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Hort Update for August 1, 2022

Hort Update for August 1, 2022, Nebraska Extension,
August through September is a good time of year for control the crawler stage of magnolia scale.

Serious ConcernsMajor Symptom
1. Kentucky bluegrass yellowing Weather conditions favor denitrification and yellowing of turfgrass
2. August 1 growing degree days (GDD) Several Nebraska sites below, Understanding Growing Degree Days
3. Pest update Pests to watch for based on growing degree days (GGD)
4. Magnolia scale August to September good time for crawler control
Timely Topics
5. White grub rescue treatments May be needed if damage is extensive, grub numbers are greater than 5-8 per square foot and early season insecticide was not applied
6. Fall weed control Fall best time to control perennial, biennial and winter annual weeds
7. Fall lawn fertilization Fall fertilizer recommendations have changed based on turf age
8. Fall lawn seeding Best time for seeding - August 15 to September 15
9. Tree defoliators Be on the lookout for fall webworm, mimosa webworm and walnut caterpillars
10. Flagging in oak trees Small terminal twigs turn brown; variety of trees affected; culprits include twig girdlers and/or kermes oak scale

1. Kentucky bluegrass yellowingWeather conditions favor denitrification and yellowing of turfgrass

Yellowing of Kentucky bluegrass and creeping bentgrass is a common site in July and August in Nebraska. In most cases the issue is iron chlorosis (a lack of iron in plant tissues) in part due to high pH soil. But why do we only see turf yellowing later in summer if soil pH is a factor?

Soil pH & Iron
Plants obtain the majority of their nutrients from soil but not all nutrients present in soil are readily available to all plants. This is because soil pH affects the form and therefore the solubility of nutrients, according to Purdue University Extension.

Soil pH is a measure of soil acidity or alkalinity. The pH scale ranges from 0 (very acid) to 14 (very alkaline) with 7 being neutral. Most nutrients have optimum availability to plants when soil pH is between 6 and 7. High soil pH can lead to deficiencies in micronutrients like iron and manganese.

In Nebraska, landscape soils tend to have a pH range of about 6.5 to 8 and so we often see iron or manganese chlorosis in some plants, especially pin oak and red or silver maple trees, all season.

Causes of Summer Yellowing
So why do we only see turfgrass yellowing later in summer? And why does nitrogen fertilization tend to make it worse?

While iron (Fe) availability in alkaline soils is low, grasses have special mechanisms in their roots to extract trace amounts of Fe from the soil. These mechanisms typically satisfy the plant's need for Fe and any greening response to Fe fertilization is the result of Fe oxide formation on the surface of the leaf.

Intensification of turf yellowing following nitrogen fertilization also suggests a micronutrient deficiency. Nitrogen fertilizer promotes turfgrass growth which increases plant demand for other nutrients like Fe. When Fe availability is low, nitrogen applications can increase deficiency symptoms.

The fact that yellowing occurs in late July and August is believed to be due to a root dysfunction caused by warm and wet soils that limits iron extraction from soil. It is thought that iron extraction systems in grass roots, called phytosiderophores, stop working normally under these conditions.

Iron - Soil vs. Foliage Applications
The University of Nebraska-Lincoln studied iron chlorosis correction at Heritage Hills Golf Course in McCook around 2014. Different chelated iron products were applied and then watered in. None of those treatments improved leaf yellowing. They believe this was because the products were watered in (on purpose).

They then re-applied different iron products to grass foliage and did not water them in. This resulted in a positive greening response further suggesting the issue is root related. All of the different iron treatments fixed the issue and chelate form wasn't that important.

Here are some tips to help combat iron chlorosis issues in summer:

  1. Avoid excessive irrigation. Chlorosis is usually worse around irrigation heads. Try to dry the soil down between irrigation events. Encourage customers not to use a "set it and forget it" method with automatic systems.
  2. Avoid nitrogen fertilization. Extra nitrogen increases growth rate and further dilutes iron concentrations within plants. This will intensify chlorosis.
  3. Apply liquid iron products to the foliage. Chelated products or iron sulfate applied at 1-2 oz/1000 sq. ft. can help. Don't water the iron in after application.
  4. Avoid driving over the fertilizer application (cart, mower, etc.) because that can leave black track marks in the turf.

