Our Approach to Flaming

Not all flame weeders are created equal.

As an organic farmer you cannot use chemicals. Each growing season you spend a lot of quality time with harrows, rotary hoes, and mechanical cultivators. However, mechanized treatments are limited by wet weather and in how tight they can be set to the crop row for fear of crop root injury and the all too common cultivator blight. As a result, these methods cannot control weeds that are within the row, competing directly with the crop for limited nutrients. Flaming, done with well-designed equipment like AFI’s line of row-crop flamers, can eliminate all weeds, even those that are within the row. As a result, flame weeding can lead to major yield increases when compared to mechanized weed control alone. Using an AFI flamer, the maximum corn yield increase that one of our organic farmers obtained was 60-70 bushels per acre! It is hard to believe sometimes how beneficial flaming can be for a farm operation. The more weed pressure a farmer has in a field, the greater the potential for a large yield increase. As a result, it does not take many acres to pay off an AFI flamer in one year! For example, if one gets a yield increase of 27 bushels/acre, using a price of $7/bushel, it would take only 216 acres of corn to pay off an AFI 8-row unit within one year (based on 2020 prices). This ridiculously fast return period is achieved through all of the great features we have built into AFI equipment, but before we dig into those, let’s get a little more background on what flaming actually is.

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What is
Flaming?

How do you use an AFI flame weeder?

What is the benefit?

How much does a treatment cost per acre?

What is flaming?

Flame weeding (a.k.a. flame cultivation, flaming) is just one of many methods that have been developed to control and manage pests. Flaming is a direct control method that is a subset of non-chemical, thermal-based pest control methods. Its name is a misnomer. Done properly flaming does not burn anything. It is not a controlled burn or a flamethrower out in your field.

Rather, flame weeding harnesses the intense heat of combustion to kill undesirable pests, or more specifically, weeds and insects. One of the major applications of flaming is controlling weeds in agriculture. Thinking that weeds are literally ignited and burned is a common misconception. It is better described as a desiccation of the weeds resulting from exposure to the extreme heat. The intense heat disrupts the plant cell function and ruptures cell walls, killing the plant within a few days of treatment (Ascard, 1995a). Flaming is a very versatile tool, having applications in a large number of agronomic and horticultural crops. It also has several uses outside of agriculture; for example, weed control on the concrete surfaces in urban areas.

Large velvetleaf before flaming treatment.
Velvetleaf wilted and dying one day after flaming treatment

Flame weeding is a certified organic method of weed control. It does not use any chemicals, and no harmful residues are left in the field to runoff into nearby water sources. Modern, LPG or propane-based flame weeding equipment was first developed in the 1930s. Interest in the method slowly gained popularity until the 1960s when chemical herbicides took over the weed control market. Interest in flaming was very limited for the next 30 years (1965-1995). It was not until the start of an organic movement in the 1990s brought about a rebirth in flame weeding (Edwards, 1964).

United States patent awarded to P.C. McLemore in 1950 for a flame weeder design.
United States patent awarded to P.C. McLemore in 1950 for a flame weeder design.

The modern fuel source for flaming equipment, propane, is one of the cleanest burning hydrocarbons. A byproduct of that propane combustion creates the ‘active ingredient’ or mode of action for a flaming treatment: heat energy. Heat is non-selective by nature and will treat whatever it contacts. The very high temperature combustion gases exiting a flaming torch are able to quickly transfer a lethal dose of heat to a weed. Ignition of the weed biomass is not required for an effective flaming treatment; rather, a lethal dose of heat is one that sufficiently damages the leaf microstructure. The damage leads to water loss and the weed slowly dies over 1-3 days. Immediately after a flaming treatment, sufficient damage can be tested by using the fingerprint test. Looking at the figure to the right, sufficient damage can be verified by a visible fingerprint remaining in a plant leaf after pressing it between the thumb and forefinger. Kang (2001) found that raising the temperature of the plant to 100°C (212°F) for 0.1 second is lethal. That goal can be achieved by exposing the plant to an estimated 800-900°C (1450-1650°F) environment for a similar time period (Thomas, 1964).

