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Hort Update for December 5, 2022

Hort Update for December 5, 2022 - Understanding compartmentalization of decay in trees.
The wall 2 compartmentalization barrier follows the tree's growth rings.
Serious ConcernsMajor Symptom:
1. Understanding how trees manage decay Trees don't heal, they compartmentalize
2. Salt damage mitigation Be alert for areas with high potential for salt intrusion
3. Poisonous holiday plants Poinsettias are not poisonous, but advise care with other holiday plants
4. Whiteflies Common pest of poinsettia; can infest other houseplants
Minor Issues
5. Insects on Christmas trees Hitchhiking insects become active under warm indoor temperatures; use of insecticides indoors not recommended
6. Leaf burn on houseplants Low indoor humidity, fluoride/chlorine in irrigation water
Timely Tips
7. Spotted lanternfly Not in Nebraska yet; good time to scout for egg masses
8. Tree sap Sap oozing and pruning recommendations
9. Tree assessment & pruning Best practices for tree health
10. Mulch Simple method to improve plant health
11. Garden clean-up & pollinator habitat Balance pollinator habitat and plant disease control
For Your Information
12. Commercial/Non-commercial pesticide applicator certification New license and recertification options
13. Digital Diagnostic Network - Need help with diagnostics? Submit pictures and questions for diagnosis by Nebraska Extension experts
14. Spanish for horticulture professionals Learn phrases and vocabulary related to plant and landscape maintenance at Metropolitan Community College

1. Understanding how trees manage decayTrees don't heal, they compartmentalize

It's important for tree management professionals to understand how trees respond to wounding, and the resulting wood rot decay which often follows, so we can keep trees healthy and safe. Alex Shigo, United States Forest Service scientist, studied wood decay in trees extensively during his career (1959-1985). Additional researchers have continued the work on understanding decay in trees.

Shigo developed a concept widely utilized in the arborist industry today called "compartmentalization of decay in trees" or CODIT. (Also referred to as Compartmentalization of Damage in Trees.)

The starting point for wood decay in living trees is a wound, which can happen in many different ways including the following.

  • Pruning
  • Physical damage – mowers, string trimmers, etc.
  • Trunk injections
  • Storm damage – hail, ice, snow, wind
  • Temperature extremes
  • Wildlife – birds, squirrels, deer and others

Compartmentalization in Trees, Dr. Alex ShigoCompartmentalization of Damage
Once an injury has occurred, the tree reacts by creating boundaries around the damaged area. This limits the loss of healthy wood function and inhibits the spread of wood decay fungi. The creation of these protective boundaries is called compartmentalization.

The CODIT model identifies four different boundaries, or "walls", trees use to compartmentalize damaged areas. Each wall is defined in part by the structure and orientation of the cells of the wood and in part by the reactions that occur in the cells following wounding. Each wall is progressively stronger; so for example, Wall 2 creates a stronger barrier than Wall 1.

Wall 1 – Cell walls between xylem cells stacked on top of each other make up the first wall. At the time of wounding, the tree reacts by filling xylem cells above and below the damaged area with defensive chemicals (tannins, resins, phenols, terpenes). Openings between cells close off and water conducting columns plug up. These actions help slow the movement of wood rot fungi into cells above or below the damaged area. However, Wall 1 is the weakest boundary and it's very common to find trees with long columns of rot in a trunk extending far above and below the initial site of injury.

Wall 2 – Growth rings present at the time of injury create the boundary of Wall 2. They prevent the spread of fungi into the central core of a tree's trunk. The mechanism of prevention is similar to what occured in Wall 1 - i.e. closing of openings between cells, cells in the boundary zone fill with defensive chemicals and water conducting columns plug up. Decay fungi often have a harder time moving across tree rings, which may slow their movement, however this wall is still relatively weak.

Wall 3 – Groups of ray parenchyma cells make up the next wall and they slow the movement of decay fungi around the trunk. Often simply called "rays", these cells move water and nutrients horizontally across the tree's trunk. Looking down at a trunk cross section, rays start from the trunk's center point and grow toward the outer edge. Again, the mechanism of defense in the cells is similar to what occured in Walls 1 & 2. 

Wall 4 is the final and strongest defense. Unlike Walls 1, 2 & 3 the cellular components of Wall 4 form only after wounding. This wall develops from the cambium layer just outside of the most recent annual ring. Its extremely strong anti-fungal properties prevent decay fungi from moving into new growth rings which develop after the wound occurred. It's common for a damaged tree to have extensive decay in the center of the trunk, but to have an undamaged healthy layer of tissue around the outer edge. These are growth rings which developed after the initial injury occurred and have been protected by Wall 4.

