Horticulture Principles and Practices

square yard (10 to 68 grams per square meter). The lightest covers are used primarily for
insect exclusion while the heaviest of the covers are used for frost protection. The most
common weights of row cover used by growers are 0.5 to 1.25 ounces per square yard
(17 to 42 grams per square meter). With row covers under 0.5 ounces per square yard
there is minimal heat retention at night and with row covers over 1.75 ounces per square
yard, there is a significant reduction in light transmission. The heavy covers are basically
used for nighttime frost protection since they do not transmit sufficient light for optimum
crop growth.
Floating covers require much less installation labor than hoop supported covers.
The wider and longer the covers, the less labor required per unit area since only the
edges are secured. These covers vary in width from 6 to 50 feet and up to about 800
feet long. Regardless of width, the cover is secured by weighting down the edges (sides
and ends) with soil, sand bags, stones, or long pins. In extremely windy areas,
additional weighting in the middle of the cover is advisable. Even though most crops
can be grown without damage under floating covers, tomatoes and pepper are an
exception. If the spunbonded material is not supported with wire hoops for these crops,
flapping of the cover in the wind will damage the growing points of young plants. Also,
with summer squash under windy conditions, many of the leaves might be broken by
the cover. For these three crops, a series of strategically placed wire hoops will prevent
crop damage.
At the Penn State Center for Plasticulture, evaluation of floating row cover materials
is being conducted on crops in the field as well as crops being grown in high tunnels
(Table 1).
HIGH TUNNELS
What is a High Tunnel? High tunnels, although resembling a traditional plastic covered
greenhouse, are a completely different technology. With no electrical service or automated
ventilation or heating systems, high tunnels are classified as non-permanent (removable)
structures, an important distinction for taxation purposes. Unlike greenhouses traditionally
covered with a double layer of polyethylene plastic, high tunnels are covered with only a
single layer of 6-mil thick greenhouse grade plastic. Traditional high tunnels do not require
a permanent heating system, although it is advised to have a standby portable propane
heater to protect against unexpected frosts in the spring and fall. High tunnel ventilation is
accomplished by manually rolling up and down the length of each side of the high tunnel
by using the roll bar with T-handle. Many high tunnels do however make use of a drip irrigation
system for water management. The purchase and construction cost of a high tunnel
is approximately $1.50 per square foot compared to a permanent automated greenhouse at
a minimum cost of $30.00 per square foot (Figure 7).
(This item omitted from WebBook edition)
FIGURE 7 Production of a
mixed vegetable crop in a 17’ x
36’ high tunnel located at the
Penn State High Tunnel Research
and Education Facility, Rock
Springs, PA. (Photo courtesy of
Dr Mike Orzolek – PSU)
20.3 Regionalized Production of Vegetables 621