Fingerprint test

Flame weeding can reliably control weeds within the crop row, making it an excellent complement to mechanical cultivation. Adding flaming to existing cultivation methods can eliminate the need for hand weeding, which can lower operating costs for the farmer. Even more important, flame weeding can, through improved control of weeds growing within the crop row, increase yields up to 25-30% compared to yields obtained through cultivation alone.

Ascard (1995a) categorized flame weeding treatments into two main types: selective and non-selective. Selective flaming treatments are done after the crop has emerged, and the objective is treating the weeds while minimizing crop damage. Non-selective treatments (also known as broadcast treatments) cover a specified path and do not distinguish between weed and crop; everything in the treatment path is fully exposed to the hot gases. Due to the fact that heat, the active ingredient for flaming, will treat whatever it contacts, crop growth stage and equipment design are the two critical factors that must be optimized to obtain successful flaming treatments. Pre-planting, pre-emergence, and early crop growth stages are the optimal times for a non-selective treatment. Obviously, the crop is not exposed to high temperatures during pre-planting and pre-emergence and is at no risk. During early growth stages, however, the crop must either be able to recover quickly from the flaming damage (e.g. young corn), or be much more tolerant of the high temperatures than the surrounding weed species (e.g. young soybeans). In non-selective treatments, the main role of equipment design is in minimizing energy input while still obtaining the desired level of weed control.

Young corn with 3 true leaves (V3 stage)
Young soybeans in the crook state (VC, cotyledon stage)

Selective treatments are done when the crop is at a later growth stage. Most crops have a longer and much more heat tolerant stalk at later growth stages like mid-season corn with 5-6 leaves (V5-V6) and soybeans with 4-5 trifoliates (V4-V5)(see picture to the right of V6 corn 1 day after treatment). This combined with the fact that any of the lower crop leaves damaged by the flaming treatment are no longer critical for crop production. Regardless of flaming damage or not, crops will drop lower leaves as the season progresses because the upper crop canopy shades these leaves from the sun. Once a leaf is no longer productive for the plant, it must go.

Equipment design for selective flaming treatments is more challenging. A flamer must always provide a high quality treatment, and in the case of selective treatments, must also minimize crop injury through optimal torch configuration and shielding with hoods. Lastly, selective and non-selective treatments can be applied in two ways: banded flaming and full flaming. Defined by the amount of area that is covered during the treatment, banded flaming applies heat to a small band (8-12 inches) that is centered on the crop row; whereas, full flaming treats all of the area within a treatment path (e.g. a full 30 in. wide crop row)

V6 corn 1 day after treatment
AFI 8-row flamer doing a banded selective treatment in V4-V5 corn.

How do you use an AFI flame weeder?

Not all flame weeders are created equal. Flame weeding is like a surgery, where a malignancy needs to be removed without causing damage to the surrounding normal tissue. Research at the University of Nebraska-Lincoln guided the development of AFI flame weeders and AFI flame weeding recipes which kill the weeds without causing lasting damage to the surrounding crop. AFI’s optimized designs have proven to maintain the crop yield at or near the same level as hand weeding. Poorly designed flame weeders with no proven treatment recipes, although they may eliminate all the weeds, can cause lasting damage to the crop and drop the yield below the capacity of a given field. AFI’s hood technology is key in achieving these goals. Hoods minimize crop damage by keeping the hot gases near the ground and away from the crop. Referencing the picture below, hot gases from flaming torches are directed underneath hoods during a selective treatment in soybeans (V4-V5) and the taller crop is protected from the high temperature treatment zone.

Hood gap in field

Timing is everything in organic weed control and flaming is no different. The early flaming treatments are the most critical. Looking at the most common row crop, corn (including all field, popping, and sweet varieties), we recommend the early flaming treatment be conducted when the crop has less than 4-5 leaves (VE-V5, 2-10” in height) and with the hoods closed.

Young corn approaching the end of the early flaming stage (V4-V5)
Below ground growing point location in early stage corn.

The growing point at these early stages is still below the soil surface and is very well protected from the flames. It is almost impossible to kill a field of young corn with flaming. The exposed crop leaves are injured during these early treatments, but the crop quickly recovers within 2-3 days. Remember, nothing is burned. A flaming treatment does NOT equal a controlled burn.