In the image above, the four protection zone walls are labeled. Shigo also experimented with wounding a tree through an improper pruning (cutting into the branch protection zone, right side of image) vs. wounding at an internode site between branches (wound on left side of branch). Significantly more decay initiated at the poor pruning site than at the internode wound.  

Tree species vary in their ability to generate defensive chemicals and build boundaries to stop the advancement of decay fungi.

In his publication, Ability to Compartmentalize Decay Differs Among Trees, Dr. Ed Gilman provides the following two lists of tree species and their compartmentalization abilities.

Poor compartmentalizers - Aesculus, Acer rubrum (may be intermediate), Acer sacharinum, Betula, Celtis, Cercis, Fagus, Malus, Populus, Prunus, Quercus palustris (may be intermediate), Salix

Good compartmentalizers – Acer saccharum, Carpinus, Gleditsia, Juglans, Quercus macrocarpa, Q. robur, Q. rubra (regarded as poor by some), Robinia pseudoacaia, Tilia (regarded as poor by some), Ulmus americana, U. parviflora

So What?
Why is it necessary to understand how trees respond to injury and their mechanisms to prevent wood rot? There are several good applications for this knowledge.

  • Understand the importance of wound prevention.
  • Understand that pruning cuts are wounds and if done poorly create large wounds, may damage defensive structures and are more likely to begin the process that leads to trunk decay.
  • Understand that trunk injections can breach Wall 4 and allow internal decay fungi access to outer wood tissues. 
  • Understand tree species vary in their ability to compartmentalize.
  • Realize the prevention of wood decay in trees is achieved through tree selection and wound prevention, not fungicide applications.

Further Reading
Decay Development, Ed Gilman, University of Florida
Wood Decay in Living and Dead Trees: A Pictorial Overview, U.S. Department of Agriculture, Forest Service, Northern Research Station

Image credit - Shigo, A. L. (2002). Chapter One. In A New Tree Biology: Facts, photos, and philosophies on trees and their problems and proper care (pp. 44).


2. Salt damage mitigationBe alert for areas with high potential for salt intrusion

Salt damage to lawns or landscape plants growing near pavement where deicing agents are used can be an issue. High salt levels change soil structure, leading to compaction and restricting availability of nutrients, water and oxygen. In summer, high salt levels decrease a plant's ability to absorb water, even when water is available. Symptoms of salt damage include stunting, leaf burn, root damage, and plant death.

To limit issues, the smallest amount of de-icing product needed to manage ice should be used and products known to cause less soil or plant damage selected (see link below). Also, avoid piling snow containing salt onto on planted areas; and mix kitty litter or sawdust into deicing products to reduce the amount needed. In areas affected by high salt levels, flushing the soil with 2 inches of water over a 2 to 3- hour period in early spring may help leach salt from soil. Repeat this procedure 3 days later. If the soil in the area is not well drained, leaching will not work.

Salt Damage in Landscape Plants, Purdue Extension 


3. Poisonous holiday plantsPoinsettias are not poisonous, but advise care with other holiday plants

During the holiday season, clients may ask about poisonous plants. Poisonous holiday plants include holly and mistletoe. The berries of these plants are very poisonous if eaten. Other poisonous holiday plants are amaryllis, azalea, boxwood, Christmas rose, Crown of Thorns, English ivy and Jerusalem cherry. Contrary to popular belief, poinsettias are not poisonous but can cause skin irritation and stomach distress.

Be aware of plants that are poisonous to people and animals; and their potential level of toxicity. For example, small amounts of Japanese Yew can be deadly to cattle and have killed full grown cattle when pruned branches were tossed into a pasture.

For information on poisonous plants, contact your local Extension office or go to the Nebraska Poison Control Center website. If a plant has been ingested, the Nebraska Regional Poison Center number is 1-800-222-1222.    

Common Poisonous Plants and Plant Parts, Texas A&M AgriLife Extension


4. WhitefliesCommon pest of poinsettia; can infest other houseplants

Whiteflies are a fairly common pest of poinsettia. If brought into the home on infested plants, whitefly populations multiply rapidly and spread to other plants. Signs of whiteflies are sticky leaves or the insect itself. Whiteflies are not true flies. They are related to aphids, mealybugs and scale insects and feed on plant sap. Adult whiteflies are tiny and have wings covered with a white powdery wax. Adult females can lay between 200 and 400 eggs. Upon hatching, flattened nymphs or crawlers attach themselves to leaf undersides and feed for about four weeks before pupating. Whitefly feeding leads to weakened plants and leaf yellowing and dropping.

