Hort Update for February 25, 2019

Hort Update for February 25, 2019, Nebraska Extension, http://communityenvironment.unl.edu/update2252019
Serious ConcernsMajor Symptom:
1. Cold temperature injury Prevention is best; don't prune out damaged branches too quickly
2. Wind & ice damage to trees Remove hazards such as broken or hanging branches first; use good pruning techniques
3. Winter desiccation on evergreens Watch for browning of foliage on the south or east side of evergreens
4. Winter desiccation of turf Dry winter conditions, particularly in western Nebraska, may result in winter desiccation damage by spring
Minor IssuesMajor points:
5. Snow mold Fungal diseases of turf, seldom warrant chemical control
Timely TopicsMajor points:
6. Cummulative effects of winter stress Cummulative stress can lead to plant death; good management practices are necessary if plants are to recover
7. Insect survival and winter temperatures

Don't expect massive insect winter dieback due to cold temperatures

8. Herbicide drift issues on trees & other plants Reduced growth, increased susceptibility to pest damage, and/or plant death
9. Pruning fruit trees Good pruning practices - timing and techniques - essential for tree health & productivity
10. Dormant oil applications Apply in March to control overwintering insects



1. Cold temperature injuryPrevention is best; Don't prune out damaged branches too quickly

If a plant is healthy, dormant and fully hardy, it should tolerate the cold temperatures this winter. Factors that can lead to injury include:

  • a plant not being fully hardy to an area,
  • plants going into winter stressed,
  • dry conditions,
  • lack of snow cover,
  • and late summer and fall fertilization.

These factors increase the potential for dieback in herbaceous and woody plants such as small fruits and roses. General prevention of cold temperature injury includes wise plant selection for the growing environment of the planting site, correct summer care, avoiding late season fertilization, and applying a winter mulch to tender plants in fall.

When spring arrives, remind homeowners not to be in a hurry to remove plants suspected of having winter injury. A plant that does not leaf out in spring when expected may not be dead. It may have suffered cold temperature injury to primary leaf and flower buds; however, secondary buds can open later. As a rule, it is recommended to wait until about June 1 before replacing a plant with one better adapted to the site. 

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2. Wind & ice damage to treesremove hazards such as broken and hanging branches first; use good pruning techniques

Heavy ice or snow loads on trees, especially accompanied by wind will result in tree damage. When tree damage occurs, homeowners should turn to professional arborists for assistance with large trees. Damaged trees require immediate attention to remove broken branches that pose the greatest hazard. Broken but firmly attached branches posing no immediate danger can be pruned after more hazardous branches are removed.

Correct pruning cuts are important. These include no flush cuts on the trunk and not leaving branch stubs. If hazardous branches are removed in a hurry for safety purposes, and correct pruning cuts are not made at the time, such as leaving branch stubs, it is important these stubs be removed in early spring or early summer.

Pruning wounds should not be treated with any kind of paint or wound dressing. Damaged trees should not be fertilized. In most cases, fertilizer will not benefit the tree and can inhibit the tree's ability to recover.

The Nebraska Forest Service has a series of publications with information on dealing with storm damaged trees, such as leaning trees, trees with split trunks and more. The storm damaged tree series can be found at Storm Damaged Trees.

Resource to help clientele understand this issue:

  • Ice, Nebraska Extension

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3. Winter desiccation on evergreenswatch for browning of foliage on the south or east side of evergreens

Cold, dry, windy winter conditions with little snow cover and extreme winter temperature fluctuations increase winter dessication injury on evergreens, especially arborvitae and boxwood, but also pine, spruce, fir, juniper and yew, because evergreens lose more moisture from green foliage during winter than deciduous plants with no foliage.

Damage occurs when the amount of moisture lost is greater than what can be replaced by roots, often due to frozen or dry soil. Plant tissue dries out resulting in browning of foliage and dieback, which is often not seen until spring. Injury is found on the outer portion of the branches and is most severe on the side of the tree facing the wind or a source of radiated heat, such as a south or west-facing brick wall or street.

