Horticulture Principles and Practices

OR
WA
ID
MT
WY
ND
SD
4
2
3
MN
WI
MI
MI
PA
ME
VT
NH
NY
MA
RI
NJ CT
CA
NV
UT
CO
NE
KS
5
IA
MO
IL
IN
KY
OH
WV
VA
DE
MD
6
NC
11
11
AZ
NM
OK
7
AR
TN
AL
GA
SC
TX LA MS
8
FL
1
AK
9
11
10
Range of average annual minimum
temperatures for each zone
8
6
5
4
3
2
7
10
11
HI
11
10
Zone 1
Zone 2
Zone 3
Zone 4
Zone 5
Zone 6
Zone 7
Zone 8
Zone 9
Zone 10
Zone 11
Below -50°F
-50° to -40°
-40° to -30°
-30° to -20°
-20° to -10°
-10° to 0°
0° to 10°
10° to 20°
20° to 30°
30° to 40°
Above 40°
FIGURE 4–3
The USDA plant hardiness zone map. (Source: USDA)
Hardy Plants
Plants adapted to cold
temperatures or other
adverse climatic conditions
of an area.
Hardening Off
Adapting plants to outdoor
conditions by withholding
water, lowering the
temperature, or altering
the nutrient supply.
the U.S. Department of Agriculture (USDA) has developed a plant hardiness zone map
that shows crop adaptation in the United States (Figure 4–3). Hardy plants are those that
are tolerant of cold temperatures and adverse climate. Most plants can live and grow
within temperature ranges of 0 to 50°C (32 to 122°F). However, sensitivity to cold
temperature limits the regions in which crops can be cultivated successfully. High
temperatures may kill plants outright or reduce production when they coincide with
flowering and fruiting periods. Plants representative of those adapted to the various
hardiness zones are presented in Table 4–1.
The damage caused depends on the physiological state of the plant. If actively growing,
succulent tissue in plant parts such as flower buds are more susceptible than dormant
tissue to cold damage. Frost damage is critical when flower buds start to open. Warm-season
crops are more prone to frost damage. Although low temperatures may harm some plants,
others flower only after receiving cold treatment to break dormancy. Chilling is part of the
culture of some ornamental bulbs such as daffodil (Narcissus) and hyacinth (Hyacinthus).
Biennial plants frequently require cold treatment or winter chilling to flower.
To improve plants’ response to cold temperatures, growers may put them
through a process called hardening. This process involves exposing plants gradually
to an increasingly harsh environment so that they slowly acquire resistance to the adverse
conditions.
Protection against Cold Temperature–Related Losses The best protection
against frost damage is to plant after the threat of frost has passed. However, this
98 Chapter 4 Plant Growth Environment