ProHort Update - January 2, 2024

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Nebraska's drought status, 12/28/23 Map updated weekly.

1. Cold injury in landscape plantsMany factors affect severity of damage; symptoms often not seen until spring growth begins

Some Nebraska winters have relatively mild temperatures, or at least a lack of extreme cold. But it is not uncommon for temperatures to dip to near record-breaking levels, even if only for a few hours or days. Following periods of extreme cold, many ask about the impact on landscape plants. This question is difficult to answer with certainty as there are too many variables to be definitive.

Factors Affecting Severity of Cold Damage
Cold injury varies with plant species, age, plant vigor, planting site and soil characteristics. Still, a review of potential damage to trees and shrubs can be beneficial. Keep in mind much of the damage which occurs in winter will not be apparent until spring when new growth begins. By then, people will have forgotten about a cold snap and might not relate damage seen in spring to an extreme winter event.

Prior to considering potential cold injury, keep in mind two factors.

1. First, if weather conditions the previous fall favored hardening of plants, it is less likely cold injury will occur in winter. However, if a warm period occurred prior to the onset of frigid weather, there is increased potential for physiological changes in plants that can result in a reduced hardiness. (Note: Nebraska did experience such conditions in the fall of 2023.)
2. Second, it is unlikely extreme cold would have much impact on native plant materials in their natural habitat. The geographic range of native plants is determined by extreme temperatures, not average temperatures. That said, it is possible native species planted in urban or suburban landscapes where soils and environmental factors are vastly different from their natural habitat could experience cold injury due to plant stresses imposed by these habitats.

Many woody ornamentals used in Nebraska are non-native. Nursery operators and landscape contractors rely on hardiness ratings to determine survivability of each species. Ratings are usually based on field trials of species in USDA-defined hardiness zones. Hardiness zones are determined by average low temperatures for a given region of the country, and not by extreme low temperatures. As such, planting a species rated hardy to Zone 5 in a Zone 4 area does not guarantee survival when the plant is exposed to extreme temperatures. Species marginally hardy to a particular zone are especially vulnerable to cold injury. Keep this in mind when assessing plant problems in spring.

Potential Root Injury
It is known with certainty plant roots are much less hardy than above ground parts, which means in-ground plants are affected differently by cold than plants in above ground containers. Soil by itself is a good insulator that provides enough cold protection for roots. Once frozen, soil temperature remains fairly constant, at least at depths greater than 6 inches.

In-ground plants - If low temperatures affect roots, it is often feeder roots near the surface that are killed. While loss of some feeder roots is unlikely to kill established plants, chronic loss of roots adds stress and increases a plants susceptibility to opportunistic pests like root rots and insect borers.

How much root damage occurs depends on many factors, 1) root hardiness, 2) general depth of rooting for a plant species, 3) soil texture (most root kill from cold seems to occur in sandy soils rather than clay soils), and 4) the presence or absence of mulch.

It should be noted that death of feeder roots during winter is not unusual. Kozlowski and Pallardy (Physiology of Woody Plants.1997. Academic Press) state that "the greatest mortality of small roots occurs during the cold months." The question is not whether roots have been killed but to what extent they've been killed.

Above ground planters - While roots of landscape plants are afforded some protection against low temperatures by surrounding soil, roots of plants in above ground planters or containers are much less protected. All of the roots within the container are subject to direct injury or death from the cold.

It may be the greatest damage to roots will happen as a result of frost heaves. The lifting of soil that occurs when it freezes can break apart roots. Frost heaving occurs in both in-ground and above-ground containers. Un-mulched fall planted trees and shrubs are most susceptible to injury from frost heaves because of the absence of extensive anchoring roots.

Above Ground Injury
The amount of cold temperature injury to above ground plant parts is as uncertain as is root damage. As with roots, shoot injury will usually not become apparent until spring.

Flower bud damage - One type of cold injury is killing of flower buds on marginally hardy trees and shrubs. Flower buds are typically less hardy than leaf buds. It has often been stated flower buds of Forsythia and peaches are prone to winter kill when temperatures drop to minus 15º F. The precision of that temperature threshold can vary with cultivar and site factors.

Vascular tissue damage - Buds are not the only structures where hardiness differences exist. In mid-winter, living xylem tissue tends to be less hardy than cambium and phloem tissue. Xylem in smaller branches is more prone to damage from low temperatures. If such damage occurs, affected branches will be slow to leaf and/or flower in spring, or die. The xylem of cold damaged branches will appear black or darkened.

