Feb 5, 2008

Irrigation Water Can Provide Freeze Protection – Sometimes

Locking the barn after the horse has gone is futile, everyone agrees – unless you recover the horse and can expect the same behavior in the future.

That’s the way it is with making the decision whether to provide frost and freeze protection for fruit crops. If you’ve had freeze damage before, chances are you’ll have it again. Global warming doesn’t mean the problem will go away. In fact, evidence suggests it will grow worse as we “enjoy” more and longer winter warm spells that encourage wood and buds to break dormancy, creating more potential for damage as spring arrives.

After a bad year – and 2007 and its Easter freezes certainly qualifies as a bad year for growers across the Midwest and South – growers revisit the problem.

Mark Longstroth, the Michigan State University Extension fruit educator in Paw Paw in southwest Michigan, focused mainly on the subject of providing freeze protection by using sprinkler irrigation on blueberries. He spoke during the Great Lakes Fruit, Vegetable and Farm Market EXPO in Grand Rapids, Mich., in December.

Whether economics justify installing sprinkler irrigation for frost protection alone depends on outcomes that are hard to predict in advance. With blueberries now more widely grown, having berries in a short crop year caused by freezes in a small region is less likely to provide the windfall profits it once did.

But since blueberries are also highly sensitive to drought and more growers are installing irrigation to prevent damage from dry weather, the decision could be made to install somewhat more expensive sprinkler irrigation instead of a cheaper drip system – and then use overhead sprinkler irrigation for both frost protection and summer watering.

Longstroth has collected a wealth of information of frost protection for fruit. It’s presented at the Van Buren County Extension Web site, www.canr.msu.edu/vanburen/frost.htm. It includes a table that shows the sensitivity of fruit and leaf buds of several fruit crops at various stages of development.

“Blueberry growers can protect against spring freezes by using sprinklers to reduce damage to blueberry flowers,” Longstroth says in a paper he co-authored with Michigan State University small fruit specialist Eric Hanson.

“Sprinklers do not protect blueberries in all circumstances. In 2002, many growers used sprinklers to protect their blueberries when they were at swollen bud and the temperatures were forecast to fall to the upper teens. Sprinklers will not protect crops under these extreme conditions.

“Sprinklers should be used near bloom time when the flowers are visible and the lows are forecast to be in the upper to mid 20s. Unless your system is specifically designed to provide a lot of water in a short period of time, it is doubtful that it will protect below 24? F.”

In the Easter freezes in early April last year, high winds – an advection type freeze – and temperatures in the high teens produced a freeze against which irrigation, wind machines or ever orchard heaters would not have been effective, Longstroth said.

But most of the freezes that occur in southwest Michigan are radiation freezes, and protection can be provided against these.

“In a radiation freeze, the temperature falls during calm, clear nights and then rises again after dawn when the sun comes up and warms the earth,” he said.

Radiation freezes are often associated with inversions, in which the air above is warmer, providing an opportunity for wind machines to pull it back down into the orchard.

Protecting with ice

“It seems counterintuitive that you can protect plants with ice, but the system works because water, when it freezes, give up heat to the environment,” Longstroth and Hanson wrote. “If the ice is wet so that the water on the outside is freezing and the ice on the inside is melting, the temperature inside the ice is 32?F, the freezing point of water, and as long as we keep the ice wet we can keep the temperature in the ice at freezing. This is warmer than the temperature that will cause damage, which is 28?F at bloom time.”

Keeping the ice wet means providing enough water to offset temperature and wind. If the cold overwhelms the capacity of the system to provide water, damage can be greater than if no water were applied at all, Longstroth said. So it’s important to have an idea of how cold it may get, how windy it will be and how much water the system can provide.

“Most systems in Michigan are designed to deliver 0.12 to 0.15 inches of water per hour,” he said. “These systems can protect to 22? F under very still conditions, but only to 26? F if there is a slight breeze (2 to 4 mph).

“Most systems cannot easily be changed to deliver more water and protect to lower temperatures,” he said.

Key elements in freeze protection using irrigation include applying enough water to protect the plant, watering fast enough to keep ice wet, overlapping sprinkler patterns and using closer sprinkler spacings in windy conditions.

In addition, Longstroth said, growers using irrigation for freeze protection should have a good source of weather information and watch it closely, perhaps subscribing to a weather service that issues freeze warnings.

