Thought Questions
Explain the science behind the following gardening questions:
1. Why did I have so much winter injury on my trees and shrubs? While it was dry and windy, temperatures were not extremely cold.
2. My arborvitae are bleached tan from the winter. Will they green-up with spring temperatures?
3. With the rather hot summer, will the apple and peach crop be as sweet as normal?
Temperature Considerations
Temperature factors that figure into plant growth potentials include the following:
- Maximum daily temperature
- Minimum daily temperature
- Difference between day and night temperatures
- Average daytime temperature
- Average nighttime temperature
Microclimates
The microclimate of a garden plays a primary role in actual garden temperatures. In mountain communities, changes in elevation, air drainage, exposure, and thermal heat mass (surrounding rocks) will make some gardens significantly warmer or cooler than the temperatures recorded for the area. In mountain communities, it is important to know where the local weather station is located so gardeners can factor in the difference in their specific location to forecast temperatures more accurately.
Examples of factors to consider include the following:
- Elevation – A 300 foot rise in elevation accounts for approximately 1°F drop in temperature.
- Drainage – At night, cool air drains to low spots. Valley floors may be more than 10°F cooler than surrounding gardens on hillsides above the valley floor. That is why fruit orchards are typically located on the benches rather than on the valley floor.
Figure 1. This magnificent garden on the hillside above Steamboat Springs, Colorado (a mountain community with a short frost free season) has great drainage giving it a growing season that is several weeks longer than down in town.
- Exposure – Southern exposures absorb more solar radiation than northern exposures. In mountain communities, northern exposures will have shorter growing seasons. In mountain communities, gardeners often place warm season plants, like tomatoes, on the south side of buildings to capture more heat.
Based on local topography, buildings, fences, plantings, and garden areas may be protected from or exposed to cold and drying winds. They may also be exposed to or protected from warm and drying winds.
Figure 2. Temperatures and growing season vary greatly based on exposure. A north facing exposure will typically be cooler and moist. A south facing exposures will typically be hot and dry.
- Thermal heat mass (surrounding rocks) – In many Colorado communities, the surrounding rock formations can form heat sinks creating wonderful gardening spots for local gardeners. Nestled in among the mountains, some gardeners have growing seasons several weeks longer than neighbors only a half mile away. [Figure 3]
In cooler locations, rock mulch may give some frost protection and increase temperatures for enhanced crop growth. In warmer locations, a rock mulch can significantly increase summer temperatures and water requirements of landscape plants.
In Phoenix, Arizona, the urban heat island (with all their rock mulch instead of grass and trees) has significantly raised day and night temperatures. The upward convection of heat has become so strong that summer storms go around the city, not raining on the urban heat island.
Figure 3. The sidewalks and stone walls of this intercity plaza creates a heat pocket with a frost free periods three months longer than the surrounding neighborhood.
Figure 1. This magnificent garden on the hillside above Steamboat Springs, Colorado (a mountain community with a short frost free season) has great drainage giving it a growing season that is several weeks longer than down in town.
Based on local topography, buildings, fences, plantings, and garden areas may be protected from or exposed to cold and drying winds. They may also be exposed to or protected from warm and drying winds.
Figure 2. Temperatures and growing season vary greatly based on exposure. A north facing exposure will typically be cooler and moist. A south facing exposures will typically be hot and dry.
In cooler locations, rock mulch may give some frost protection and increase temperatures for enhanced crop growth. In warmer locations, a rock mulch can significantly increase summer temperatures and water requirements of landscape plants.
In Phoenix, Arizona, the urban heat island (with all their rock mulch instead of grass and trees) has significantly raised day and night temperatures. The upward convection of heat has become so strong that summer storms go around the city, not raining on the urban heat island.
Figure 3. The sidewalks and stone walls of this intercity plaza creates a heat pocket with a frost free periods three months longer than the surrounding neighborhood.
Influence of Heat on Crop Growth
Temperature affects the growth and productivity of plants, depending on whether the plant is a warm season or cool season crop.
- Photosynthesis – Within limits, rates of photosynthesis and respiration both rise with increasing temperatures. As temperatures reach the upper growing limits for the crop, the rate of food used by respiration may exceed the rate at which food is manufactured by photosynthesis. For tomatoes, growth peaks at 96°F.
