Early Season Extension Using Hotcaps

Hotcaps are covers used to protect individual plants from low temperature stress early in the season. They usually are 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.

Laurie Hodges, Extension Horticulturist

Hotcaps ideally 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 caps and plastic water-filled tubes — affected the growth and development of tomato transplants. Opaque plastic jugs readily are available and inexpensive, even if purchased new. Waxed paper hotcaps commonly are 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.

Study Methodology

Four-week old tomato transplants, cv. Early Girl, were transplanted in a field in Blacksburg, Va. in early April. Standard production practices were used for field preparation. Plants were hand-watered as needed. Three forms of protection were compared: Opaque, plastic one-gallon milk jugs with caps and bottoms removed; plastic, water-filled tubes (Wall-o-Water); and nine-inch tall waxed paper hotcaps (source: Wetzel Seed Co, Harrisburg, Pa.). Household bleach was added to the water used to fill the plastic tubes to reduce algal growth. The final concentration was a 2.5 percent bleach solution. Waxed paper caps were secured by burying the edges in soil. Plastic jugs were attached with wire to bamboo stakes adjacent to the plants. When the threat of frost was over, plastic jugs were removed, waxed paper hotcaps were slit open on top, and plastic tubes were spread open at the top and narrowed at the base to allow additional room for growth. Excess rainwater accumulating in the hotcaps was not removed. Extensive microclimate data was recorded from each system. Each system also was evaluated in the growth chamber under controlled conditions.

Study Findings

Periodically ventilating the plastic tubes by spreading the top on warm days may reduce the problem of succulent growth caused by the high humidity and low light intensity. The plastic tubes provided about the same level of protection as plastic row covers used on larger plantings of tomatoes; however, the plastic tubes require a water source to fill them and the process was time consuming. Once filled, plastic tubes were difficult to transport, and the water supported algal growth unless a bleach solution was used (2.5 percent final concentration). It is easier to fill the tubes after they are in place around a transplant by using a hose or portable water source. Although plastic tubes are reusable, they require cleaning between seasons and are expensive (approximately $3 each compared to 5 cents for waxed paper hotcaps).

Study Conclusions

In the growth chamber, the rate of temperature decline was fastest for plastic jugs, slightly less for waxed paper hotcaps, and slowest for plastic tubes. All hotcap designs transmitted less than 70 percent of the available solar energy, although plastic tubes transmitted less than either of the other designs. Photosynthetically active light and soil temperatures under sunny conditions were highest inside the plastic jugs. Under cloudy conditions, no design provided warmer soil temperatures than found with no protection. Air temperature inside the waxed paper hotcaps was as much as 32 degrees warmer than the outside air on a sunny day and almost as high for the plastic tubes and plastic jugs. On cloudy days, internal air temperatures were only three to six degrees warmer. The waxed paper hotcaps provided the highest average increase in air temperature over ambient temperature (21 degrees); however, they did not retain heat best at night, averaging only one degree warmer than the ambient night air. Highest night air temperatures were retained by the plastic tubes, 3.4 degrees above the outside air.

When hotcaps were removed after danger of frost had passed, the plants grown in the hotcaps were tall, succulent, spindly and had poor color. The control plants (not covered) were shorter at flowering, set more fruit on the first cluster, had higher dry weights and were a darker green than plants grown under hotcaps. The nearly constant high humidity and low light levels under the various hotcaps contributed to the poor plant development. When the first flower cluster develops inside the hotcap, the lack of air movement and excessively high temperatures (greater than 85°F) may result in poor pollination.

From this information, it is evident that two types of commonly used hotcaps provide little or no frost protection and have adverse effects on tomato growth and development. The best option for home gardeners and small-scale market gardeners to protect young transplants from erratic spring temperatures is to use a water-filled plastic tube hotcap, opening it wide on warm days to increase ventilation, reduce humidity, and increase the amount of light reaching the transplant.


G. E. Welbaum, Effects of Three Hotcap Designs on Temperature and Tomato Transplant Development, HortScience 28(9):878-881, 1993.

Reference to commercial products or trade names is made with the understanding that no discrimination is intended of those not mentioned and no endorsement by University of Nebraska–Lincoln Extension is implied for those mentioned.

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Index: Lawn & Garden
Issued August 2007