Hort Update for September 17, 2019

Hort Update for September 17, 2019, Nebraska Extension, http://communityenvironment.unl.edu/update9172019
Late summer pruning of this yew shrub resulted in death of the new growth following a November cold snap.
Serious ConcernsMajor Symptom:
1. Emerald ash borer confirmed in Saunders County Insect confirmed near Ashland; continue to wait until insect is found within 15 miles of a location before beginning to treat
2. Protecting young thin bark trees from winter frost cracks Plan to apply tree protection in November
3. Frost cracks and tree species prone to damage Common on maple, sycamore, linden, white ash, apple, willow and walnut
Minor Issues Major Symptom:
4. Weedy vine control Wild cucumber, burcucumber and honeyvine milkweed
5. Oak twig girdler Small terminal twigs turn brown; variety of trees affected
6. Anthracnose in shade trees Irregularly-shaped brown leaf lesions; early leaf drop; fungicide applications not effective now
7. Landscape fungal leaf spot infections Many pathogens; fungicide applications in fall will not provide control
Timely Topics Major Points:
8. Bagworms Chemical control no longer effective
9. Lawn fertilization recommendations for fall Still time to make September application; use combination of slow/fast release Nitrogen
10. Fall weed control and timing Fall best time to control perennial and winter annual weeds; control of summer annual weeds now is a waste of time and chemicals
11. Root starter products and fall planting Little benefit to plant establishment rate
12. Consequences of late summer/early fall pruning Pruning now stimulates new growth; increases potential for cold damage to new growth



1. Emerald ash borer in Saunders CountyInsect confirmed near Ashland; continue to wait until insect is found within 15 miles of a location before beginning treatment

The most recent confirmation of emerald ash borer (EAB) was on private land near Ashland in Saunders County. This new finding does not change the current quarantine as Saunders County was already included due to EAB findings in nearby counties. Information about this new finding is in the news release linked below. The recommendation to wait until EAB has been found within 15 miles of a tree to begin treating is still in place. For ash owners who do not plan to treat their ash, it is wise for them to consider tree removal once EAB has been found within 15 miles. Waiting until an ash tree dies from EAB increases the risk of brittleness in ash trees making removal more difficult; as well as there could be a higher demand for tree removal services.

Nebraska Department of Agriculture News Release

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2. Protecting young thin bark trees from winter frost cracksPlan to apply tree protection in November

Prepare now for wrapping the trunks of young, tender barked trees, such as maple, to protect from frost cracks and sunscald, but don't apply wraps until November. Wait until after a hard freeze or after leaves have dropped to apply tree wrap. On young trees, wrap covers photosynthetic tissue that produces food the plant needs for root establishment. If wrapped too early, less stored food may be produced, moisture may build up beneath the wrap, or insects may use wraps for overwintering.

When wrapping tree trunks, begin at the bottom of the tree and wrap upwards, being sure to overlap the wrap so no part of the trunk is exposed. Remove all wraps in early spring. If left on too long, wraps can girdle young trees; and moisture may build up beneath tree wrap to promote decay organisms. Not all young trees need to be wrapped. When tree wrap is used, only use it on 1) tender barked trees during their first winter after transplanting, 2) when moving trees, 3) or if the nursery guarantee requires it.

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3. Frost cracks and tree species prone to damageCommon on maple, sycamore, linden, white ash, apple, willow and walnut

Frost cracks are vertical cracks in tree trunks. They occur during winter on the south or southwest sides of trees out in the open. Cracks occur when sunlight warms the bark and inner wood on the south or west side of young, thin-barked tree. As the sun sets or is hidden by clouds, temperatures drop quickly causing shrinkage in the bark while the inner wood takes longer to contract. This unequal shrinkage or contraction between the bark and the inner wood causes the bark to split and along with it the wood directly below the bark. Scientists believe it actually results from water moving out of cells and freezing during sudden drops in temperature. The wood closest to the surface shrinks as water is lost quickly while inner wood is not affected. The sudden change creates pressure between these two zones resulting in the wood cracking. The sudden splitting causes a loud report or bang. Some trees are more prone to frost cracks. These include maple, sycamore, Linden, white ash, apples, willow and walnuts.

Frost cracks and sunscald damage are two separate tree problems

Frost Cracks in Trees, Michigan State University
Sunscald and Sunburn on Trees, Washington State University

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4. Weedy vine controlWild cucumber, burcucumber and honeyvine milkweed

Wild cucumber and/or burcucumber seem to be taking over some shelterbelt trees. Both are annual vines that grow from seed each year and vines can grow from 10 to 25 feet long. They can shade out parts of evergreens and may cause some branch dieback. At this time of year, pull vines off of trees by hand or with a rake to allow sunlight to reach tree foliage; or cut the vine off at the base to kill it. The first killing frost will also kill the vines. For herbicide control, apply a preemergence during May next spring. Simazine (Princep 4L) is labeled for preemergent control in shelterbelts to kill weed seedlings as they germinate. Do not apply more than 4 qt. Princep 4L per acre (4 lb. a.i./A) per calendar year. Do not apply more than twice per calendar year. In place of herbicide control, or along with a herbicide, periodic scouting of the area to clip off or hoe out young vines is effective. 