Commercial Greenhouse and Nursery Production: Soil pH, Purdue University


2. August 1st growing degree days (GDD)

Location Accumulated Growing Degree Days
Grand Island, NE - Airport 2233
Lincoln, NE - Airport 2246
Omaha, NE - Airport 2264
Norfolk, NE - Airport 2099
North Platte, NE - Airport 2058
Scottsbluff, NE - Airport 1880


3. Pest updatePests to watch for based on growing degree days (GDD)

Examples of insects that have or may emerge soon in Nebraska based on growing degree days for your area. Understanding and Calculating Growing Degree Days.

GGD (base 50) InsectLifestage present at this GGDType of Damage
950-2150 Japanese beetles Adult emergence leaf feeding
1000-2000 Emerald ash borer Peak adult emergence D-shaped holes on trunk
1200-1800 Fall webworm Caterpillar feeding Leaf feeding & webbing
1250-1350 Pine needle scale 2nd generation egg hatch Needles turn yellow/brown
1375-1500 American plum borer 2nd generation Cambium tunneling on trunk, especially around wounds
1500 Pine needle scale 2nd generation hyaline stage (control target) Needles turn yellow/brown
1500-1600 Cooley spruce gall adelgid 2nd generation adults active (control target) Enlargement & browning on new growth shoots
1600-1700 Walnut caterpillar egg hatch; caterpillars Leaf feeding
1700 Zimmerman pine moth Adult flight Tunneling at branch junctions; popcorn masses of sap and sawdust
1800-2200 Banded ash clearwing borer Adult emergence Tunneling in phloem & sapwood
1900-2050 Euonymus scale 2nd generation egg hatch Yellow spots on leaves; defoliation; branch death
1925-1950 Magnolia scale egg hatch Sticky honeydew on foliage; black sooty mold; branch dieback


4. Magnolia scaleAugust - September good time for crawler control

If clients have a saucer or star magnolia in your landscape, one insect to watch out for is magnolia scale. August through September is a good time of year for control, so it’s worth your time now to scout for potential problems.

Heavy infestations weaken plants, cause leaf yellowing or kill entire branches, but the first symptom usually noticed is a black moldy coating on the magnolia’s leaves. Insects feed by inserting their needle-like mouth into a stem and sucking up plant sap. They secrete excess plant sugars, which drop to foliage and branches beneath. This creates a sticky, shiny coating which is quickly colonized by a black sooty mold. Look for sticky leaves and branches, or plant parts with a black moldy coating.

The insects feed on plant stems, not on the foliage, so that’s where to look for them. Females do not move once they have found a feeding site on a stem, so insects can build up to the point that stems are completely encrusted with scale. Usually at this point the stem dies. But these insects blend into the plant so well, many gardeners overlook them even after the plant starts to have visible symptoms.

Magnolia scale is one of the largest and most conspicuous scale insects found in the United States. Adult females reach up to ½” diameter at maturity in late July and early August. Each female insect is covered by a soft, irregularly-shaped shell, shaped somewhat like a contact lens, which is shiny and light brown. By mid to late-August, the female’s shell turns white as it is covered by a thin coating of wax. Mature males have a similar shell, although smaller. They pupate under their shell in late July and early August then emerge to resemble tiny flies, which fly to the females for mating.

Females give birth to tiny, dark nymphs in mid to late August. These nymphs are called “crawlers” because at this point in their life they have legs and can move around on the plant to find a feeding site. Once nymphs begin feeding, they create a protective shell and stay in place until the males mature or the females die. But they are susceptible to insecticidal control during the crawler stage. If crawlers are not controlled, they will overwinter on plant stems and complete their lifecycle the following summer.

