The potential for cover crops to lower nighttime temperatures has long been a concern among citrus producers. It is well known that bare soil has a greater capacity to absorb incoming radiation during daylight hours and transfer heat back to the surface at night (Gradwell 1963; Cochran et al. 1967; Fritton et al. 1976; Fritton and Martsolf 1981). Cover crops reflect more solar radiation, allowing less to reach the soil surface. They also evaporate more water from the surface soil layer, reducing its thermal conductivity and heat capacity. As a result, less energy is captured at the drier soil surface, and heat transfer and storage are impeded. Consequently, bare soil has better heat storage during the day and improved heat transfer during the day and night.
In addition, limited observations and trials suggest that citrus orchards with groundcover of sufficient height and density have lower air temperatures than orchards with bare ground (Pehrson 1989). Similar observations have been made for deciduous trees (Snyder and Connell 1993) and in grape vineyards (Donaldson et al. 1993).
The extent of freeze damage is related to how far the temperature drops below the damage threshold and the duration at the minimum temperature. Cover crops increase reflection of incoming radiation and shade the soil surface underneath. Groundcovers also remove water from upper layers of the soil and reduce thermal conductivity and heat capacity. Therefore, soil heat storage during daylight is reduced when a cover crop is present.
Similarly, USDA, UC and the University of Arizona have historically maintained that leaving shredded prunings on the orchard floor lowers the orchard temperature. Like cover crops, shredded material theoretically will reduce the amount of solar radiation reaching the soil surface. Therefore, the absence of cover crop and shredded material should enhance daytime heat storage. In light of these phenomena, citrus growers have been reluctant to allow vegetation on the orchard floor.
Recently, there is new interest in using cover crops in citrus orchards because of perceived beneficial effects on erosion and pest management. However, the benefits from using cover crops may be offset by the potential for freeze damage.
evaluate the effect of orchard floor management on minimum temperatures and on
the potential for freeze damage, we conducted trials in citrus orchards in
Tulare and Kern counties during the winters of 1994 and 1995.
Pruning and Orchard Temperature
established an orchard floor management trial in a commercial Valencia orange
orchard in northern Kern County on Feb. 3, 1995. The 10-year-old trees were in a 22-foot-by 22-foot
(6.7-meter-by-6.7-meter) spacing with rows facing north to south. The orchard floor was not tilled, and herbicides were applied for weed
suppression. All trees were pruned
in September, then the prunings were shredded, leaving a substantial residue
approximately half an inch (1.25-centimeters) depth on the soil surface. Two treatments, raked and
unraked, were imposed. In the raked treatment, the shredded material was removed from an area 44
feet (13.5 meters) in all directions from the centrally located tree where
temperature was monitored. A second
sample tree was located in the unraked plot. Temperature was monitored using model 107-thermistor temperature probes
and a datalogger. Thermistors were
placed inside of Gill shields and mounted on tree trunks at a height of 5 feet
(1.5 meters) above the ground. Temperatures
were recorded hourly to evaluate treatment differences.
Cover Crop Effect on Temperature
study the effect of cover crops on minimum temperature, we also initiated an
experiment on March 2, 1995, in southern Tulare County in two adjacent
commercial navel orange orchards. Both
orchards had 20-year-old trees planted on a 20-foot-by-20-foot (6-by-6-meter)
spacing. There was approximately 40
feet (13 meters) between the two plots. One
block was nontilled with herbicides applied for weed suppression. The cover-crop block had an established cover crop planted in fall 1994.
The cover crop —bell bean, lana vetch and oats — averaged
approximately 30 inches (about 0.75 meter) tall, with complete ground shading
between the tree rows. Within the
tree rows, volunteer weed growth was 12 to 16 inches (0.3 to 0.4 meters) tall
with partial to complete shading.
One tree was instrumented in each treatment block in the same
manner- as the pruning experiment.
Minimum Temperature Analysis
recorded temperatures after sunset and before sunrise and analyzed them to
determine treatment effects. Rainfall was recorded at a nearby California Irrigation Management Information System (CIMIS)
station (Snyder and Pruitt 1992) and days with rainfall were eliminated from the
analysis. Comparisons were made by
computing the regression of the raked versus unraked, and the no-cover-crop
versus cover-crop nighttime temperatures. If
there are no treatment effects, the temperature at the Y-axis intercept should
be 32°F (0°C).
If the slope of
the regression line is less than unity, the difference between treatments is
increasing as the temperature drops.
Raked vs. Unraked Prunings
In the pruning experiment, the raked area had a higher temperature than the unraked area, and the difference was increasing at lower temperatures. When water evaporates, the air temperature decreases because heat is removed from the environment to break hydrogen bonds between the molecules. When water vapor condenses (i.e., dew or fog formation), heat is released as hydrogen bonds form between molecules and the air temperature rises.
The temperature increased at about 6 P.M. on Feb. 14, so it is likely that fog formation was the cause. If dew was forming, we would expect a more rapid temperature drop after the initial temperature rise. If clouds were passing over, the rate of temperature drop would decline but the air temperature would not increase.
The treatment temperatures began to separate at about 11 A.M. on Feb. 15 as the fog lifted; the sunlight began to warm the raked treatment more than the unraked treatment. Most likely a short period of fog or cloud passage blocked the sunlight between 1 P.M. and 3 P.M. until the sunlight again began to heart the soil and air, again more in the raked than the unraked plot. Because more heat was stored in the soil of the raked plot, there was more energy to keep the surface warmer during the night of Feb. 15. Where the soil surface is warmer, the air temperature is also warmer.
Feb. 3 until Feb. 15 the raked plot also stored additional heat in the soil.
Even when there was a small difference in the soil heat storage on a
daily basis (such as, if soil under the raked treatment accumulated 0.1% more
heat per day), we found that the mean soil temperature would increase relative
to the unraked treatment over time.
any given day, the surface temperature range may be the same for the two
treatments, but because the mean soil temperature is higher for the raked
treatment, the minimum surface temperature will be higher.
In both the long term (over weeks and months) and the short term (on a
daily basis), removing prunings improves freeze protection.
Cochran P. H., Boerma L., and Youngberg C. T. 1967. Thermal properties of a pumice soil. Soil Sci. Soc. Amer. Proc. 31:454-9.
Donaldson D. R., Snyder R. L., Elmore C. and Gallagher S. 1993. Weed control influences vineyard minimum temperature. Amer. J. Enol. & Vitic. 44(4):431-4.
Fritton D.D. and Martstolf J.D. 1981. Solar energy, soil management and frost protection. Hort. Sci. 3:205.
Fritton D.D., Marstolf JD, and Busscher W.J. 1976. Spatial distribution of soil heat flux under a sour cherry tree. Soil Sci. Soc. Amer. J. 40:644-7.
Gradwell M.W. 1963. Overnight heat losses from the soil in relation to its density. New Zealand J. Sci. 6:463-73.
Pehrson, J.E. 1989. Groundcovers and orchard temperatures. Citrography 75:14.
Snyder R.L. and Connell J. 1993. Groundcover height affects predawn orchard floor temperature. Cal. Ag. 47(1):9-12.
Snyder R. L. and Pruitt W. O. 1992. Evapotranspiration data management in California. Session
Proceedings/Water forum 1992, ACSE. Baltimore.