If a plant is heavily infested, owners should consider discarding it as whiteflies are extremely difficult to control. For smaller whitefly populations, insecticidal soaps or insecticides labeled for use on houseplants indoors may provide some level of control, but they are often not effective.


5. 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 may 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. Most insects are in the egg stage and if they hatch, they do so in small enough numbers that they go unnoticed. Many 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. However, aerosol insect sprays are flammable and should not be sprayed on a Christmas tree.


6. Leaf burn on houseplantsLow indoor humidity; fluoride in irrigation water

When the tips and edges of houseplant leaves turn brown, it’s usually due to low humidity or fluoride/chlorine in water. Most houseplants are injured when humidity is under 20 percent. Humidity levels between 40 and 60 percent are preferred for houseplants. The best way to increase humidity is to use a room humidifier or a whole-house humidifier attached to the furnace. Syringing (spraying plants with clean water) removes dirt from leaves and increases humidity, but only to a very small degree for a short time.

Excessive fluoride & chlorine levels in water can cause tip and leaf scorching. Sensitive plants like Dracena, Cordyline, and Chlorophytum are best watered with rain water when possible. Tap water can be used but let it stand for at least 24 hours in containers to allow chlorine and fluorine to dissipate.


7. Spotted lanternflyNot in Nebraska yet; good time to scout for egg masses

While spotted lanternfly has not yet been found in Nebraska, winter is a good time to monitor for egg masses. Freshly laid egg masses appear as if coated with a white substance. As they age, the egg masses look as if they are coated with gray mud, which eventually takes on a dry/cracked appearance. Very old egg masses may look like rows of 30-50 brown seed-like structures aligned vertically in columns. The preferred host is tree of heaven, Ailanthus altissima, but spotted lanternfly also feeds on over 70 other plant species including apple, plum, cherry, peach, apricot, grape and pine. When working with these plants, monitor for egg masses. If suspected egg masses are found, report them to the Nebraska Invasive Species Program. Phone: 402-472-3133. Email:

Spotted Lanternfly, Nebraska Invasive Species Program


8. Tree sapSap oozing and pruning recommendations

Based on client questions, some still think tree sap “goes down” in winter and that’s when trees are best pruned. Sap does not move into roots during winter. The amount of sap in a tree is essentially the same year-round. It is only the flow of sap that changes in winter, with sap flow slowing or stopping.

When some trees, like maple and birch, are pruned in spring, they ooze a lot of sap. While this causes concern to homeowners, loss of sap is harmless to trees.


9. Tree assessment & pruningBest practices for tree health

Early to mid-winter is not the best time to prune shade trees, but it’s a great time to assess trees for pruning needs later in the season. Without leaves on trees, it is easier to see branch structure and locate issues. Pruning creates a wound the tree must expend energy to respond to. There should be a purpose to every cut that benefits the tree. If not, don’t wound a tree by pruning for the sake of pruning.

However, pruning is an important maintenance practice, especially in the first 5 to 10 or 15 years after a tree is planted to create good branch architecture. Stand back and assess a tree. Branches that are not beneficial and may need pruning include dead or damaged branches, branches crisscrossing and touching another branch, branches growing closely parallel to another branch, or those with very narrow angled attachments and which have included bark.

Winter, or anytime during the dormant season, has commonly been the recommended time to prune shade trees. We know trees can be pruned most anytime without killing them, but there are ideal times to prune and times when pruning is best avoided. 

New research shows the optimum time to prune living branches is late spring and early summer because pruning at this time promotes the quickest sealing of pruning wounds, known as CODIT or compartmentalization of decay in trees. Late spring and early summer is when tree cells are most active during the growing season, hence sealing occurs the quickest. 

Professionals may not have a choice on timing, due to client work load or after a wind/ice storm when broken branches need to be removed for safety. But when a choice is possible, aim for the ideal time; especially if you are a do-it-yourselfer pruning smaller branches off a smaller tree. Pruning of large branches in large trees should always be left to professionals.

ANSI A300 Pruning Standard 


10. MulchSimple way to improve plant health

Landscape plants are often mulched. A benefit often not considered is how mulch protects soil from temperature extremes and how this benefits plants. Mulch keeps soil cooler in summer and warmer in winter. This benefits plants because temperature extremes can kill very fine roots responsible for much water and nutrient absorption. According to Penn State University, of the total root surface area, root hairs can contribute up to 67%. While soil temperature extremes rarely kill established plants, they can cause a chronic stress as plants expend energy to generate new root hairs. Modifying soil temperature is especially important near the soil surface where fine roots can be killed by hot soils in summer and by freezing and frost heaving during winter. The correct use of organic landscape mulch benefits plants year-round. Coarse mulches, like wood chips, tree leaves or recycled holiday greenery are good mulches.