Controlling Weedy Vines in Acreage Trees, Nebraska Extension

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5. Oak twig girdlerSmall terminal twigs turn brown; variety of trees affected

Twig girdler causes clusters of terminal leaves to turn brown; a symptom called 'flagging'. It also causes a small amoun of twig dieback. The girdler is a long-horned beetle that emerges in late summer. As part of egg laying, the female girdles the twig to kill it because the larvae cannot develop in healthy wood. The dead tip may fall to the ground or hang in the tree until wind knocks it out. While damage is obvious, it is rarely severe, and there is usually no need for control. Larvae overwinter inside twigs. Pick up and discard dead twig sections that fall to the ground to reduce this insect. Squirrels clipping tree twigs can be confused with girdler damage.

Twig Girdler, Kansas State University Research and Extension 

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6. Anthracnose in shade treesIrregularly-shaped brown leaf lesions; early leaf drop; fungicide applications not effective now

Trees and shrubs heavily infected by Anthracnose diseases this summer are dropping leaves now. Anthracnose is a fungal leaf infections occuring in many landscape trees and shrubs, caused by a group of fungi including Colletorichum sp., Kabatiella sp., Elsinoe and Apiognamonia sp. Fungi become active in spring during frequent rainfall and cool 60º - 65º F temperatures. Later summer infections can also occur if weather conditions are cool and rainy. 

It's too late for fungicide control this year. Prune out dead branches/twigs. Rake up and destroy infected leaves to reduce disease pressure for next year's growing season. Fungicide applications are only warranted on severely affected plants with a history of repeated infections or those with severe shoot death. Applications should be planned for next spring, starting at budbreak. This fall improve tree health through proper mulching and watering, approximately 1 inch per week, until the ground freezes. 

Anthracnose of Trees and Shrubs, Cornell University
Diseases of Broadleaf Trees, Nebraska Forest Service

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7. Landscape leaf spotsMany pathogens; fungicide applications in fall will not provide control

Many fungal and bacterial pathogens infect landscape trees and shrubs, causing leaf spots, twig dieback, or branch cankers. Some of the most common pathogens are anthracnose, Entomosporium, rust (cedar-apple, cedar-quince, cedar-hawthorne), Mycosphaerella, Phyllosticta, Pseudomonas, (angular leaf spot)  Septoria (Septoria leaf spot and canker of dogwood) and many more. Most leaf spot diseases are favored by wet conditions at some point in the growing season which enable the disease pathogen to spread and infect additional leaves. Leaves may have orange, tan, brown, reddish, or black spots on the foliage. Some expand to cover a large area of the leaf. Heavily infected leaves usually fall from the plant early, often resulting in some level of defoliation of the host plant. Infections may spread to stems or twigs causing cankers; infected areas of stems may turn a darker or light color than normal. 

Important Points

  • Most leaf spot diseases only affect a small portion of the plant's total foliage and are only a minor stress on plant health
  • Leaf spot diseases reduce plant vigor by reducing the plant's ability to photosynthesis and store carbohydrates for growth, defense and storage
  • Several years of heavy leaf loss may affect plant health enough to warrant control

Control

  • Rake up and destroy infected leaves in fall. 
  • Prune out dead branches or twigs.
  • Prune trees and shrubs to increase light penetration and air circulation, which promotes leaf drying after rain or heavy dew. 
  • Do not overcrowd plants, which increases humidity around the foliage, shades leaves and slows leaf drying. 
  • Avoid wetting foliage during irrigation. 
  • Maintain good plant vigor by watering during dry periods, maintaining a 3-4 inch mulch layer beneath plants and not fertilizing unless indicated by a soil test. 
  • Identify specific leaf spot disease and apply fungicides at the proper time. 

Leaf Spot Diseases in Trees and Shrubs, University of Minnesota Extension

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8. BagwormsChemical control no longer effective 

This year's bagworms are no longer feeding and have pupated into adults. Insecticide control is no longer effective, because the insecticide (even systemic products) will not move through the bags to reach the adults or next year's eggs. Mark calendars for next June to control the next generation with insecticides.

For now, removing and destroying bags on high value, smaller evergreen trees will help reduce next year's bagworm population. Bags can be removed from now until insects hatch next year, approximately mid-May. Destroy bagworm eggs by removing bags from the plant and crushing or immersing them in soapy water. If bags containing eggs are discarded on the ground, eggs may survive winter fine then hatch and larvae return to the surrounding plants next summer. As many as 500 to 1000 eggs can overwinter in one female bagworm's bag.

Bagworms, Nebraska Extension   

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9. Lawn fertilization recommendations for fallStill time to make September application; use combination of slow/fast release Nitrogen

On older lawns (10 to 15 or more years), the first fall application should be made very soon if it has not yet been done. A fertilizer with a slow release nitrogen source can be used.

On younger lawns, two fertilizer applications during fall are recommended; one in late August/ early September and one in mid-October.

