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Winter pruning ideal for tree health

He’s not talking about the holidays, but rather, the health of your living trees in the wake of the discovery of oak wilt in Douglas and Bayfield counties, and the further spread of it in Sawyer County.

This year, oak wilt was found in Barnes, Cable and Drummond in Bayfield County; Gordon and Dairyland in Douglas County; and in Couderay, Hayward, Hunter, Round Lake and Spider Lake in Sawyer County. For the first time, oak wilt was also found on federal and tribal land in Northwest Wisconsin in the Chequamegon-Nicolet National Forest, Washburn Ranger District, and Lac Courte Oreilles.

The discoveries prompted the Douglas County Forestry Department to add restrictions concerning when loggers can harvest timber to protect the county’s oak trees.

“It has a lot of bearing on how we do business as far as seasonality of timber sales,” said Craig Golembiewski, Douglas County forestry operations manager. “We had a meeting with our partners, our neighbors, the Forest Service, Bayfield County, some state partners, just how we’re going to attack this thing.”

Since the discovery of oak wilt, a fungal infection spread by beatles that can kill an oak tree within months, Douglas County now prohibits timber removal in affected areas from April 15 to July 15 to protect the county’s oak trees, Golembiewski said.

Even without the restrictions, it’s not in the interest of loggers to cut during that period, according to Max Ericson of Ericson Logging in Minong.

“Most of your good loggers, if there’s oak wilt in the area, they’re going to wait even if they are under contract … they wouldn’t want to take the chance of spreading oak wilt, I think,” Ericson said.

The county is monitoring the situation internally, and plans to send forestry staff for training to control the spread of the disease, Golembiewski said. He warned the forestry committee earlier this month the Douglas County Forestry Department could be seeking exemptions to the county’s herbicide ban to prevent the spread of the disease.

Slowing the spread means killing healthy trees around the infected one to keep the fungal spores from spreading through root systems.

Pruning trees in the winter can also help stop the spread.

“Winter is the ideal time to prune not only oaks but other tree species as well, and there are a few reasons for that,” Cigan said. “When it comes to trees in general, when they are in dormancy and don’t have any leaf material on them it’s much more easy to see where the pruning cuts … need to be focused.”

During the winter months, it’s easier to shape the crown of the tree and see dead limbs that need to be removed in the absence of leafs. It’s also the time of year when less sap flows from the fresh cut and few insects are out, which could jeopardize the health of the tree.

“Generally with trees, as they age, from sapling to mature tree, pruning is a way of improving the overall shape of the tree so you get a better aesthetic … those decisions are better made when you can see the whole branching structure of the crown,” Cigan said. He said growing a strong central stem for some species can be beneficial because it promotes a stronger limbs that that are less susceptible to wind breakage.

When it comes to oak wilt specifically, Cigan said the high risk period — when oak wilt is most likely to spread — is April through July. After July 15, the risk of spread is low, but not absent completely. He said there is still a small risk that the infection can be spread because beatles responsible for spreading the spores and the infectious spores themselves may still be around. However by end of November, he said there is basically no risk at all.

“At that point we would have encountered probably at least several frosts, killing frosts where temperatures get quite low, beatles become dormant or killed and the spore mat that produces the spores are going to be shut down and not viable at that point,” Cigan said.

Pruning when the trees are dormant means a lot less sap is flowing from the wound, resulting in less insect-attracting scents and fewer insects are active, which means spreading oak wilt or developing an insect infestation is less likely, Cigan said.

In pruning trees in the winter, it is also unnecessary to use a sealant on the fresh cuts.

Cigan said while people should definitely use a sealant if they are addressing storm damage during the growing season to protect the health of the tree, those sealants aren’t as necessary after July 15, and aren’t necessary at all in the winter months. They could even be counterproductive to healing.

“The pruning sealants are often phytotoxic, and they can prevent some of the healing wood tissue — what’s called the callous wood — from forming around that branch,” Cigan said. “They hinder the production of that issue and can really prevent that wood from sealing up over seven to 10 years after that wound was made … pruning sealants are really best avoided, unless we’re talking oak in the growing season. Then they become a useful tool. They are less necessary after mid-July.”

For more information, go to dnr.wi.gov/topic/UrbanForests/treeCenter.html.

“It’s really going to improve forest health if pruning is done at the right time, in winter,” Cigan said.