11. Garden clean-up & pollinator habitatBalance pollinator habitat and plant disease control

With the trend of leaving herbaceous plants standing over winter for pollinator nesting and overwintering, clients may question if this won’t also increase the number of harmful insects. A general answer is that the majority of insects are harmless or beneficial and so the risk of this happening is minimal. However, if a plant did have a harmful pest attack it during the summer, it would be wise to cut back and remove that plant debris for the current winter to reduce the potential for that pest to overwinter.


12. Commercial/Non-commercial pesticide applicator certificationNew and recertification options

If you have a pesticide applicators license expiring April 2023 or you need to get a new license — classes begin soon. Make plans now to attend the training option that fits your needs.

Commercial/noncommercial applicators are professionals who apply restricted-use pesticides for hire or compensation. Anyone who applies pesticides to the property of another person, either restricted- or general-use products, for control of pests in lawns, landscapes, buildings or homes must also have a commercial pesticide applicators license. Public employees (those employed by a town, county, state) applying mosquito control pesticides whether restricted- or general-use, must also hold a commercial or noncommercial certification.

Commercial/noncommercial applicators have several options to recertify or get a new license.

Traditional In-person Classes
Classes begin in late January. Visit for dates, locations and registration. Preregistration is required; cost $95 per on-line registration.

In-person trainings are a supplemental learning opportunity; they DO NOT replace pre-class studying of category manuals or flipcharts for test preparation. Study materials for all commercial categories must be purchased online

On-line Self-Paced Option - Recertification ONLY
This is a 100% online training, which includes watching the General Standards recertification video, plus a video for each additional category to be renewed. Cost is $80.00 per on-line registration. Visit to register, beginning December 19, 2022. 

Conference Options - Recertification ONLY

  • Nebraska Turf Conference - For commercial/noncommercial applicators needing recertification only for General Standards (00) and Ornamental & Turf (04). Visit for more information.
  • Urban Pest Management Conference - For commercial/noncommercial applicators needing recertification only for General Standards (00), Structural Health (08), Wood Destroying Organismas (08w) and Fumigation (11) categories. Visit more information.

Testing-only Options

  • Closed-book exams are given by the Nebraska Department of Agriculture (NDA). Preregistration is not required an there is no cost. Visit the link below for a list of available test-only dates, times and locations -
  • NDA computer-based testing is provided through the Pearson-Vue company. Click here for a list of testing sites, categories available, dates, and registration information. Cost $55 per exam. (For applicators with multiple categories on their license, each category is charged the full testing fee.)


13. Digital Diagnostic Network - Need help with diagnostics?Submit pictures and questions for diagnosis by Nebraska Extension experts

Do you or your clients have questions you need help answering? Maybe you are a lawn care person and they're asking about trees, shrubs, or flowers? While you can refer them to their local Extension office, another option is Digital Diagnostic Network. Homeowners, lawn care professionals, pest control operators and others are invited to submit questions and photos through this website or with the assistance from an Extension professional at any Nebraska Extension office. All offices are equipped with high-resolution digital image capturing technology. Whether the question is about a lawn weed, insects on a plant, diseases in a shrub border or other, an expert panel of Extension professionals will review and respond to the question. To get started, create an account so the question can be reviewed and responded to via email. For more information and to create an account, go to Digital Diagnostic Network.

Bugging Out With Your Camera Phone - Tips on how to get a good picture.


14. Spanish for horticulure professionalsLearn phrases and vocabulary related to plant and landscape maintenance at Metropolitan Community College

Our instructors have developed a series of noncredit courses, Spanish for Horticulture Professionals. The first in the series will have a focus on the phrases and vocabulary related to maintenance of plants and landscapes. No base level of Spanish proficiency is needed; this class is for beginning learners.

Future series will focus on communication related to hardscaping, softscaping, and contracts/labor.

The class is aimed at individuals working in the horticulture industry, who would benefit from learning to communicate more effectively with their Spanish-speaking team members.

Class information: Spanish for Horticulture Professionals: Maintenance
Location: Metropolitan Community College, Fort Omaha Campus, 32nd and Sorensen Parkway
Dates: January 11, 18 & 25
Time: 5:30-7:00pm
Cost: $40 for all three classes in this first program series

Register now! 


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.