Since magnolias bloom in spring, one of the best ways to control magnolia scale without harming pollinators is to target crawlers in fall with a contact insecticide. Pollinators will not be present on plants in fall since they are not blooming.

Contact Spray. Horticultural oils, also known as summer oils, are a good product to use. Since they are not traditional insecticides, but instead are highly refined oils, they are very safe to use around human, pets, wildlife and other beneficial insects. For good control, it’s important to get thorough oil coverage on plant stems. Oils can be applied from mid-August until freezing temperatures occur in fall and again in early spring before the flower buds begin to swell. Commonly available homeowner products include Bonide All Seasons Horticultural & Dormant Oil Spray, Ortho Volck Oil Spray, or SunSpray Ultra Fine Year-Round Pesticidal Oil.

Use caution when applying oils in late summer; they can burn leaves if conditions are too hot or when applied to drought-stressed plants. Never spray landscape plants with a pesticide if air temperatures will reach 85° F or above during that day. Water plants well a day or two before application and wait until moderate temperatures occur to make your applications. Read and follow all pesticide label directions before use.

Systemic Control. Soil drench application of imidacloprid in early May also provides good control. Wait until flowers are finished blooming to make the application to minimize insecticide impact on beneficial insects and pollinators. Commonly available homeowner products containing imidacloprid include BioAdvanced Tree and Shrub Insect Control, Bonide Annual Tree and Shrub Insect Control, Monterey Once a Year Insect Control, and others. 

Magnolia Scale, Morton Arboretum


5. White grub rescue treatmentsMay be meeded if damage is extensive, grub numbers are greater than 5-8 per square foot and early season insectide was not applied

If grub damage is extensive in August or September, with five or more white grubs per square foot, a rescue application of either carbaryl (Sevin) or trichlorfon (Dylox) can be applied. It must be watered in for acceptable control. Moving the insecticide into the root zone requires applying one-half inch of water immediately after application. 

Managing White Grubs in Turfgrass, Purdue University


6. Fall weed controlFall best time to control perennial, biennial and winter annual weeds

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, biennial and winter annual weeds are best controlled in fall 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. newly germinated or first-year biennials are now in the rosette stage which is ideal for control,
  4. perennial broadleaf weeds are translocating stored energy (and properly applied herbicide) below ground, and
  5. cooler temperatures reduce the likelihood of injuring turf or ornamental plants.

Winter Annuals - Preemergence herbicide should be applied by early September for control of winter annuals like annual bluegrass, downy brome, goosegrass, henbit, little barley and speedwell.  Winter annuals are or will soon be germinating; preemergence herbicide should be in place beforehand so it kills the seeds as they germinate.  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.  

Biennials - Weeds with this lifecycle grow into a flat rosette of foliage by the end of the first year. These rosettes are the ideal stage for post emergent herbicide spot spray control in fall. Common biennial weeds in home landscapes include common burdock, common mullien, Damesrocket,  poison hemlock, speedwell, sweet clover, some thistles (bull, Flodman, musk, plumeless, scotch, tall and yellow-spine), western salsify, wild carrot, wild parsnip. Any of the herbicides listed below for controlling perennials weeds can also be used against biennial weeds, as long as the site is listed on the label. Make applications anytime after September 1, while plants still have green foliage. 

Perennials and germinated winter annuals - Apply herbicide beginning September 1st 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. 


7. Fall lawn fertilizationFall fertilizer recommendations have changed base on turf age

Early September is a one of the best times to fertilize cool season turfgrass like Kentucky bluegrass and tall fescue. On older lawns (10 to 15 or more years), one fall application may be all that is needed as older lawns typically only require two fertilizations per season. A fertilizer with at least a 50% slow release nitrogen source is recommended.

On younger lawns, two fertilizer applications during fall are recommended; one in late August/ early September and one about mid-October. For the first application, use a slow release nitrogen source. For the last one, use a fast release nitrogen source. Avoid fertilization after late October. Applications much past this time 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.