Bark damage - Cold injury such as sun scald and frost cracks often occur in response to sudden and wide fluctuations in the temperature of wood. This typically takes place on cold days when the sun warms the sunny side of the trunk to a temperature above freezing, causing some expansion of wood. According to Harris (Arboriculture. 1992. Regents/Prentice Hall Publishing), the temperature of wood may differ by as much as 18º F from air temperature on sunny days. For expansion of wood to take place due to warming, the temperature of the wood would have to rise to above freezing.

If temperatures during a cold spell remain far below freezing throughout the period, then warming and expansion of wood on sunny days may not be widespread. Nevertheless, such injury could occur and practitioners should examine the trunks and branches of woody plants for sun scald and frost cracks each spring.

When damage occurs, it should not be treated with any kind of wound dressing or covered with tree wrap during the growing season.

Assessing Winter Injury
Because of limited root growth, it is not unusual to see newly planted specimens die after a hard winter. In the worst years, long established plants could die, including some native species. In most cases the most loss is with marginally hardy species.

Winter damage usually does not become apparent until spring when growth resumes. Winter damaged plants may be slow to initiate growth, show distorted growth, death of leaf and flower buds, or dieback of shoots and branches. In challenging years, the plant species damaged are often remarkably consistent.

Below is a list of plants observed to have died as a consequence of winter damage or have had significant winter injury (brown foliage, branch dieback, etc.). This list is compiled from observations by Nebraska Extension staff:

• Arborvitae
• Boxwood
• Butterfly bush (and other suffrutescent shrubs)
• Juniper (Eastern Red Cedar)
• Spirea
• Perennials - many species, especially if hardiness zone is 5 or 6
• Rhododendrons
• Roses - Note new shoots appear at the base of heavily damaged roses. On grafted roses, these shoots often originate from the rootstock resulting in roses not true to variety.
• Japanese Yew
• Weeping Willow

Managing Winter Injured Plants
The options for dealing with severely winter injured plants are limited. Basically, care at this point involves pruning out dead portions of affected plants. In addition, it is necessary to provide a good growing environment to eliminate further stress. That means correct watering and mulching, but fertilizer is not needed.

With pruning, the shoot to root balance shifts to the roots. Food reserves in roots - assuming there are ample roots that have not been winter killed - should support new growth in the remaining living parts of the shoot system. Addition of fertilizer will not influence the transport of food reserves to growing points on the shoot system.

In conclusion, the impact of extreme winter events on the health and survival of woody plants is difficult to predict prior to spring growth. However, such events and the type of damage they could cause to plants is a factor to keep in mind when evaluating plant growth or death in spring.

Source: Effects of Cold on Landscape Plants, Ron Kujawaki, University of Massachusetts-Amherst. Adapted for Nebraska and ProHort Update by Kelly Feehan, Nebraska Extension.

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2. The mechanical effects of bulges developed around bark‑included branch junctions of hazel (Corylus avellana L.) and other treesFrom the journal Trees.

A currently prevalent rule in European arboriculture - if a bark-included branch junction is associated with a large bulge in-line with the plane of the included bark, then it is more likely to fail than if there is a smaller bulge or the absence of bulging. This rule for arborists originates from an initial suggestion with no associated data. And is not logically consistent with recent research into the effects of natural bracing in trees, nor guidance provided by the International Society of Arboriculture.

This also raises the question of how to correctly interpret the function of these bulges formed at bark-included junctions: either as secondary growth that has been pushed to the side by internal growth pressures, or as compensatory growth developing around a weakened component.

To test the veracity of this rule, 117 branch junctions of common hazel (Corylus avellana L.) were subjected to tensile tests, comprising of controls with no bark included within them and bark-included specimens exhibiting a range of bulge sizes. In addition, photographs from the failure of 110 bark included junctions were categorized to assess the frequency of failed specimens with different degrees of bulging. The results of mechanical testing identified three significant factors affecting the maximal bending moment of these branch junctions:

1. their categorized morphology,
2. the diameter ratio of the branch junction and
3. the width of the included bark at the apex of the junction.

Overall, and in each category of branch junction tested, the extent of bulging was not found to be a significant predictor of the junctions’ maximal bending moment. This finding was reinforced by the analysis of the images of bark inclusion failure where the most frequent bark inclusions to fail were those associated with little to no bulging. Both findings identify the bulging would be better interpreted as compensatory growth. This study highlights the need for further research on the load-bearing capacity of bark-included branch junctions to better inform arborists and tree managers as substantial variations in their biomechanical performance have not yet been elucidated.

