Ideally, top vents should span the entire growing area, and bottom vents should be placed as close to the ground as possible. With this pattern, natural convection forces hot air out of and fresh air into the greenhouse.

Shading, too, can prevent overheating, while small fans and evaporative cooling systems can augment ventilation and improve air circulation. A simple method involves hosing down pathways, terraces, and walls to reduce temperatures and to elevate relative humidity during the warm months. (For various methods of controlling air temperature, consult the books in the reading list.)

Soil temperature is another key to a productive greenhouse, especially for organic gardeners. Most of the microorganisms that gobble up organic matter and turn out plant nutrients require soil temperatures above 55°F. A drop below this level results in no nutrient release, cold plant roots, and a slower metabolism for both plants and microbes.

Soil temperatures can be improved by using insulated growing containers and boxes . . . hot-water systems sunk below the growing beds . . . or hot air forced through a subterranean duct system. Rootzone heating with electric heating cables or tapes can also ensure the proper soil temperatures. But talk to the manufacturers, and a few of their customers, before buying any of these products.

HUMIDITY: In the winter, if the air in the house is dry, the extra humidity from the attached greenhouse will be welcome. Most plants prefer a relative humidity of 55% to 60%.Excess moisture, however, can cause problems, because many pathogenic fungi germinate when the relative humidity rises above 80%.

High humidity can be controlled with proper ventilation and air circulation. Sometimes a small fan inside the greenhouse can speed up the air exchange between home and garden. Watering in the morning and applying the water to the soil— not to the leaf surfaces—will minimize the symptoms of excess moisture. Providing proper spacing between plants and avoiding overwatering will further reduce the possibility of fungal diseases.

Condensation can also become a problem as more and more moisture is held in the air. Moreover, condensed water on the glazing scatters and reflects incoming light, reducing the amount that passes into the greenhouse by as much as 30%. A few glazings are coated with a film that beads up condensed water so that it quickly runs of the glazing. (The 3M product- Sun-Gain is one of the best for reducing condensation.)

Low humidity is a summertime condition. Unless managed properly, the warm air will rob the greenhouse soil of its moisture, promoting physiological stress on plants. Shane Smith describes "A Homemade Burlap Swamp Cooler" in his excellent book The Bountiful Solar Greenhouse