Aquatic plant wastewater treatment systems, mainly those involving water hyacinths, have been demonstrated to be cost-effective in warm areas of the U.S., such as southern Texas and Florida. In this report, three geographic zones in the U.S. where various types of aquatic plant systems would be likely to provide reliable year-round treatment are identified. The coolest of these zones extends northward to a latitude of about 35 degrees N. The costs of using greenhouse structures to extend the geographic range of effective treatment are evaluated. It is concluded that greenhouse construction and heating fuel costs would tend to increase total costs substantially above those for conventional treatment alternatives.