Horticulture Principles and Practices

pests and reduce/ eliminate hail damage to the crops growing under the low tunnels.
Generally, number 9 wire that is six feet long is bent into a semi-circle and placed in
the soil every eight feet over the crop row. Polyethylene film with either 300 or 600
holes per square yard or polypropylene row cover is than placed over the wire hoops
to form the low tunnel. Clear polyethylene provides increased temperatures in the
low tunnels because of the greenhouse effect. In most northern locations, the
polyethylene as a low tunnel cover works best when applied after transplanting crops
in mid-April to early June. Because of the unpredictability and fluctuation of
temperatures, low tunnels with polyethylene over the top must be monitored daily to
prevent plant growth inhibition or crop loss because of elevated temperatures inside
the tunnel that could be in excess of 100°F. Low tunnels made with clear plastic
mulch usually increase air temperatures too high for most days during the spring in
southern locations, so the tunnels need to be ventilated by perforating the plastic or
rolling back one side of the low tunnel between noon and 4:00 PM. White polyethylene
can provide some growth enhancement while avoiding the extremely high temperatures
in the tunnel. Polypropylene and non-woven covers are fabric-like and can be
placed over rows in tunnel fashion or can be laid on the crop in a floating fashion.
Since the fabrics are porous, temperatures in the low tunnel are generally 5° to 10°F
lower compared to the plastic film covers (Figure 5).
ROW COVERS AND FLOATING ROW COVERS
Floating row covers were developed to provide season extension for vegetable producers
in areas that are susceptible to early and late frost events (temperatures between 28°F
to 36°F) during the growing season. Row covers will increase both soil and ambient
air temperatures in the crop canopy, maintain higher levels of available soil moisture,
act as an insect barrier, and reduce wind stress on especially young and immature
plants.
Insect control is effective with spunbonded row covers when all of the edges are
completely sealed. For example, maggots in radish (and in other crops in the cabbage
family) are controlled when a cover is applied at seeding with a complete seal around
the edges. When the adult maggot flies search out the young seedlings for laying eggs, the
flies will be unable to get under or through the cover. Placing the outside edge of the
floating row cover in a continuous shallow furrow and than covering the row cover
FIGURE 5 Low tunnels used
in conjunction with high tunnel
for the production of table beets
in November in central Pennsylvania.
(Photo courtesy of Catie Rasmussen,
Penn State Center for Plasticulture)
(This item omitted from WebBook edition)
20.3 Regionalized Production of Vegetables 619