The Challenge of the Freeze
Our last issue covered fall and winter crops grown in Skagit County and the challenges that wetter and wetter falls bring to their successful cultivation. This time we'll explore cold—sometimes severe cold—that affects crops in our "temperate" climate.
How Plants Adapt to the Cold
Whether a crop is damaged to the point of irreversible loss or remains unscathed when a freeze hits depends on many factors. Among these are the variety of plant grown, the topography where it's sown, the quality of the soil, moisture levels in the soil, and the degree and duration of the freeze in a specific microclimate.
Given time to acclimate to steadily decreasing temperatures, plants have a number of ways to cope. One is "nature's antifreeze." This happens when water moves out of the plant's cells and into the gaps between cells.
You see, it's the water in cells that freezes and when it does, it forms shards. Shards that pierce and slice the cell walls and destroy the pathways for water and nutrients the plant needs to live. By moving water out of the cells, the plant exists in a state of partial dehydration.
What remains in the cells' sap is a concentrate of sugars and other compounds which lowers its freezing point. In effect, antifreeze. In a limited period of freeze, this is often enough to keep a crop going.
When a Freeze Hits Matters Most
The greatest damage to a crop comes when there isn't time to acclimate. When weather patterns are erratic, warm a few days, then shockingly cold, the plant cannot adjust quickly enough. That's why an unseasonably warm early spring is so dangerous.
Warmer temperatures encourage growth, tender unprotected growth. If a freeze hits then, a plant is defenseless and crop loss follows.
How Farmers Adapt to the Cold
The first line of defense is accurate, precise weather data. Assuming farmers' intimate knowledge of their cropland and any conditions that demand special concern (for example, low lying areas that hold cold pockets), timely weather information tells them when intervention is required and how much to use.
One source of such data is AgWeatherNet (AWN), a service through Washington State University that provides local reporting at 15-minute intervals of air temperature, relative humidity, soil temperature at depths of two and eight inches, rainfall, wind speed, wind direction, and dew point.
According to Texas A&M Extension, "The dew point is the temperature at which the air releases moisture. It is a function of temperature and relative humidity." They go on to say, "The dew point is considered the low temperature for the night because the condensation of moisture or formation of ice slows or halts the rate of temperature drop. A sub-freezing dew point signals the need to consider protective measures."
Specific Strategies to Combat the Freeze
Measures to protect crops include adding an appropriate mulch to act as insulation and making sure the crop is well-watered. Again from Texas A&M Extension, "Wet soil and water puddles will capture radiant energy during the day and release it upward and around the plants during the night. Further, water gives off heat as it freezes, providing more warmth for the plants nearby."
Row covers or frost blankets protect a crop not by insulating it from the cold night air, but by holding the ground's heat. For that reason, covers are placed over crop rows in the warmest part of the day when the most heat is available to trap. Depending on the type and weight, the covers can offer up to five degrees Fahrenheit protection, even more if covers are doubled up.
Photo: Bryn Hulbert
To further protect the crop in times of severe cold, a cool wind is blown over the tops of the plants, but under the covers. This breeze redistributes warmth from the ground and prevents cold air from settling.
In some cases, a well-directed blower can even eliminate the need for covers at all. Cold air is heavier than warm air; left alone it pools at the ground. Using a blower keeps that from happening.
High tunnel hoop houses protect a crop from the effects of freezes as long as there are no dramatic temperature fluctuations that cannot be mitigated within the structure.
An added benefit is the planting density a high tunnel hoop house allows. Plants grown closer together keep each other warmer.
There May Be Loss, but It Need Not Be Total
Say a farmer has done everything that can be done, but the freeze is simply too deep or too long lasting to completely avoid damage. Not all may be lost. Depending on the crop, top growth may be affected, but the roots themselves may survive. If so, hope remains for new growth.
The key issue is the temperature of the soil. Fortunately soil usually doesn't get as cold as the air when there's a freeze. Nor does soil temperature fluctuate as much as air temperature. By nature it tends to stay constant and there's nearly always warmth in the earth, especially in a zone like the Skagit Valley. The water at the surface of the soil may freeze, but the soil itself is fine.
The Worst Conditions/The Best Conditions
Here's what a farmer doesn't want to see develop: A period of warmth that encourages a plant to relax its natural protection against cold, followed by a cloudy day and a clear, windless night of deep cold.
First of all, the plant's defenses are weakened, then the warmth of the day is curtailed by cloud cover, and finally a lack of clouds at night provides no insulation to keep whatever warmth there was close to the ground. The lack of wind allows that heavier cold air to settle right where it will do the most damage.
On the other hand, an ideal transition to severe cold takes enough time that a plant's natural defenses are primed, the daytime weather is clear to absorb the maximum amount of radiating sunlight, and the night is cloudy and slightly breezy to insulate and move the air.
Now, does that seem like too much to ask?
By Teresa Bennett: info@skagitonians.org