For the first one, use a slow release nitrogen source. For the last one, use a fast release nitrogen source. Applications much past November 1 are less efficient because plant uptake is low. This causes nutrients to leach away during winter or linger in soil until spring; resulting in too early succulent growth.  A specific winterizer-type fertilizer is not needed, but have homeowners buy a fertilizer with their particular fertilizer spreader settings listed on the label.

Improving Turf in Fall, Nebraska Extension Turf iNfo
Fertilizing Home Lawns, Nebraska Extension Turf iNfo

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10. Fall weed control and timingFall best time to control perennial and winter annual weeds; control of summer annual weeds now is a waste of time and chemicals

Identification of problem weeds is critical before attempting control. Annual weeds, such as crabgrass, foxtail, knotweed, oxalis and spurge, will naturally die in just a few weeks. Applying herbicide to kill them now is a waste of time and chemicals. Preemergent herbicide should be applied next year the first week of May to kill germinating summer annual weeds. 

However, perennial and winter annual weeds are best controlled right now because:

  1. winter annual weeds germinate in late summer/early fall and can be controlled with preemergent herbicide applied at that time,
  2. winter annual weeds that have already germinated are small and more easy to control than when they mature in spring,
  3. perennial broadleaf weeds are translocating stored energy (and properly applied herbicide) below ground, and
  4. cooler temperatures reduce the likelihood of injuring turf or ornamental plants.

Winter Annuals - Preemergence herbicide should be applied now for control of winter annuals like little barley, henbit and speedwell.  Winter annuals are or will soon be germinating.  Common landscape preemergent herbicides include bensulide (Bensumec), dithiopyr (Dimension), oxadiazon (Ronstar G), pendimethalin (Pendulum) and prodiamine (Barricade). Other management options for winter annual control include maintaining a healthy and vigorously growing lawn or landscape bed plant cover to crowd out weed invaders. A good layer of organic mulch 2-3 inches thick will also minimize weed germination.  

Perennials and germinated winter annuals - Apply herbicide now and again in 4 to 5 weeks if any new growth emerges or green tissue remains.  If weeds are not killed completely, weakening them with fall herbicide applications makes them more susceptible to winterkill. Weeds are killed more slowly as daytime temperatures drops. Ideally daytime temperatures should be between 65-85º when applications are made. Selective broadleaf weed control products include triclopyr, quinclorac, carfentrazone, sulfentrazone, dicamba and 2, 4-D. 

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11. Root starter products and fall plantingLittle benefit to plant establishment rate

Fall is a great time to plant woody trees, shrubs and perennial herbaceous ornamentals. According to Professor Ed Gilman, University of Florida School of Forest Resources and Environmental Horticulture, tree establishment is determined by many factors. Fertilization and the addition of root stimulant products have little or no effect on how quickly a plant re-establishes on the new site.

Practices that encourage growth

  • loose soil, proper irrigation
  • mulch ring 8 feet in diameter or more around planting hole
  • root flare slightly above soil surface
  • leaving top of tree intact (no pruning at planting)

Limits growth

  • compacted soil
  • little or no irrigation
  • grass and weeds close to trunk
  • planting too deeply
  • pruning at planting

Little or no effect

  • peat or organic matter addition as backfill
  • root stimulant products
  • fertilizing at planting
  • adding mycorrhizae
  • adding water absorbing gel crystals

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12. Consequences of late summer/early fall pruningPruning now stimulates new growth; increases potential for cold damage to new growth

Pruning is an invigorating process which stimulates new growth. Whenever pruning is done during the growing season, new growth will quickly follow. Late summer/early fall pruning of evergreens, such as yew or boxwood, and spring or summer-flowering shrubs leads to the development of new growth. It takes time for this new growth to harden off and develop full cold hardiness, making the new growth very prone to damage from freezing temperatures in October/November or winter injury. If pruning needs to be done before winter, wait until plants are dormant. The EXCEPTION to this rule is removal of dead, diseased or damaged branches; this can be done at any time. 

Ideal pruning times:

  • spring flowering trees & shrubs - immediately after flowering
  • summer flowering trees & shrubs - late winter or early spring
  • oaks - dormant season pruning is recommended, no pruning from April through June
  • most shade and non-blooming ornamental trees -during dormancy, just before growth begins in spring, February to early April
  • fruit trees - January through March; start with most cold hardy first and prune the least cold hardy just before new growth begins. Most cold hardy - apple, pear, tart cherry, plum; least cold hardy - sweet cherry, peach, apricot
  • evergreen trees (spruce, fir) - late winter while still dormant, February through early April
  • pines - during candle stage, late May to early June
  • evergreen shrubs (juniper, yew, boxwood) - before new growth begins, late March to early April; light pruning in late June or early July if needed

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  • Emerald Ash Borer Resources - EAB has been found in the following Nebraska locations. 2016 - Omaha and Greenwood, NE; 2018 - Mahoney State Park and Lincoln, NE. 2019 - Ashland. Homeowners are encouraged to wait to begin treating their ash trees until the insect is confirmed within 15 miles of their location. 

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