Actions that can be taken now are placing burlap wind screens between plants and prevailing winds or radiated heat sources; applying antidessicants according to label directions when temperatures are above 40º F; and watering if soils are not frozen and air temperatures are above 45º F.

Prevention includes wise plant selection for the planting site's growing environment, correct summer and fall watering, and avoiding late season fertilization.

When spring arrives, it will be important to remind homeowners not to be in a hurry to prune damaged tissue. While green needles may be brown, the buds on the branches may still be viable and will eventually open. If damage is not too severe and twigs are not killed, the area may eventually fill in. With evergreens, pruning cannot be done past where there is green leaf tissue. If this is necessary, consider replacing the plant with one better adapted to the site.

Winter Dessication Injury, Nebraska Extension

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4. Winter desiccation of turfDry winter conditions may result in winter desiccation damage by spring

While eastern Nebraska has had good amounts of snow fall this winter, western Nebraska has received much less moisture. In the absence of rain or snow cover turf may experience winter dessication injury, particularly turf with a high percentage of perennial ryegrass. The risk is greatest for golf and sports turf growing on sand soils. Stands with lots of thatch are more likely to have issues with winter desiccation (i.e. tees and fairways). Lawns of predominately Kentucky bluegrass, fescue, or buffalograss are much more tolerant of winter desiccation stress.

On lawns, the risk of desiccation is usually greatest on exposed or elevated areas where water surface runoff is greatest, and on poorly rooted turf that cannot take up water that is deeper in the soil profile. An example would be turf growing in heavy clay soils that were compacted and not prepared properly before seeding/sodding.

If needed on exposed sites, water only when the soil is not frozen and air temperatures are above 40º F. Apply water at mid-day so it has time to percolate into the soil before potential freezing overnight. Water just enough to moisten the crown of the plants. Apply water slowly enough that it soaks in and does not run off or pool and freeze around plant stems or crowns overnight.

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5. Snow moldfungal diseases of turf, seldom warrant chemical control

With heavy amounts of snow received in many parts of Nebraska this winter, snow mold damage may be a common sight this spring. Snow molds are fungal diseases active in winter, resulting in nearly circular patches of dead matted turf blades. Damage does not become evident until snow melts in spring.

  • Gray snow mold requires snow cover for infection and patch development. Favored by cool conditions - 32° to 36º F. Mild patch development can occur with 40-60 days of snow cover, moderate damage with 60-90 days, and severe damage with over 90 days of snow cover.  
  • Pink snow mold does not require snow cover and is active under a wider temperature range - 30º to 60º F - so the fungus can be active in late fall and early spring, as well as during winter. 

Snow mold is most likely to develop on tall turf, over 3", that becomes matted under snow after it fell. Continuing to mow lawns in fall, at 2.5 to 3" height, until growth ceases can reduce the potential for snow mold. 

Chemical control of the fungal pathogens is usually not necessary. Rake up matted patches of turf in spring and allow the turf to regrow naturally. A lawn would need to have a history of serious snow mold damage to justify the use of a fungicides.

Gray Snow Mold, Purdue Extension
Pink Snow Mold, Purdue Extension

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6. Cumulative effects of winter stresscummulative stress can lead to plant death; good management practices necessary if plants are to  recover 

Trees and other landscape plants can experience winter injury in several ways as mentioned above.  Common causes of winter injury in Nebraska include the following. Plants may suffer one or more different types of damage each year. 

  • Branch breakage from heavy ice or snow loads, which is particularly severe when accompanied by high winds. (see #2 above)
  • Excessive drying of foliage or stems, called winter desiccation. Evergreens and broadleaf evergreens are most at risk, but deciduous plants can be affected too. For example, shrub roses are often affected by winter desiccation. (see #3 above)
  • Bark death from sunscald - caused by unseasonably warm days, sometimes as high as 60º F or more, in January or February followed by normal cold evening temperatures. 
  • Bark death from extreme temperatures - often occurs in fall when plants have not yet attained complete winter dormancy due to extended warm fall conditions. This happened in November 2014 when night temperatures suddenly drop to single digits after a very long warm fall and trees had not become fully winter hardy.

