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Hort Update for December 1, 2021

Hort Update for December 1, 2021, Nebraska Extension,

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Serious ConcernsMajor Symptom
1. Open wounds & decay in trees Considerations when addressing homeowner concerns
2. December growing degree days (GGD) Lincoln Airport 12/1/21 - 4030 GGD,  Understanding Growing Degree Days
Timely Topics
3. Tree wrap pros and cons  Their use is controversial; if used, apply in November and remove each spring
4. Voles and bird feeders Active all winter; bark feeding can girdle tree and shrubs stems
5. Winter color on evergreens May be darker green to purplish-brown
6. Fungus gnats & indoor plants  Tiny dark-colored gnats commonly infest overly wet soil
7. Snow removal tips Spread snow out; avoid tree & shrub branch damage
8. Insects on Christmas trees Hitchhiking insects become active under warm indoor temperatures; use of insecticides indoors not recommended

1. Wounds & decay in treesConsiderations when addressing homeowner concerns

Trees can be wounded in many ways - storm damage, poor pruning, hail, insects, sunscald, mower or string trimmer injury - just to name a few. Wounds breaking through the outer bark and damaging interior wood, such as the xylem (water conducting tissues) and phloem (food conducting tissues), impair the tree's ability to move water and nutrients through the damaged section. These wounds also open the tree to entry of wood rot fungi and bacteria.  

Trees respond to wounds, not by repairing the damaged tissue, but by sealing it off in a process called compartmentalization. New tissue, called callus tissue, grows over the wound. Woody plants also develop barriers within the wood present at the time of injury which limits the spread of wood rot fungi. In the best case scenario, the tree will completely seal over the damaged area within a few years. However, sometimes damage is too extensive for the tree to seal off or the tree's vigor is low and it cannot grow fast enough to seal off the wound. In this instance, the inner wood is susceptible to the entry of wood rot fungi. 

Homeowners are often concerned when they find an open wound or decayed hole in their tree. They want to "do something" to help the tree, so what should we recommend? 

  • Do not cover the wound or hole with tree wrap. Covering a wounded or decayed area creates the perfect hiding spot for insects to further attack the tree and does nothing to prevent the entry of wood rot fungi. 
  • Do not paint the damaged area with wound dressing or paint. The idea of wound dressing helping woody plants recover was proven incorrect by Alex Shigo, US Forest Service tree researcher, many years ago. However, wound dressings are still promoted and sold today as a way to prevent the entry of insects, decay and fungi into a tree wound, but research has shown they do more harm than good. Wound dressings 1) seal in moisture and decay, 2) inhibit compartmentalization of wounded areas, 3) prevent callus tissue from forming, and 4) may serve as a food source for pathogens. They do not prevent the entry of decay organisms or stop wood rot. Note: There is one exception to the "no wound dressing" rule. In areas where oak wilt is present, wound dressing or paint can be used to prevent insect spread of the oak wilt fungi if trees are pruned during the growing season. But ideally oaks are only pruned during the dormant season, when no insects are present, for this same reason. Oaks pruned in winter will seal off wounds before insects become active the following summer, eliminating the need for wound dressing. 
  • Do not fill the hole with cement, expandable foam or any other material. These products will not stop moisture from entering the damaged area and will hold moisture against the damaged wood, which may speed the growth of wood rot fungi. They obstruct the growth of callus tissue and impede the natural compartmentalization process. Oxygen is essential for the fastest growth of callus tissue; wound dressing, cement and other fillers slow or stop oxygen from getting to the wounded area. Once buried in the trunk by callus growth, they become a hazard for arborists during pruning or tree removal. 
  • Do not scrape the inside of a cavity to remove dead wood. This may injury live tissue around the cavity and allow wood rot to spread. Only remove loose wood or debris. 
  • Do not drill holes in the tree's wood below a cavity to drain water standing in the cavity. This opens up a new area for further wood rot. 