8. Fall lawn seedingBest time for seeding - August 15 to September 15

Overseeding is used to increase density of thinned lawns, introduce disease resistant cultivars, and to convert Kentucky bluegrass lawns to turf-type tall fescue. The optimum time to seed cool-season grasses is between August 15 and September 15. Keys to success include using quality seed, obtaining seed to soil contact, and uniform irrigation following seeding.

Quality Seed
Purchasing quality seed is essential for long-term performance of a turf area. Seed cost is insignificant compared to long-term maintenance costs (you get what you pay for with seed). Poor quality seed can lead to problems like introducing rough bluegrass, Poa trivialis to the site. If good quality seed is used, a lawn may last for twenty years or more with no reseeding if maintained properly.

Purchase seed from local, high quality seed wholesalers or retailers and purchase their more expensive seed; which virtually insures quality seed and dependable cultivars. Pay attention to label details on the seed bag such as purity, germination, and other crop seed. Know what the preferred ranges are for these details. This information is available in the publication Improving Turf in Fall.

Site Preparation
When seeding into an existing turf stand, it is important to ensure good seed to soil contact. Seeds that get hung up in thatch or other foliage can dry out and not establish properly. To achieve seed to soil contact while overseeding, mow the area to 1.5 inches or as low as feasible to reduce competition from established grass. Aerify the area, punching 20 to 40 holes/sq. ft. with the largest tines available, making at least 2 to 3 passes over the lawn. Power raking prior to aerating will also help increase seed-soil contact. Apply a starter fertilizer over the entire lawn at 1.0 - 1.25 lbs. P2O5/1000 sq. ft.

Seed with a drop seeder or a power overseeder (also called a slit seeder or slicer-seeder) which is a machine that drops seed into small grooves cut into the soil. Make 2 to 4 passes over the lawn in different directions with either type of seeder to insure uniform seeding. Use recommended seeding rates for the type of turfgrass being planted. For Kentucky bluegrass, seed at a rate of 1.5 to 2.0 pounds per 1000 sq. ft. and for turf-type tall fescue, seed at 6.0 to 9.0 pounds per 1000 sq. ft.

After Seeding Care
After seeding, lightly water the newly-seeded area as much as 3 to 4 times daily to keep the soil surface moist. Light, frequent irrigation is the rule until seedlings are rooted. Once rooted, irrigation frequency can be reduced and irrigation depth increased.

Mow frequently to limit the competition from the established turf. Mow at 1.5 to two inches until new seedlings have been cut at least two times, probably 4 to 6 weeks after seeding. After that, gradually raise the mowing height back to 3.0 to 3.5 inches where it should remain. Four weeks after germination, apply a starter fertilizer again at 1.0-1.25 lbs P2O5/1000 sq. ft. unless a soil test indicates this is not needed.

Most broadleaf herbicides should not be applied until after the second mowing of the seedlings, which may be 4 to 6 weeks after seeding. However, some broadleaf herbicides like carfentrazone (QuickSilver from FMC) can be used sooner after seeding. Purchase herbicide products accordingly and be sure to follow label instructions for your specific herbicide.

When converting KBG to tall fescue, multiple years of overseeding may be required to increase the percent of tall fescue in the lawn.

Improving Turf in Fall, Turf iNfo


9. Tree defoliatorsBe on the lookout for fall webworm, mimosa webworm and walnut caterpillars

Several tree defoliators are active in late summer, including mimosa webworm, fall webworm and walnut caterpillar. Homeowners often spot the silken webs created by the first two insects at this time of year. The webbing provides protection from some predators and the caterpillars feed inside the web until all leaves are devoured, then additional leaves are encased in the web. Webbed areas of leaves grow larger as the caterpillars mature, becoming a messy, ugly eyesore as it is filled with shed skins, excrement and leaf fragments.