The severity and extent of damage are key to the future of the plant.

  • Extensive bark death reduces a plant's ability to move water the following summer during hot conditions and can lead to continued decline or death. 
  • Canopy reduction due to branch breakage or winter desiccation reduces a plant's ability to photosynthesize, producing food for growth, which can lead to a gradual decline in plant health. 
  • Reduced photosynthesis also reduces a plant's ability to create protective chemicals. Stressed plants are commonly targeted by borers and other insects, in part due to a lack of defensive chemicals. These secondary pests can lead to the plant's death. 

What can be done for damage or stressed trees to help them to recover? Best practices include the following. 

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7. Insect survival and winter temperaturesDon't Expect Massive Winter insect dieback due to cold temperatures

Cold winter temperatures and heavy snow don't automatically result in fewer garden insects next summer. Insects have evolved many coping mechanisms enabling them to tolerate winter conditions just fine.

How Do Insects Survive Winter, Nebraska Extension Gro Big Red Blog

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8. Herbicide drift issues on trees and other plantsReduced growth, increase susceptibility to pest damage, and/or plant death

There has been a noticeable increase in herbicide injury to trees and other nontarget plants from herbicide drift in recent years. Symptoms include curling and cupping of leaves, and leaf and small twig distortion. Homeowners often blame lawn & landscape professionals for misapplying herbicides, but drift can just as easily come from surrounding neighbor's properties or nearby farmlands. 

As a rule for otherwise healthy trees, shrubs and herbaceous perennials, plants would need to sustain injury a number of years in a row for serious damage to occur. However, herbicide injury leads to stressed plants resulting in slower growth and increased susceptibility to pests.

To reduce issues, professional pesticide applicators should always read and follow label directions - the label is the law. Use herbicides with lower drift potential and pay attention to weather conditions when spraying. Wind speeds should be below 10 miles per hour but completely calm, which can create conditions for the development of a temperature inversion.  Temperatures should also not be too high. The use of many herbicides is not recommended at temperatures above 85F.

Suggest homeowners consider alternative weed control where feasible, such as using mulch, hand pulling, hoeing or digging. They can also have conversations with neighbors about pesticide usage and drift issues, if drift seems to be coming from nearby properties.

Common Landscape Herbicides and Their Effects on Trees, Mississippi State University
Air Temperature Inversions: Causes, Characteristics and Potential Effects on Pesticide Spray Drift, North Dakota State University

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9. Pruning fruit treesgood pruning practices - timing and techniques - essential for tree health & productivity

Productive fruit trees with an abundance of high quality fruit do not just happen. They result from good cultural practices, including pruning. However, fruit tree pruning is often neglected either due to a lack of pruning skills and knowledge, or a fear that the tree will be damaged or killed by incorrect pruning.

Most fruit tree pruning is done during the dormant season when no leaves are on the tree. March is the best time to prune. Cultivars or varieties of trees susceptible to winter injury are best pruned in late spring before growth begins, rather than in January or February. Regardless of the cultivar grown, do not prune any tree before January or winter injury can occur.

Besides dormant pruning, trees may be prune at planting; during July and early August to restrict growth; remove water sprouts; or remove diseased or damaged wood. Once the basic structure of a fruit tree is developed, avoid pruning until fruiting occurs. For information on how to prune different fruit trees, see the resource links below.

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10. Dormant oil applicationsapply in march to control overwintering insects

March is the month to apply dormant oil sprays to fruit, nut and ornamental trees to kill insects and mites overwintering in cracks and crevices on trunks and branches. Temperatures need to be above 40F when applying dormant oils. Pests controlled include aphids, scales, spider mites, insect eggs and some hibernating caterpillars. A few dormant oils are labeled for use on white flies, mealybugs and lacebugs.

Dormant oils kill by suffocating insects and mites. They are most effective if applied as late in winter as possible, but before spring growth begins. At this time, insects are weakened and easier to kill. Also, dormant oils can damage tender plant tissue and should not be used once flowers or leaves begin to grow. Follow label directions when using any pesticide.

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