Vietnam War: Forest Fire as a Military Weapon - Paperless Archives

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Applied to woody vegetation, the contact sprays affect both the leaves and small twigs. These plant parts-with high surfae.volume
ratios can be killed because a large proportion of the tissue is exposed to contact with the chemical.
Larger branches, stems, and buds commonly arc protected with resinous or waxy coverings, or by corky bark.
Typical drying of leaves and twigs after application of contact desiccants is shown in Figure 37. Most leaves die within
2-4 weeks, but additional leaves and twigs continue to die and dry f.•r as long as two months. A few succulent stems may
die. But most stem tissue remains alive, capable of producing new foliage when weather becomes favorable.
Under humid tropical climate, as with guava in Puerto Rico, the regrowth starts on most stenis soon after the original
leaves have dried. After a few months the plants show little or no effect from the desiccant spray. In a tropical climate
with a long dry season the plants recover during the next wet season after the leaves have been desiccated. Under climates
with cool or cold winters, the leaves remain dry and brown until replaced by new foliage during the following spring or suminer.
Most of the desiccated leaves drop quickly from the plants under tropical climates and under humid climates with
cool or cold winters. Leaves remain attached for months on evergreen vegetation under climates with a long dry season.
Contact desiccants are effective in making b'irnable fuel out of the leaves and a small proportion of the twigs. But
since understory vegetation is usually small-stemmed, this represents only about 35-40 percent effective desiccation of the
potential small fuel (Figure 38). Although contact desiccants are relatively ineffective in increasing the amount of available
fuel. they do reduce the dampening effect of green leaves and open shrub crowns to speed drying of litter. This, along with
adding dry leaf fuel, is all that is needed in certain situations, and will promote good burning if adequate litter and dead
stems already are present. But contact desiccants are not adequate as the only treatment for use in climates where total
available fuel weight is less than 0.3 pounds per square foot (see Appendix B). The contacts are recommended as a supplement
to systemics in situations where a quick kill of green leaves is essential. One common situation is where new green
foliage has regrown short;y before the planned date of burning.
Several different cherricals will serve equally well as contact desiccants.
Paraquat - II'-dimethyl-4,4'-bipyridinium salt
Diquat - 6,7-dihydrodipyrido( 1,2-a: 2', i'-c) pyrazidiinium salt
Cacodylic acid - hydroxy dimethylarsine oxide
Dinitro - 2' s butyl-4,6-dinitrophenol
Paraquat and cacodyllc acid were used in most trials and produced consistent results. The best dosage for each
chemical was not apparent from out trials: in some cases the lowest applied dosage was as effective as the highest. We
recommend any one of the following contact desiccants: paraquat at 4 pounds acid equivalent per acre, cacodylic acid at
12 pounds, diquat at 12 pounds, or dinitro at 12 pounds. The dinitro should be mixed with diesel oil, and the others with
water, to make 5 gallons per acre total volume of spray mix.
If weather will be sunny and warm (daily mean temperature above approximately 60 0 F.) the contact desiccant should
be applied 3-4 weeks ahead of the planned date of burning. If cloudy, cool weather is expected, the contact desiccant can
be applied two months in advance of burning. Longer periods of drying are not advisable because additional regrowth of
green vegetation may occur before burning.
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