Time To Check Your Crops For Winter Injury

After freeze, if a vine has suffered a winter injury, the tissues will be brown. If a vine has survived the cold without injury, it will be bright green. (Photo credits: Michigan State University Extension Bulletin)

After freeze, if a vine has suffered a winter injury, the tissues will be brown. If a vine has survived the cold without injury, it will be bright green. (Photo credits: Michigan State University Extension Bulletin)

In 2012, USDA updated the plant hardiness zones in the U.S. This was thought to be due, in part, to a response to warmer winters being experienced. However, this winter brought a new term — polar vortex — and with it extreme cold to the Midwest and Eastern states. Growers saw temperatures unlike they’d seen in 10 or 20 years.

Many cultivars thought to be grown successfully in the new hardiness zones could have sustained winter injury. And, with every temperature plunge into the sub-zero digits, the potential for winter injury increases. What complicated the matter was not just the temperature, but the duration of those extreme lows. The daily temperatures (warmer than normal) preceded the sudden drop, and the temperature fluctuation between freeze and thaw impacted the state of cold hardiness of plants at the time of exposure to extreme lows.

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The extent of the damage, however, will depend on the variety planted.

“It’s a winter where you might see pretty dramatic differences between two different varieties you have planted that would now indicate that one variety was more cold tender than another,” says Mark Longstroth, a Michigan State University Extension small fruit educator.

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As spring nears, now is a good time to assess what type of damage you have to your plants and take the necessary steps to start the growing season off right.

Check For Bud Damage
Before pruning, growers should check each cultivar to see how many buds survived the winter. This way they will have a good plan of attack going into the spring.

“(Take a cutting) and put it in a glass of water and let it bud out. When it gets ready to bloom, (growers) can examine the flowers. In addition, they can collect a shoot with flower buds and let it warm up and splice it with a razor blade, then look to see if the interior of the flower bud is brown or black, which would indicate that the flower had died,” says Longstroth.

In the vineyard, growers should “collect canes from each variety block and assess 100 buds for damage to come up with a percent of primary bud damage,” says Imed Dami, associate professor and Extension viticulturist with Ohio State University. Dami suggests that if you find extensive primary bud damage, assess secondary buds as well.

Luke Haggerty, a viticulture Extension associate with the Lake Erie Grape Region of Cornell Cooperative Extension, recommends growers only make assessments on viable wood expected to have live buds. Growers should also only count buds 1-10 on the grapevine when doing the assessment. Keeping in mind the buds that are healthy among the 10, growers should prune accordingly.

“If you have 50% or higher bud damage, you may have vascular damage and can’t expect that vine to push the wood it normally does. Leave 40% to 50% more buds than you normally do. However, plan to walk the rows and assess trunk damage. Shoot-thin the vines that have cracked or bleeding trunks in an attempt to avoid vine collapse later in the season,” says Haggerty.

The tip dieback on this blueberry branch may have been caused by cold injury. (Photo credit: Gary Gao, Ohio State University)

The tip dieback on this blueberry branch may have been caused by cold injury. (Photo credit: Gary Gao, Ohio State University)

Blueberries, although more cold hardy than other cane crops, could have been hit by the extreme cold temperatures this year. As spring nears and temperatures start to climb, Gary Gao, Extension specialist and associate professor of small fruits with The Ohio State University, suggests growers get out in the field and see what damage their blueberry canes suffered over the winter.

“Look at the flower and the leaf buds to check on the percentage of damage,” says Gao. “Typically, the first three to five buds are large. When you cut them open, the inside should be a light green. The dead buds will turn brown or black.”

Gao says that winter injury in the stem of blueberry bushes will visually show. Healthy blueberry stems are reddish in hue, whereas stems that sustained cold injury will be brownish or black.

“Cold damage shows up as browning in the terminal growth or tips of new growth from last year,” says Gao. “If they’re injured by cold weather, the terminal growth will turn brown. The top portion will be brown and the bottom portion could be reddish.”

“Growers should be checking to see how much bud damage there is just before they begin pruning each block or variety. (In the Finger Lakes region of New York), much of the pruning for vinifera varieties gets underway in March, when we’re (hopefully) past the worst of the cold temperatures,” says Hans Walter-Peterson, a viticulture Extension specialist with the Finger Lakes Grape Program at Cornell University. “Most native and hybrid varieties will probably be pruned by then.”

To read more about how Hans Walter-Peterson recommends to check grapevine buds for winter damage, and the steps to take before pruning, see “Evaluating Grapevine Buds For Winter Injury.”

When Trees Suffer Trunk Injury
Trees can also suffer trunk injury due to the cold. Haggerty notes that winter injury affects grapevines by damaging vascular tissues in the trunk and/or cordons. Longstroth says he expects to see the most damage at the snow line. He warns that young trees, planted within the last two years could sustain trunk damage if they did not harden off as well as they should have.

If a grower suspects trunk damage, Longstroth suggests cutting through the tree bark into the wood to see if the layer between the wood and bark is green.

“The cambium is the layer that is going to make wood to one side and bark to the other. If it hasn’t been hurt, it will be a pale green; if it has been hurt it will be brown; and if it’s been hurt pretty badly, it will be really brown,” says Longstroth. “If it’s turned brown, those trees will struggle and probably not be any good for four or five years until they build up enough wood to move stuff around. I’ve seen young trees recover from some damage as long as they’re not really hurt.”

