Early Season Extension Using Hotcaps

Hotcaps are covers used to protect individual plants from low temperature stress early in the season. They are usually used by home gardeners or market gardeners with limited production. Growers with more acres or more extensive production tend to use row covers or high tunnels that protect more than one plant.

Ideally, a hotcap will transmit sufficient solar energy for photosynthesis and to warm the air inside, but not so much that plants are damaged by overheating. Hotcaps should retain sufficient heat during the night to protect plants from low-temperature injury. They also protect the plant from wind stress early in the season, which can be a significant factor in Nebraska, reducing plant vigor and yields. Research conducted in 1954 showed that rigid plastic hotcaps provided less frost protection than waxed paper, but that melons started under either system matured at the same rate and produced the same total yield. Production technology has changed a bit since 1954. With new plastics and new product designs, claims are being made for superior heat retention and frost protection.

A study by researchers at Virginia Polytechnic Institute evaluated how three common hotcap designs - opaque plastic milk jugs, waxed paper, and plastic water-filled tubes - affected the growth and development of tomato transplants. Opaque plastic jugs are readily available and inexpensive, even if purchased new. Waxed paper hotcaps are commonly available through garden supply stores and would be considered a "standard" method for protection from spring frosts. Water-filled plastic tubes, such as the Wall-o-Water?, are based on the theory that the water in each tube collects solar heat during the day and then releases it at night. The Virginia study was the first to scientifically investigate the effectiveness of these three methods for frost protection. Microclimate sensors were used to monitor air temperature, relative humidity, and light quantity and quality inside and outside the hotcaps. Tomato transplants were evaluated for vigor, color, height, and yield.