The weeds even remain green immediately after a flaming treatment but start wilting within minutes of heat exposure. After about 30 minutes the very small weeds (< 1”) are flat on the ground. The broadleaf weeds will die and will not come back. The grasses, however, will come back within several days.

A common broadleaf (waterhemp) before flaming
Taken one day after treatment, this broadleaf (waterhemp) will not recover.

We further recommend that the early flaming is followed by aggressive cultivation of the inter row space if soil conditions allow it and the crop is not in danger of being buried. By aggressive cultivation we mean that enough dirt will be thrown to cover the wilted grasses and prevent them from post-flaming recovery. In some cases this treatment (banded flaming followed by aggressive cultivation) may turn out to be adequate for the entire season.

Pictured below is an example of the power of a well-done banded flaming treatment completed with an AFI flame weeder in early stage corn (V3-V4, ~6” in height). Taken 1 day after the treatment, the picture on the left has a red line marking the transition from treated (left side of picture) to untreated (right side of picture). The second picture is a close up of the flame treated band (highlighted between red lines). If you look closely there are many small (1-2”) velvetleaf that have wilted significantly and will never recover. The grasses have also been wilted significantly and a subsequent aggressive cultivation was very successful in covering up those wilted grasses. The injury that can be seen on the corn does not have a negative impact on the crop or the final yield. The corn recovers quickly and within 14 days it is difficult to tell flame-treated corn from untreated corn, and final yields are comparable to the optimum yield of a hand-weeded crop.

Moving on to another very common row crop, soybeans, early flaming is conducted during the cotyledon stage (a.k.a. shepherd’s crook or crook stage, VC, <1” in height). Sunflower is another broadleaf crop like soybeans where this timing applies. It is a window of about 1-5 days, depending on soil moisture and temperature, and it starts the moment the soybean cracks the soil surface.

Young soybeans at emergence
The end of the early stage window for soybeans
Sunflower

During this period the growing point is well protected from the flames. As with early flaming in corn, we recommend banded flaming with the hoods closed. The broadleaf weeds will die, but the grasses will come back. Again, cultivation should follow as long as there is no risk of physically damaging the young soybean or sunflower plants. Full flaming can be used to avoid cultivation or when the field is too wet for effective cultivation.

The cotyledon stage window for early flaming in soybeans and sunflowers is very short and one can extend this window as follows: The field can be prepared and the weeds can be allowed to germinate for about a week. The crop can then be planted. The soybeans or sunflowers will emerge about 4-7 days later, depending on planting depth, soil temperature, and soil moisture. One can do the early flaming from the moment the soybeans or sunflowers have been planted until the end of the cotyledon stage. This window is about 10-12 days long. Weather permitting, it is still preferable to do the flaming during the cotyledon stage, which will give the crop the greatest competitive advantage over the existing flush of weeds. After completing a flaming treatment in early stage soybeans or sunflowers, it is fairly common to see scorching of the cotyledon skins the next day (see pictures below, soybeans on the left and sunflower on the right). This should not be a concern. The first leaf stage (V1, unifoliet) will emerge and your crop will thrive with no weeds around to compete with.

VC soybean scorched
VC sunflower scorched

Weeds never seem to rest and a new flush is just a small rain shower away from germinating. These later season flushes can be addressed with a second and final late flaming. Based on the University of Nebraska research, a maximum of two post-emergence flaming treatments can be applied to a crop before the injury starts to affect final yields. Any pre-planting or pre-emergence flaming treatments do not count towards this maximum. Also, by this time in the season cultivation has already been done at least once and there is likely a soil ridge running along the crop row. It is important to note here that flaming torches must be raised to account for this soil ridge. Torches are not cultivator sweeps. They will malfunction if they are pulled through the soil. As for crop growth stages, the crop will be much taller than it was for early season flaming. AFI equipment accounts for this by configuring a gap to be easily opened between the hoods allowing those taller corn, soybean, or sunflower plants to safely flow through that gap (see drawing to the right). The gap can vary from four to eight inches, and similar to setting cultivator sweeps, how tight you set the hood gap depends on how comfortable you are with driving the tractor through the field.