“Consider purchasing a monitor that calls you when the temperatures get low,” he said. “Consider purchasing a hand-held wind meter and a sling psychrometer to measure wet and dry bulb temperatures, relative humidity and dew point.”

When to try

Fruit trees enter and leave a condition called dormancy each winter. During that time, stems and even buds can develop the ability to tolerate temperatures of -20? F. or even lower, Longstroth said. As days shorten in the fall and temperatures drop, cold hardiness develops.

Until plants have completed their chilling requirements, dormancy can’t be broken. For blueberries, that’s usually a thousand hours or so for northern plantations but, depending on the cultivar, may be as low as 100 for southern plantings.

Once that chilling requirement is met, plants are ready to resume growth as soon as warm weather encourages them. That’s the danger of late winter warm spells. Cold induces cold hardiness, Longstroth said, but cold hardiness is more easily lost than gained. So the worst conditions for blueberries are warn winter temperatures followed by quick cold snaps. Once they break dormancy, cold hardiness is not recovered.

“Blueberry flower buds and flowers become more sensitive to cold as they develop,” he said. “Swollen but closed flower buds tolerate 15? to 20? F. At tight cluster or early pink bud, injury will occur between 18 and 23? F. Once flowers have separated from one another but the petals are still closed, 22 to 25? F may be lethal. By the time the petals are half their full length, they are damaged at 25 to 26 F. Fully open flowers are killed at 27? F. The most sensitive stage is just after the petal fall, when 29?F may cause damage. At that temperature, green fruit is equally susceptible.”

Starting and stopping

The decision to freeze protect needs to be made early by weighing several factors, Longstroth said: Will it get cold enough to injure my crop? Can I protect to that temperature? Can I run the system long enough to protect the planting all night?

“If the answer to any of those questions is no, then don’t frost protect,” he said. “Save the money, time and effort for another time, when protection is worthwhile. It is better to turn the system on when it will save part of the crop in a marginal freeze than to turn it on when it will fail and only compound the damage.”

When irrigation first begins, air temperatures drop due to evaporative cooling, Longstroth said, so it’s important to start before temperatures fall too low. The amount of cooling depends on the relative humidity. If the air is very dry (dew point 15 – 20? F), start the irrigation when the air temperature drops to 36? F. If the relative humidity is higher (dew point above 24? F), you can wait to start irrigating until air temperature falls to 34? F.

Stop irrigating when the ice is melting and temperature is rising. Ice breaking free from branches indicates water is forming under the ice and it is likely safe to quit, he said. Normally this is when temperatures are above freezing and rising. Beware of sudden dips in the temperature soon after sunrise.

Other choices

Freeze and frost injury is affected by other factors besides temperature.

Variety, for example, makes a difference. Last year, the early variety Bluecrop was damaged by the Easter weather because the leaf buds were advanced and killed by the cold. Fruit buds were less affected – and they produced fruit on bushes that had too few leaves to support the fruit.

The later season varieties Jersey and Eliot were fine, Longstroth said.

Fruit trees can be damaged in the fall when freezing weather comes when plants are shutting down but not yet dormant. To avoid this kind of injury, discourage late growth by discontinuing irrigation by October and avoiding late fertilization.

“Some frost avoidance can be gained by keeping the soil surface clean of vegetation, moist and packed,” he said. “But don’t go out and disk or rototill the day before a freeze. A layer of loose soil acts to insulate the soil and reduces the heat storage of the soil and release during cold conditions. If you are going to work the soil to reduce freeze damage it needs to be done early so the soil can settle before the freeze.”

Moist soils have a greater capacity to capture and store heat energy during sunny days and release heat to maintain air temperature during cold nights. Weeds, sod and plant residues insulate the soil from the sun and reduce heat capture.

In addition, tall grass and weeds raise the effective ground level. Since cold air is heavier than warm air and settles in the lowest areas, vegetation effectively raises the ground level and flower buds a foot higher in the canopy may be injured during a frosty night.

“Mowing fields with tall weeds is worthwhile,” he said.

Moist soils have a higher heat capacity than dry soils, and packed soils absorb more heat than recently cultivated soils.

“The bottom line is that clean, moist, and packed soil surfaces absorb the most radiant energy during the day, and protect from frost by releasing this heat during the night,” he said.