How to cut a branch

  • DO clean up branch stubs and ripped bark caused by physical injury or storm damage. Branches broken during a storm or from heavy ice or snow load often leave behind a jagged stump. Using good pruning techniques, make cleanup cuts outside the branch bark ridge and branch collar to remove rough stumps. If either the branch bark ridge or branch collar were damaged, then make the best cuts possible without causing further damage. If bark was ripped by a falling branch, carefully remove any loose bark flaps. With a sharp knife, cut away the loose bark back to a point of firm attachement.

Another common homeowner concern is whether the wound and/or decay has weakened the branch or trunk to the point where it becomes a hazard - meaning either the branch or entire tree needs to be removed. The benefits provided by the tree or branch must be assessed against the likelihood and potential for damage. The publications below provide a good background for understanding and beginning to assess hazards. 

Tree Risk & Hazard Assessment Concepts, Dr. Kim Coder, Warnell School of Forestry & Natural Resources
Managing Hazards and Risk, International Society of Arboriculture
Tree Hazard Awareness, Nebraska Extension

Trees should only be fertilized when a nutrient deficiency is evident, either through a soil test, small and/or yellowing leaves, a thin tree canopy or inadequate new growth each year. Heavily fertilized trees are more susceptible to certain insects, so don't allow clients to automatically fertilize a wounded tree thinking this will speed recovery. 

Wound closure occurs fastest in healthy vigorous trees and young trees, so optimizing tree health is the best way to speed recovery. Water deeply, but infrequently during dry periods. Ideally trees should receive 1" of water per week through either rain or irrigation, even older mature trees. Encourage homeowners to apply water slowly, so it soaks into the soil 18-24" deep. Eliminate competition beneath the tree from weeds or turf with a ring of mulch applied in a FLAT 3-4 inch thick layer. DO NOT pile the mulch up against the tree's trunk. Keep the mulch back 1-2 inches from touching the trunk's bark to prevent bark and crown rot, and minimizing the potential for vole damage to the bark. 


3. Tree wrap pros and consTheir use is controversial; if used, apply in November and remove each spring

Plastic or paper tree wraps are often recommended to protect the trunk of young, thinned bark trees from sunscald injury and wildlife damage. However, tree wraps are a contentious issue causing disagreement over how helpful they actually are. There are studies that show they can help and studies that show they may cause harm. If recommended to homeowners, it is important to inform them on how to use tree wraps correctly.

Sunscald occurs when sunny, warm winter days heat the bark to relatively high temperatures. Research done in Georgia has shown the southwest side of the trunk of a peach tree can be 40 degrees warmer than shaded bark. This warming can cause a loss of cold hardiness of bark tissue resulting in cells becoming active. These cells are then susceptible to freezing when temperatures drop at night. The damaged (dead) bark tissue becomes sunken and discolored and eventually cracks and sloughs off.

According to Purdue University, studies of common tree wraps have shown they do not prevent extreme fluctuations in temperature on the bark. In some cases, the temperature extremes are worse. Tree wraps are also ineffective in preventing insect entry. In fact, some insects like to burrow under them. A tree wrap can also kill a tree if left on too long and girdles to tree.

If used, Purdue Extension states that flexible, light-colored, plastic wraps appear to be the safest for providing trunk protection. These loosely-fitting wraps allow air circulation to buffer temperature extremes and prevent excess moisture from accumulating between the wrap and the trunk tissue. Light colored corrugated wraps may be better than flat paper wraps as well. If used for protection from wildlife, plastic wraps or 1/4" mesh hardware cloth around tree trunks are better choices. 

When used, place them on young, smooth barked trees during winter only. In November, apply a light-colored plastic tree wrap or corrugated wrap from the ground up to the lowest branches. Remove tree wrap in spring. Usually, protecting the tree the year of planting is sufficient but some trees may need to be wrapped a second year.

