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  • From: Applesrgoodforu@aol.com
  • To: nafex@lists.ibiblio.org
  • Subject: Re: [NAFEX] cold night
  • Date: Mon, 10 May 2010 23:10:54 EDT

Cool
 
In a message dated 5/10/2010 5:46:11 P.M. Pacific Standard Time, ossoeo@myfairpoint.net writes:
As it seems timely, here's an excerpt from today's U of Maine's Apple Pest Report.
Richard
Gouldsboro, ME

Frost Risk, Tuesday morning May 11
The following text is adapted from a April 24, 2007 article in the Penn State Fruit Times by Dr. Rob Crassweller. Some of the information is irrelevant presented is of no use this close to a frost event, but included for completeness.

In the spring flower damage is based upon the Critical Temperature for flowers. The critical temperature is that temperature measured on a properly exposed and calibrated thermometer at which flowers or fruits will endure 30 minutes or less without injury. “

Extent of Damage
The extent of the damage to flower buds can be variable even though critical temperature thresholds are reached or exceeded. The reasons are varied:
* Not all buds are equally sensitive
* Not all trees are equally sensitive
* Not all cultivars are equally sensitive (Ed. Note – but after Petal Fall, cultivars are more
similar in frost sensitivity)

Factors impacting upon Critical Temperature Damage
* Flower development stage - for apples, King Bloom and Pink appear to be more sensitive than those at Full Bloom. In general it is believed the decline in sensitivity is due to increased water content in the floral parts with a concurrent decrease in the ability to supercool.
* Wind
* Humidity
* Cultivar
* Tree vigor *
* Weather prior to freeze event - the exposure to low temperatures and dry conditions
prior to a frost event can induce greater hardiness and resistance to frost damage
* Rate of thawing – fast thawing may increase injury

TYPES OF FROST
Advective
Advective freezes occur when a large mass of cold air moves into the region. It may or may not be accompanied by clouds. The air mass then migrates into the region and is usually accompanied by winds that exceed 5 mph. Temperatures at the surface are below freezing and become colder with elevation. Little can be done to protect the trees other than possibly covering a crop. Heating is not economically feasible.

Radiational
Radiational frosts occur when heat that has been stored in the upper soil surfaces is radiated back into the atmosphere at night. Wind speeds are generally low and an inversion usually develops with temperatures near the ground surface falling to or below freezing. The rapidity and depth of the temperature drop can be influenced by the dewpoint.

Dewpoint is the temperature at which moisture begins to condense from the air mass. The general rate of temperature decrease due to radiative losses can be fairly rapid until the air approaches the dew point temperature when atmospheric water begins to condense on the colder plant tissues (which reach atmospheric dew point temperature first because they are colder).
The more water vapor in the air the higher the dewpoint. Knowing the dewpoint can help determine how quickly a grower will need to respond to frosty conditions. When the dewpoint is above freezing then the temperature drop will be much slower. If the dewpoint is below the critical temperature then the temperature drop is faster. Low dewpoints also indicate dry air and potentially more difficulty in increasing temperatures in the orchard.

Passive methods of frost prevention
Good site selection is the most economical and effective method of frost protection. No frost protection mechanism can overcome poor site selection for the orchard.

Soil moisture
On a daily basis heat is transferred into and out of approximately the top foot of soil. When the soil is wet, heat transfer and storage in the upper soil layer is better, so more heat is stored during daylight for release during the night. Considerable differences between thermal conductivity and heat capacity are observed between dry and moist soils. However, if the soil water content is near field capacity, wetting the soil is unnecessary. Wetting the soil to a depth below 1 foot is unnecessary because diurnal temperature is insignificant below that level.

Soil texture
Heavier soils with more clay retain heat better than sandy soils. Sandy soils are also often lighter in color and hence tend to reflect more sunlight rather than absorb it in the form of heat.

Ground Cover Management
When grass or weeds are present in an orchard or vineyard, sunlight is reflected from the surface and less energy is stored in the soil. A tall dense cover crop has the potential to reduce night time temperatures by up to 10F (5C) whereas, a closely mowed cover crop is usually only about 2F (1C) colder than bare soil. Therefore, during the frost season it may prove beneficial to keep the row middles in the orchard closely mowed to increase potential daytime heat absorption and reduce nighttime heat loss. Similarly, vegetative mulches usually reduce the transfer of heat into the soil and hence make crops more freeze prone. A bare undisturbed moist soil with no ground cover vegetation can release enough heat to raise the temperature 2 to 3 degrees in the plant canopy as compared to a sod mulched covered soil.
Maine comments
The critical temperatures for apple trees in bloom and after bloom are 28F for 10% bud kill, and 25F for 90% bud kill. Source: Data from Washington State University, reported by Michigan State University at http://www.hrt.msu.edu/faculty/Langg/Fruit_Bud_Hardiness.html)


On May 10, 2010, at 7:57 AM, Maria Schumann wrote:

We survived getting 14 inches of snow dumped on us a week ago; now  
they're predicting a cold night in the low twenties or even upper  
teens.  We have about 100 apple trees in early pink to late pink.   
Too many to cover or hang Xmas lights on!   We'll spray them with  
water, make some bonfires and cross our fingers.  Any biodynamic  
people out there that use valerian spray?
_______________________________________________

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