Freeze damage in citrus has longer lasting ramifications than the fruit it destroys, particularly in more severe events. Knowing when and how to properly prune trees after they’ve been damaged by severe cold is important to orchard health and future crop yields, according to a California citrus farm advisor.
Neil O’Connell, citrus and avocado farm advisor with the University of California Cooperative Extension in Tulare County, cautions against pruning too soon after a major freeze event, such as was experienced in the San Joaquin Valley in early Dec., 2013. According to O’Connell, it can take six months or longer citrus trees to reveal their true damage, depending on the severity of the freeze and weather conditions in the months following the event.
“There is a definite value to giving trees some time after the freeze for the damage to extend itself,” O’Connell said at a recent citrus growers meeting at the Lindcove Research and Extension Center in Exeter, Calif.
O’Connell said he’s seen damage from the December freeze in Central California that ranges from mild to very severe. Some young groves that were planted earlier in the year were decimated. Even mature groves saw considerable damage to the trees. That does not include the lost fruit.
Die back in citrus trees resulting from the freeze can continue for months. With the exception of trees that received only light freeze damage, pruning too early could result in misidentification of the kind of damage growers will want to prune out.
“Die back will depend on the weather conditions,” O’Connell told his audience. “You may want to wait until June to begin your pruning.”
O’Connell recommends saving as much of the tree’s framework as possible when making pruning cuts in mild to moderately-impacted trees, and then to make those cuts in good, green tissue. The risk of pruning too early is a cut can be made and die back will continue, leading to the need for another trip to the grove to prune trees.
The spring flush is not a good indicator of when and where to make those cuts, O’Connell says. Some tree damage will continue to reveal itself after the flush and can even claim some of that new growth. His point: be patient.
That does not mean do nothing. O’Connell recommends walking citrus groves now to survey the damage, and to walk them frequently to inspect for changes. One grower at the meeting said he had shoots dying on his trees for two years after the big 1990 freeze in the San Joaquin Valley, which saw temperatures remain below 25 degrees in parts of the San Joaquin Valley for three to five days, according to the Western Regional Climate Center.
Evidence of freeze damage can include dead and dying leaves, cracked wood and even delayed leaf drop in more severely-damaged groves. Twigs will appear water-soaked and discolored; older branches and the trunk bark may split, curl or develop hairline cracks and dead patches of bark may appear on limbs and the trunk.
There are also categories of tree damage, from light to very severe. In the most severe cases injury extends well down the trunk but is followed by strong sprouts above the bud union.
In those groves that can be identified with medium damage, O’Connell recommends waiting to prune trees until full extent of the damage is visible – usually six months. Then, cut below all serious bark injuries, save as much of the framework as possible and when injured limbs are removed, cut back to good, strong new shoots that are best available.
Severely damaged tree require waiting until after mid-summer to begin pruning, he says. Then, remove the entire top of the tree by cutting below all large areas of injured bark. He also recommends cutting off the trunk above the uppermost sprout of those that have since formed on the trunk. Slope that cut downward and away from the sprout.
In very-severe damage, where the top of the tree is killed and the injury extends well down the trunk with strong bud shoots above the bud union, a new trunk and head must be formed from a strong shoot coming from above the bud union. Pinch back the other shoots.
O’Connell says there is mixed opinion on the need to seal cut surfaces after drying in these trees in order to prevent secondary organisms from getting into the wood. In drier climates such as those seen in the San Joaquin Valley, this may not be necessary. Coastal regions with higher humidity may require these kinds of measure.
For those young groves that were essentially decimated by the freeze, O’Connell says decisions will have to be made whether to remove those trees altogether or to attempt in-field rehabilitation, which requires delicate work and much skill.
“Some severely damaged young trees can be rehabilitated,” he said.
Another issue borne from these kinds of measures will be sunburn.
“Any wood on the tree that has been shaded from the sun for a lengthy period of time is very susceptible to sunburn,” he said. Trees can die if exposed to sunburn.”
O’Connell recommends a good latex paint, diluted with water at a 50-50 rate, to whitewash citrus trees exposed to the sun. Sunburn risk is not as much a factor of temperature as it is length of day and time of year. Any time after late April is critical for sunburn, he says.
Nutrient and water applications can be cut back during these times as well. For instance, trees where the canopy was pruned by 30-50 percent can likewise have their nutrient and water applications cut by similar amounts, O’Connell said.
“The trees are going to need nutrient, no question, but for one thing you don’t want to push growth too fast,” he said. “What happens with these trees is the root system is damaged too, and we never think about that. So if you push the tree too vigorously you don’t have the root system to support this growth.”
Weed control is another factor to consider during post-freeze pruning and rehabilitation operations. It’s important during this time to do a good job of controlling weed growth.