Tree Support Systems, Purdue University


4. Voles and birdfeedersActive all winter; bark feeding can girdle tree and shrub stems

Voles do not hibernate. All winter, they eat seeds as well as leaves and stems of grasses and occasionally roots and bulbs. The most economic damage they can do is feeding on and girdling young trees. Vole tunneling can also undermine retaining walls and cause them to collapse. Voles are often attracted to spilled bird seed, a favorite food. Removing or limiting this food source can help reduce vole populations. Also, keep weedy areas mowed and avoid deep mulch layers near trees. Clean up spilled bird seed on a regular basis or move bird feeders to areas less susceptible to vole damage or easier to clean up areas, such as over pavement.

Voles and Their Damage, Nebraska Extension 
Controlling Vole Damage, Nebraska Extension 


5. Winter color on evergreensMay be darker green to purplish-brown

Some evergreens, both conifers and broadleaves, develop winter color. As we tend to be seeing an increase in evergreen diseases, a plant changing color during winter may concern homeowners. This may prompt questions about winter color and concern that the tree is diseased.

Winter color is natural and not harmful. It develops in late fall and early winter and ranges from darker green to purplish-brown to bronze. Eastern red cedar, Juniperus virginiana, ranges from bronze to yellow-brown in winter naturally. 

In contrast, a Scotch pine infected with pine wilt turns a dull green or grayish-green first, then fairly rapidly turns completely brown.

Browning caused by winter dessication typically shows up in late winter or spring, not during fall or early winter.

It is always important to positively identify the cause of plant concerns for homeowners. It may be something natural and not an issue at all. Information about expected winter color can also be shared with clients when they purchase plants.  


6. Fungus gnats & indoor plantsTiny dark-colored gnats commonly infest overly wet soil

Tiny gnats flying around houseplants, our faces or windows during winter are most likely fungus gnats. Adults lay 100 to 150 eggs on soil of houseplants and the larvae feed on fungus and organic matter in potting mixes or soil. They may also feed on plant roots. They are most often associated with overwatered or poorly drained soil; especially during winter when plants continue to receive the same amount of water as during summer, but they are not using as much water.

Fungus gnats do not bite or harm anything in the home. To eliminate a fungus gnat infestation, the life cycle must be broken. This can be done by removing the fungus in which they breed while simultaneously reducing the number of adults able to mate. The following things can be done to reduce the moisture in topsoil.

  1. Water less frequently and allow the soil to dry out more completely between waterings. 
  2. Blow air over the soil with a fan.
  3. Change the planting medium to provide better drainage.

Yellow sticky cards can be purchased at garden stores to catch flying adult gnats. In addition, there are biological control products such as Bacillus thuringiensis that can be applied to the soil to control the larvae. Look for fungus gnats on the insecticide label and follow the directions.

How to Control Fungus Gnats, Nebraska Extension


7. Snow removal tipsSpread snow out; avoid tree & shrub branch damage

When doing snow removal, spread the snow out as much as possible instead of piling it in one place. Piled snow may lead to branch breakage in shrubs, matted turfgrass, and buildup of salts in soil where de-icing products are used. Turf areas with piles of snow are prone to snow mold and the longer snow covers the turf surface, the more time for vole damage to occur.


8. Insects on Christmas treesHitchhiking insects become active under warm indoor temperatures; use of insecticides indoors not recommended

When fresh-cut Christmas trees are brought into warm homes, insects and spiders overwintering on them can become active. The most common are aphids and spiders. Those that do become active indoors are considered accidental invaders. They will not harm people, pets or anything in the home. They might lead to sticky presents or frightful screams. Most insects are in the egg stage and if they hatch, they do so in small enough numbers that they go unnoticed. Many dessicate and die from dry indoor air. Christmas trees should not be sprayed with insecticides. Available household insecticides are not a serious health risk but there is no benefit to exposing people or pets to pesticides that aren't needed.

Insects on Christmas Trees, Iowa State University


Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Nebraska Extension is implied. Use of commercial and trade names does not imply approval or constitute endorsement by Nebraska Extension. Nor does it imply discrimination against other similar products.