$! Flaming Weeds

This week a Jersey grower asks why his high BTU vapor phase propane weed flamer doesn't kill all his weeds. He reports that it seems to work fair - most of the time - on broadleaf weeds, but doesn't do an adequate job on grasses. Why are his results so variable?

Weed control implement with vapor phase propane burners

Optimizing Propane Flaming Weed Control 

Here’s a weed control tip: If you use flaming weed control with vapor phase propane burners, perform your work in the afternoon, beginning at least eight hours after sunrise.

That’s right. Flaming the same weed, like a difficult to control grass at 6 leaves, results in 62% injury early in the morning vs. 82% injury by simply waiting 8 hours or more after sunrise on the same day. We know from weed science experience that while 62% injury sounds like a big percent, it is inadequate, ineffective control. All weed species show more injury when flamed in the afternoon. Broadleaf weeds, more sensitive to flaming than grasses, may experience 90%+ injury rates when flamed 8 hours after sunrise or later.

There is a documented–but not well-understood–daily variation in the leaf water content of plants, which affects their susceptibility to flaming weed control used to prepare stale seedbeds. These findings come from studies by Stevan Knezevic and colleagues at the University of Nebraska.

The bottom line is that flaming should always be conducted in the afternoon, on smallest weeds possible, to improve reliable control and reduce propane consumption rate when making stale seedbeds. 

On long season crops like organic processing tomato, we know from field trials in Italy that even after preparing stale seedbeds by flaming, additional tillage and hoeing will be required. Total labor inputs will average 50% higher than conventional weed control methods, and the weeds in the field at harvest will be higher.

Propane flaming is not a job to be taken lightly. Flaming is potentially hazardous, expensive, slow, and labor intensive. It may involve nurse tank storage and hazardous transportation regulations. But mostly, flaming is frustrating because the results are frequently unreliable, particularly on grasses and larger weeds.

So, if you are going to perform flame weeding, stack the deck in your favor, and forget the idea of going out in the cool, calm, early morning hours to perform the work.

-Jack Rabin