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There have been tantalizing rumors ova the past few years about new hot-water based weed control technologies for controlling weeds in areas where complete vegetation removal is sought. Hot water can also be applied as a spot treatment for weeds in turf or cracks. The principle behind these machines is simple: hot water melts the waxy coating on weed leaves, or breaks down the plant's cellular structures. Treated plants are unable to retain moisture, and dehydrate within hours or a few days. Hot water kills new as well as mature plants.

Hot water technology offers many benefits over chemical herbicide applications, as it does not contaminate water, soil , or air, and it eliminates the potential for human or wildlife exposure to pesticide residues. Although herbicidal in effect, the U.S. Environmental Protection Agency has exempted the hot water technology from pesticide registration requirements.

Two companies, Waipuna Systems, Limited from New Zealand, and Aqua Heat, based in Minneapolis Minnesota, have developed equipment that delivers super-heated water from a boom or spray nozzle reached to a diesel-fired boiler. The smaller Waipuna machine has been available for lease in New Zealand for several years, and has been adopted for use by numerous New Zealand cities for weed control in parks, on school grounds, and more. According to trials by the New Zealand Crop & Food Research Centre in Levin, the Waipuna system killed all annual weeds and many perennial weeds, although some like dock and dandelion, regrew after 41 days. The system was as effective as the herbicide Roundup (glyphosate) after 41 - 49 days.' Larger (agricultural scale) Aqua Heat machines have been tested in citrus groves, vineyards, and corn fields in Michigan, California, and Florida, and along highway right of-way in California and Florida. While both of these machines have been shown at trade fairs and conferences and demonstrated in a few areas in the U.S., neither has yet become commercially available in this country. This may be changing very soon.

Aqua Heat is now doing demonstrations of a smaller machine, Aqua Heat jr., intended for use in urban settings like parks or schools Twenty machines are currently available for purchase, and the company plans to market the system more widely by year's end.

Enthusiastic endorsement of the technology comes from one Northwest user, Tony Idczak a horticulturist with the Whatcom County Parks Department in Bellingham Washington. Idczak used a Waipuna machine last summer and fall in a number of settings and was impressed with the results he achieved. He found that the hot water treatment kept weeds down as well or better than Roundup. He said the hot water technique yielded particularly impressive results on weeds coming up through gravel, crushed brick and concrete surfaces Just one treatment eliminated the annual weeds and grasses. As with chemical herbicides, several treatments were necessary to gain control of certain perennial weeds.

Idczak notes that the initial Waipuna hot water application is slower than applying herbicides on large areas. However, he also noted that the time required for subsequent applications on a given site was comparable to herbicide treatments. He feels that the machine has many advantages over herbicides It can be used in windy or rainy conditions with no concern about drift, runoff, or loss of efficacy. Treated areas can be used immediately.

What are the drawbacks to this technology? Like many chemicals hot water is a broad spectrum biocide that kills beneficial soil microorganisms as well as insect pests and weeds. Some critics don't like the idea of having to haul around a large reservoir of water. They point out that use of the machine may not be practical or ecologically sound in dry areas Some believe that direct searing of weeds with infrared heat or propane flamers is more energy efficient and preferable to hotwater, although it is unclear if there is data to show this. In any case, for large - scale applications the hot water technology is not yet cost effective

As with any pest control technology, hot water is not likely to be an answer to all weed problems However, it has great potential to become one important tool in integrated vegetation management strategies especially in settings like schools and parks Realization of this potential is a step doser with the commercial availability of small-scale hot water machines in the U.S.

Becky Riley

1. Ingle, A. 1992. Performance of the non-toxic vegetation control process. Crop and Food Confidential Report No. 3. Levin, NZ.: New Zealand Institute for Crop a Food Research, Ltd.

Citation : Riley, Becky. 1995. "Hot water : a "cool" new weed control method", Vol. 15, No. 1, 1995, pp.9.

Copyright 1995 Northwest Coalition for Alternatives to Pesticides.

Reprinted with permission.

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