Dami says to expect trunk and cordon damage on grapevines if there is a high percentage of bud damage.

“If there is trunk injury, the vines will either grow new shoots only from below the soil line or the protective mound that got put up in the fall, or the new shoots will start to emerge from the retained buds but then collapse at some point in the season because the remaining healthy trunk tissue cannot support them,” says Walter-Peterson.

Hold Off On Pruning
Once growers have a good idea of how many flower buds survived the cold temperatures, they should prune accordingly.

“Generally, increase the number of buds retained proportionally to the percentage of bud damage up to 75%. Hedging (leaving approximately five bud spurs) is recommended for higher percent of bud damage,” says Dami.

Peter Hirst, professor of horticulture in the Horticulture and Landscape Architecture Department at Purdue University, suggests growers take a wait-and-see approach when it comes to pruning.

“I think this may be a spring to be a little more conservative and see how things look once they start to bud out in the spring,” says Hirst. “On peaches especially, if they’re going to delay their pruning, they can ease off of their pruning if they have only a few flower buds.”

Apply A Foliar Spray, As Needed
Hirst says that along with taking a wait-and-see approach to pruning, the same should be taken with plant nutrition sprays in early and late spring.

“The danger is if they apply their normal nutrition program, and they don’t have much of a crop on their trees, then the trees are going to exhibit a lot of vegetative growth,” says Hirst. “Whereas without a crop, they can hold back that fertilizer application and try to avoid that problem.”

Longstroth noted that Warren Stiles, professor emeritus of Cornell University, always recommended an application of zinc and boron in early spring to strengthen the remaining buds if cold damage was suspected. Longstroth also recommends waiting until the buds open, to ensure the spray goes directly on the buds.

“Some foliar nutrition sprays might be worthwhile,” says Longstroth. “The beginning of growth is pretty much dictated by the reserves that were stored in the plant last fall. (Put the) foliar sprays of zinc and boron right on the growing point as needed.”

Longstroth suggests taking care of the damaged trees, and applying some fertilizer to encourage growth while keeping in mind that too much fertilizer could overstimulate the tree.

“While I would recommend probably some more fertilizer,” says Longstroth, “I don’t know anyone who has been able to fertilize their way out of a problem.”

Prepare For 2015 And Beyond
Longstroth suggests that once growers understand that they won’t have a crop, “lower the height of your peach trees because they’re going to have fruit on the wood that you grow this year. Rather than have the fruiting zone of the tree move up 3 or 4 feet, you could lower the tree 3 or 4 feet to make it easier to work with.”

If an orchard has been hit hard with cold injury, “you might want to think about what you might replace it with because they’ve probably got three or four years of declining yields, if they don’t recover,” says Longstroth. He also recommends placing tree orders now.

Haggerty says that a renewal program, as well as cutting out the damage, is the best plan of action if there is extensive cordon and trunk damage in your vineyard. He recommends applying Roundup before suckers start to grow and also applying a pre-emergence treatment, as done to start a new vineyard.

“The cordon can be pretty easy to replace. Next year you get a couple good canes from this year’s growth. However, if your trunk is damaged to the ground, you’re cutting that out and having to regrow a trunk and a cordon. That takes you back two to three years to full production,” says Haggerty. “Even if your vine survives, there is probably trunk and vascular damage and the vine should be phased out over the next few years.”

Haggerty stresses the need for growers to have healthy and disease-free vines going into winter in order to ensure that the vines can handle as much cold stress as they could possibly be exposed to.

“I’m finding some of these vineyards I walk into, at -10°F, most of the moderately hardy vines should be OK, but if they weren’t healthy, and didn’t have a good spray program, the stress and disease on the vine may result in more damage than they should have,” says Haggerty. “On the flip side, in moderately tender vines, I’m finding a lot of good buds out there on vines that shouldn’t have been able to withstand -10°F, but they had very healthy vines.”

Haggerty and Dami also recommend standard practices of winterizing vineyards such as covering the graft union, tilling, and possibly using mulch as an insulator. Walter-Peterson also advises growers to avoid stress on the vines and to consider installing a wind machine.

“Growers do have the option to install wind machines that can mix different layers of air that are different temperatures in order to warm the air at the vineyard level,” says Walter-Peterson. “The initial cost can be significant, but that cost can be recovered by saving one or two crops if the equipment is used under the right conditions. They only work when the air is dead calm — if there’s any wind, the different air layers are already mixed together, so they are only effective under certain circumstances.”

Walter-Peterson also says that now is the time to communicate with your crop insurance agent to “discuss the possibility of setting up a claim on their policy” if extensive damage is expected.

With the USDA cold hardiness zones being changed, Longstroth suggests growers take note of which varieties survived the extreme cold and communicate that information to their Extension agents.
“This would be a good year to see what varieties you probably don’t want to replant if you think it’s going to get this cold again in 10 years or so,” says Longstroth.
Walter-Peterson suggests along with proper cultivar selection, growers need to take note of site selection as well.

“If poor choices were made in either of those regards, then the grower will have an uphill battle,” says Walter-Peterson.

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