- Temperature influence on growth – Seeds of cool season crops germinate at 40°F to 80°F. Warm season crop seeds germinate at 50°F to 90°F. In the spring, cool soil temperatures are a limiting factor for plant growth. In mid-summer, hot soil temperatures may prohibit seed germination.
Examples of temperature influence on flowering:
- Tomatoes
- Pollen does not develop if night temperatures are below 55°F.
- Blossoms drop if daytime temperatures rise above 95°F before 10 a.m.
- Tomatoes grown in cool climates will have softer fruit with bland flavors.
- Spinach (a cool season, short day crop) flowers in warm weather with long days.
- Christmas cacti and poinsettias flower in response to cool temperatures and short days.
Examples of temperature influence on crop quality:
- High temperatures increase respiration rates, reducing sugar content of produce. Fruits and vegetables grown in heat will be less sweet.
- In heat, crop yields reduce while water demand goes up.
- In hot weather, flower colors fade and flowers have a shorter life.
Table 1 illustrates temperature differences in warm season tomatoes and cool season cole crops.
| Table 1. Temperature Comparison Between Cool Season and Warm Season Vegetables | ||
Cool Season:
broccoli, cabbage, and cauliflower |
Warm Season:
tomato, pepper, squash and melons | |
Germination
|
40°F to 90°F
80°F optimum |
50°F to 100°F
80°F optimum |
Growth
|
Daytime
65F° - 80°F preferred 40°F minimum Nighttime greater than 32°F for tender transplants down to mid-20s°F for established plants |
Daytime
86°F optimum 60°F minimum Week below 55°F will stunt plant, reducing yields. Nighttime above 32°F |
Flowering
|
Temperature extremes lead to bolting and buttoning.
|
Nighttime: Below 55°F, non- viable pollen (use blossom set hormones).
Daytime: Greater than 95°F by 10 a.m., blossoms abort. |
Soil
|
Cool
Use organic mulch to cool soil. Since seeds germinate best in warm soils, use transplants for spring planting, and direct seeding for mid-summer plantings (fall harvest). |
Warm
Use black plastic mulch to warm soil, increasing yields and earliness of crop. |
Heat Zone Map
A new concept in plant selection is heat zone mapping, a measurement of the typical summer heat accumulation. It will help identify geographic areas that have adequate heat accumulation to mature various crops.
The American Horticultural Society’s Heat Zone Map can be viewed on-line at:www.ahs.org/publications/heat_zone_map.htm.
Heat zones can be sorted by zip codes. To look up a heat zone by zip code, go on-line atwww.ahs.org/publications/heat_zone_finder.htm.
It should be recognized that in mountain communities, minor changes in elevation and exposure (for example, south slopes versus north slopes) make significant differences in heat accumulation. A heat zone for a community’s zip code may not reflect the actual growing conditions in any specific garden.
Influence of Cold Temperatures
Hardiness Zone Map
Hardiness zone maps indicate the average annual minimum temperature expected for geographic areas. While this is a factor in plant selection, it is only one of many factors influencing plant hardiness.
In 2012, the U.S. Department of Agriculture released a new USDA Hardiness Zone Map. It can be found at http://planthardiness.ars.usda.gov/PHZMWeb/ . The revised map has a large database correcting inherent problems with the 2003 version. It documents a climate zone creep, that is zones moving northwards in recent years. Zones are based on a 10°F difference in average annual minimum temperature.
Table 2.
Hardiness Zone Map: Average Annual Minimum Temperature
Zone 4
-20°F to -30°F
Zone 5
-10°F to -20°F
Zone 6
0°F to -10°F
Most of the Colorado Front Range area falls into Zone 5, with cool mountain areas in Zone 4. Warmer locations in the Denver Metro, Fort Collins, El Paso, and Pueblo Counties fall into Zone 6. Warmer areas of western, southwestern, and southeastern Colorado are in Zone 6.
Table 2.