Hood gap example

At this stage the corn stalk is very sturdy (6-8 leaves, V6-V8, 24-30” in height) and the short flame exposure (about 1/10 of a second) cannot harm the corn plant. Likewise, soybean and sunflower stems become woody (through lignification) at these more mature growth stages and are very heat tolerant (V4-V5 in soybeans and V8-V10 in sunflower).

Taller crops ready for a late flaming treatment:
V6 corn
V10 sunflower
V5 soybeans

Similar to early flaming recommendations, late flaming can also be followed by aggressive cultivation to smother the wilted grass and take care of the weeds growing between the rows. As for sorghum, it is quite similar to corn especially in the earlier vegetative growth stages. Although no full season flaming research studies were completed in sorghum like they were for corn, a similar recipe to corn could be used for flaming sorghum.

A couple final notes: first, at any treatment timing, early or late, we do NOT recommend cultivating first and then flaming. Soil moved during the cultivation pass can partially cover some weeds and protect them from the heat, reducing the effectiveness of the flaming treatment. Second, since soil surfaces are exposed to flames for such a short time period, about 1/10 of a second, heat from flame weeding does not penetrate the soil. As a result, there is no harm done to the soil microbes.

References

Ascard, J. (1995a). Thermal weed control by flaming: Biological and technical aspects (Doctoral dissertation, Swedish University of Agricultural Sciences, 1995). Department of Agricultural Engineering, Alnarp, Sweden.

Edwards, F.E. (1964). History and progress of flame cultivation. Proc. First Annual Symposium: Research on Flame Weed Control, 3-6. Sponsored by Natural Gas Processors Association. Memphis, TN.

Kang, W.S. (2001). Development of a flame weeder. Transactions of the ASAE, 44(5), 1065-1070.

Thomas, C.H. (1964). Technical aspects of flame weeding in Louisiana. Proc. First Annual Symposium: Research on Flame Weed Control, 28-33. Sponsored by Natural Gas Processors Association. Memphis, TN.

What is the benefit?

Better weed control through flaming leads to better crop yields. If those weeds are not in the row competing for nutrients, the crop can take full advantage and maximize yields. The most impressive numbers have been observed in corn. It is fairly easy to obtain a 20 bushel per acre yield bump, and the maximum yield increase can approach 60-70 bushels per acre! It does not take long to pay for an AFI flamer with that magnitude of return. An average yield increase of 5-7 bushels per acre can be obtained in soybeans. Although not as impressive as the yield increases observed in corn, there are a few additional benefits in flaming soybeans. First, flaming helps overall weed control in your soybeans and makes for a cleaner looking field later in the season. Cleaner soybean fields are prettier soybean fields, and prettier soybean fields attract the right attention from neighboring farmers driving by on the highway. Second, given the bushier soybean plant, leaves and flowers located very close to the ground can get damaged during flaming, but the soybean plant often responds well to stress by setting more flowers higher up. Fewer pods being set close to the soil surface where they are hard to reach with a combine can result in lower harvest losses. The last side benefit that has been discussed with farmers is reduced risk of fungus or mold growth. The clearing of some of the lower-growing leaves and branches through late flaming can allow better air circulation and possibly help prevent fungus or mold outbreaks later in the season. Although not fully verified in controlled research studies, flame weeding soybeans yields more benefits than just better harvest numbers.

How much does a treatment cost per acre?

An average banded flaming treatment consumes 5 gallons of propane per acre (GPA). If the weed population is out of control and/or approaching 6” in height or taller, more fuel will be needed (up to 7 GPA). Full flaming consumes 10-11 gallons of propane per acre. Farmers opt to use full flaming when they want to avoid cultivation or soil conditions remain too wet from frequent rain events. Every time you cultivate you lose about one inch of water, and especially in arid or semi-arid climates, soil moisture must be conserved whenever possible. Regarding the cost of propane consumed, the treatment cost for flaming varies from $5-10 per acre (2020 prices of propane), which is very small compared to the benefits of improved weed control and improved crop yield. As mentioned above, increases in corn yield can net a farmer up to $300-400 more per acre!