Fall webworm attacks many hosts, over 85 known species of deciduous trees, including elm, hickory, pecan, plum, chokecherry, poplar, walnut and willow. In fact, almost all fruit, shade and ornamental trees, except conifers, can be affected by fall webworm. Adults of this native insect are white moths, with reddish-orange front legs and a 1.25 inch wingspan. Immature insects are pale yellowish caterpillars with red heads and reddish-brown spots. An alternate color variation among the larva is yellow-green caterpillars with black heads a broad dark stripe on the back and black spots. The caterpillars have many long, fine hairs on their backs. There are one to two generations per year in Nebraska.

Mimosa webworms are similar, but attack honeylocust and mimosa trees. As adults, mimosa webworms are dull colored moths. They have a half inch wingspan and are grey in color with small black dots on their wings. Their eggs are small, oval, and white but will turn rose-colored as they near hatching. The caterpillars are about an inch long when mature. In terms of color they are gray to dark brown with five white stripes and a brown head. If disturbed they will drop down from trees on a silken line.

Walnut caterpillar does not create a web, but feed in large groupings in walnut, butternut, pecan, hickory and on occasion oak, willow, birch, honeylocust, and apple trees. Newly hatched larvae have black heads and are generally light green, gradually changing as they grow to become reddish brown or purple with white stripes. As worms approach maturity, they darken, becoming almost black and at that time are covered with long, fuzzy, white hairs. Caterpillars eventually reach a length of up to 2 inches. Larvae behave oddly when threatened, arching their fore- and hind-legs in a defensive posture.

All these caterpillars feed on the tree's foliage, reducing it's photosynthetic capacity. Although unsightly, feeding by these defoliators is rarely seriously damaging to large trees; however, several years of defoliation for small ornamental trees can weaken them. The web impedes most insecticides from reaching the insects, unless you can catch it early. One of the best ways to get rid of them is by taking a rake and breaking up the web. Or you can try a heavy stream of water to break up the webbing. Many of the caterpillars will be knocked out of the web onto the ground, and will be killed by predatory insects.

Biological insecticides such as Bacillus thurengiensis, B.T., or Dipel are effective. Other insecticides, such as spinosad, permethrin and bifenthrin, will also provide good control. Thoroughly cover leaves next to the nest, and as the larvae ingest the insecticide they will be killed.


10. Flagging in oak treesSmall terminal twigs turn brown; variety of trees affected; culprits include twig girdlers and/or kermes oak scale

Many oak trees can be seen in the late summer landscape with many small dead twigs at the end of larger branches. There are two potential causes of this damage.

Twig girdler causes terminal leaves to turn brown; a symptom called 'flagging'. It also causes twig dieback and the girdler can attack oak, elm, linden, hackberry, honeylocust, poplars, hickory, pecan, persimmons and some fruit trees like apple. 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

Another possible cause is Kermes oak scale. Pale brown, hemispherical scales appear as large growths attached to leaf midribs and twigs. Mature scales are very tough and gall-like. Leaves become stressed, yellow, or withered, and honeydew secretions are evident. Infested trees can suffer serious branch dieback, but infestations are usually isolated to specific trees and are rarely widespread.

This scale produces a profuse amount of honeydew that covers leaves and becomes blackened by sooty mold. Ants and many other insects feed on the honeydew, and there are a number of natural enemies that help restrain populations. Homeowners often complain of sticky sap falling from infested trees.

Nebraska records have confirmed the presence of Kermes Scale since 1921. Hosts affected have been red, pin, and bur oaks, but a wide range of oaks can be infested. Counties involved in the records include Gage, Otoe, Richardson, Pawnee, Lancaster, Douglas, Dodge, Saline, Sarpy, Platte, and Buffalo counties, but this pest is undoubtedly widespread across the state wherever oaks occur.

There is one generation per season, with females reaching maturity in June. Crawlers emerge in September then migrate to buds being formed for the following year where they spend the winter.  A dormant-season spray oil from March through mid-April is an opportunity to treat. Crawlers are also susceptible to control in September with a topical insecticidal spray.

Kermes Oak Scale, Kansas State Research and Extension


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