Hardiness Zone Map: Average Annual Minimum Temperature
Plant Hardiness
Hardiness refers to a plant’s tolerance to cold temperatures. Low temperature is only one of many factors influencing plant hardiness (ability to tolerate cold temperatures). Key hardiness factors include the following: [Figure 4]
- Photoperiod
- Genetics (source of plant material)
- Low temperature
- Recent temperature pattern
- Rapid temperature changes
- Moisture
- Wind exposure
- Sun exposure
- Carbohydrate reserve
Figure 4. Impact of temperature change on winter hardiness of trees - The red line represents a tree's hardiness. Regions A-D represent various stages of hardiness through the winter season. The blue line represents temperature. When the blue (temperature) line drops below the red (hardiness) line damage occurs. Points 1-4 represent damage situations.
- Region A. Increased cold hardiness induced by shortening length of day.
- Region B. Increased cold hardiness induced by lowering temperatures.
- Region C. Dehardening due to abnormally warm temperatures mid-winter.
- Region D. Normal spring dehardening as temperatures warm.
- Point 1. Injury due to rapid drop in temperature with inadequate fall hardening.
- Point 2. Injury at temperatures lower than hardening capability.
- Point 3. Injury due to rise and fall of midwinter temperatures.
- Point 4. Injury due to spring frost.
Figure 4. Impact of temperature change on winter hardiness of trees - The red line represents a tree's hardiness. Regions A-D represent various stages of hardiness through the winter season. The blue line represents temperature. When the blue (temperature) line drops below the red (hardiness) line damage occurs. Points 1-4 represent damage situations.
- Region A. Increased cold hardiness induced by shortening length of day.
- Region B. Increased cold hardiness induced by lowering temperatures.
- Region C. Dehardening due to abnormally warm temperatures mid-winter.
- Region D. Normal spring dehardening as temperatures warm.
- Point 1. Injury due to rapid drop in temperature with inadequate fall hardening.
- Point 2. Injury at temperatures lower than hardening capability.
- Point 3. Injury due to rise and fall of midwinter temperatures.
- Point 4. Injury due to spring frost.
Examples of Winter Injury
- Bud kill and dieback – From spring and fall frosts.
- Root temperature injury – Roots have limited tolerance to sub-freezing temperatures. Roots receive limited protection from soil, mulch, and snow. Under extreme cold, roots may be killed by the lack of snow cover or mulch. Street trees are at high risk for root kill in extreme, long-term cold.
- Soil heaving pushes out plants, breaking roots. Protect with snow cover or mulch.
- Trunk injury – Drought predisposes trunks to winter injury.
- Sunscald – Caused by heating of bark on sunny winter days followed by a rapid temperature drop, rupturing membranes as cells freeze. Winter drought predisposes tree trunks to sunscald. [Figure 5]
Figure 5. Southwest bark injury is common on trees under drought stress, such as this tree with a restricted root spread.
- Frost crack – Vertical split on tree trunk caused by rapid drop in bark temperature. [Figure 6]
Figure 6. Vertical frost crack is common on trees when the temperature drops rapidly. Along the Colorado Front Range, it is common to go from a nice spring day back to cold storm with a 40 to 60 degree temperature drop in an hour!
- Frost shake – Separation of wood along one or more growth rings, typically between phloem (inner bark) and xylem (wood), caused by sudden rise in bark temperature.
- Winter injury on evergreens
- Winter drought – Water transpires from needles and can’t be replaced from frozen soils. It is more severe on growing tips and on the windy side of trees. [Figure 7]
- Sunscald – Winter sun warms needles, followed by rapid temperature drop rupturing cell membranes. It occurs typically on southwest side, side of reflected heat, or with sudden shade.
- Photo-oxidization of chlorophyll – Foliage bleaches during cold sunny days. Needles may green-up again in spring.
- Tissue kill –Tissues killed when temperatures drop below hardiness levels.
Figure 7. Winter drought, sunscald and photo-oxidization of chlorophyll are common on arborvitae. This is a poor plant choice for this windy site with little winter moisture.
Figure 5. Southwest bark injury is common on trees under drought stress, such as this tree with a restricted root spread.
Figure 6. Vertical frost crack is common on trees when the temperature drops rapidly. Along the Colorado Front Range, it is common to go from a nice spring day back to cold storm with a 40 to 60 degree temperature drop in an hour!
Figure 7. Winter drought, sunscald and photo-oxidization of chlorophyll are common on arborvitae. This is a poor plant choice for this windy site with little winter moisture.
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