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3. Plant native Nebraska trees & shrubsAdding trees and shrubs increases carbon sequestration. Adding natives improves habitat.

Doug Tallamy, University of Delaware Entomology Professor and author of Bringing Nature Home, recommends ten things home and greenspace managers can do to increase a landscape's positive contribution to the environment, habitat potential and climate change migitation. One action is to increase landscape plant diversity, especially with native plants. Not sure what plants are Nebraska natives? Then check out Native Trees and Shrubs from the Nebraska Statewide Arboretum.

Secondly, remove invasive plant species. Trees & Shrubs That Are Invasive Species in Nebraska, Nebraska Statewide Arboretum.

These two lists are interesting though, in that some plant species are on both - like eastern red cedar. In this case, even a native tree can become invasive where it overwhelms and degrades farm, pasture or wild lands.  

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4. Avoid foot traffic on frozen turfMinimize winter traffic on any turf area and especially when frost is present on green turf

If ice crystals (frost) have formed and foot or vehicle traffic occurs, the physical abrasion can damage turfgrass. Winter traffic can cause cosmetic damage, physical abrasion, and/or soil damage depending on the situation. Too much traffic on turfgrass at a time when it cannot recover also leads to winter injury. Winter golf or over-using soccer fields during winter are examples.

Winter Traffic and Play on Turfgrass, Nebraska Extension Turf iNfo

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5. Effects of freezing or wetness on stored pesticidesImproper storage can reduce effectiveness.

Store pesticides correctly and securely.  Storage information can be found on pesticide labels.  Read and follow it for safety and to help keep pesticides from degrading so they may no longer be as effective.

In general, pesticides need to be stored in a secure, well ventilated location that can be locked. The location should be away from children, pets and food items as well as anything that might be contaminated in case of a leak or accidental spill. 

Do not store pesticides near heat, sparks, or open flames; and check that containers are tightly closed. Always store pesticides in their original containers.  A mistake made is pouring a pesticide into a container other than the original. This is against pesticide label law and has led to accidental poisonings. 

A common question about winter storage is if a pesticide is still effective after it freezes.  Most pesticides are safely stored between 40 and 100 degrees Fahrenheit, but it is best to check the label for storage temperature requirements and any warnings against freezing.  If a liquid pesticide does freeze, it might be less effective in controlling pests. 

Pesticides contain active and inactive ingredients. The active ingredient is what kills the pest. Inactive ingredients include solvents, carriers, or emulsifiers that make the pesticide more efficient. Due to some inactive ingredients, the freezing point of some liquid pesticides could be lower than 32 degree F. Read the label for temperature storage requirements and what to do if a pesticide does freeze.          

Pesticides formulated as wettable powders or granules are not affected by low temperatures. However, moisture can cause caking that may reduce effectiveness so follow label directions for correct storage recommendations.

If you have products formulated in water-soluble packets, these should not be frozen as they tend to become brittle and then break open.

Safe Transport, Storage, and Disposal of Pesticides, Nebraska Extension

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7. Commercial/Non-commercial pesticide applicatorsObtaining a new license or updating an expired license

If you have a pesticide applicators license which expired in April 2024 or you need to get a new license, commercial/noncommercial applicators have several options to recertify or get a new license.

2024 Traditional Classroom Training
Commercial/non-commercial in-person training classes are held from January through April. Training schedules will be available soon at https://pested.unl.edu/. 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 https://pested.unl.edu/

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 - https://pested.unl.edu/.
• 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.)

2024 Conference Options
Extension and industry-sponsored events also offer opportunities to renew a license. Recertification only.

• Jan. 3-23 | Nebraska Extension Crop Production Clinics - 00 general standards, 01 ag plant, demonstration/research
• Jan. 9-11 | Nebraska Turf Conference - 00 general standards, 04 turf & ornamental 
• Feb. 19-21 | Nebraska Aviation Trades Association Convention - 00 general standards, 01 ag plant, 07 right-of-way, 09 public health, 12 aerial
• Feb. 21-23 | Nebraska State Pest Control Association Conference - 00 general standards, 08 structural/health related, 08w wood destroying organisms, 11 fumigation, 14 wildlife damage control

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.

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8. 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.

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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 constitue endorsement by Nebraskas Extension. Nor does it imply discrimination against other similar products.