ProHort Update for August 2025

1. Iron Chlorosis in Trees

Iron chlorosis is a problem for many trees and our clientele often notice it this time of year when the leaves of their trees turn more of a yellow-green color than the deep green they are used to. Iron chlorosis can be corrected through cultural practices as well as iron treatments or sulfur treatments around the tree.

What is Iron Chlorosis

Iron Chlorosis is simply a lack of iron in the plant. Iron chlorosis shows up in many of our plants as lighter green leaves, the veins of the leaves are typically dark or regular green colored while the rest of the leaf is more like a lime green or medium yellow color. As it progresses, the leaves may develop brown tips or browning around the edge of the leaf. If left untreated, it can cause branch dieback or even tree death after a few years of the deficiency. This is a common issue in Pin oak trees in southeast Nebraska, but it is often found in many plants including maples, river birch, and big leaf hydrangeas. 

How pH affects that

Southeast Nebraska soil tends to have a high pH, the soils are very alkaline. Alkaline soils change the availability of certain nutrients to plants. According to the Utah State University, even though these soils typically have plenty of iron, the high soil pH causes a chemical reaction that makes the iron solid and unavailable to plant roots. Compacted soil can make this situation worse for the plant as well. 

Could it be more than just pH

Soil pH is typically blamed for the iron chlorosis problem we face, however, it could also be from excess fertilizers. High phosphorus or high nitrogen can cause these problems as well. High levels of these nutrients can bind up the iron in the soil, preventing it from being available for tree roots to absorb it, according to Sarah Browning, Forester, Nebraska Forest Service. 

According to Linda Chalker-Scott, horticulturalist from Washington State University, high phosphate levels in our soil can be detrimental to iron uptake in trees. Phosphate blocks the binding sites for iron to enter tree roots which causes an overall decrease of iron in trees. 

Linda Chalker-Scott also shows that high phosphate also decreases mycorrhizae activity around tree roots. Mycorrhizae is simply defined as fungal root. It is a symbiotic relationship between the roots of plants and the fungus. Both the fungus and the plant benefit from the relationship and neither is harmed. The fungus does not photosynthesize, so it gets food from the plant. In turn, the fungus can more easily access nutrients from the soil that it passes along to the plant. Without mycorrhizae, the tree has to work harder to find and access iron. 

How to correct Iron Chlorosis

We can help reduce iron or manganese chlorosis by first doing a soil test to check the current levels of phosphorus and nitrogen in your soil prior to applying any fertilizers, including those on the lawn because tree roots will absorb nutrients from fertilizers applied to the lawn as well. This will help you determine what nutrients, if any, are necessary to add to your soil. If phosphorus is in high supply, as it usually is in landscape soils, choose a nitrogen fertilizer for your lawn that does not include phosphorus. 

You can also treat iron chlorosis by applying sulfur to the soil to help lower the pH, making the iron more available to the tree. This is going to take a lot longer to see results than some of the other methods. Chelated iron can also be applied to the soil to improve uptake in the tree, but it doesn’t last very long and can be quite expensive. Trunk injections or implants of iron can be inserted directly into the tree trunk but it does damage the tree every time you drill a hole in it. The injections or implants the injections or implants are the quickest acting method of treating for iron chlorosis.  Overall, treatment is a balance between effectiveness, speed of recovery and potential damage to the tree. 

For more information on the methods of treating iron chlorosis in your clients trees, view this article from the Nebraska Forest Service

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2. July 29th growing degree days (GDD)

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3. Pest update

For a more complete list, visit Michigan State University GGD of Landscape Insects or GGD of Conifer Insects.

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4. Irrigation Do's and Don'ts, Michigan State University Extension

In mid-summer, in a period of high evapotranspiration, new tree and shrub plantings can struggle to stay alive.  Replacing the water that is removed by atmospheric pull will help ensure successful establishment.  There are several techniques for effectiveness for watering both new and moderately sized trees including applying a long, gradual soaking rather than several light applications per week, watering beyond the drip line and under the canopy as well as at the base of a tree, calculating the amount of water to apply based on crown spread and seasonal demand and mulching properly after application.

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5. Diversity in the Landscape

Everyone has their favorites for a reason, but too much of a good thing can quickly turn into a bad thing. We see this far too often in standard landscape plans with large swaths of the same type of plant, or multiple landscapes with many of the same plants. This is because they are the “tried-and-true” plants that deliver exactly what the customer wants without too much fuss, or maybe it’s a new specimen that someone wants because they like what they saw in someone else’s yard.

Having a lot of just a few types of plants leaves an entire landscape vulnerable to potential issues. If insect or disease pressures take hold, many plants are lost and big visual holes are left behind.

Plant diversity in landscapes provides options for visual appeal and general resilience. While specimen plants help with ‘pop’, integration with multiple types of plant, especially North America natives, gives a depth of texture, multi-season bloom and resistance to all of the plants crashing out at the same time.

Issue-prone maples and crabapples can be buffered by oaks, redbuds or serviceberries; or interplanted with shrubs like viburnum, coralberry or button bush.

Instead of focusing on summer coneflower or black-eyed Susan pop, include spring-blooming Amsonia (bluestar), late-summer to fall-blooming Liatris (blazing star) and some mid-summer-blooming Virginia mountain mint.

Hostas have plenty of issues, so break up slug-eaten foliage with native wild geranium or shade-tolerant blue-stemmed goldenrod.

Natives aren’t always readily available at national chains or even local nurseries, but the more they get requested, the more they are bound to show up. More and more places are starting to showcase native sections, because more and more, it’s what customers want.

For plant ideas, information and sourcing, check out these Nebraska institutions and companies:

• Nebraska Statewide Arboretum for information
• Great Plains Nursery for trees and shrubs
• Midwest Natives Nursery for herbaceous plants
• Prairie Legacy for herbaceous plants

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6. Magnolia Scale

Magnolia scale, Neolecanium cornuparvum, is one of the largest scale insects found in Nebraska and is beginning to show up on saucer magnolia. Like all scale insects, they are sap-suckers and often go unnoticed until signs of infestation appear—typically the sticky honeydew they excrete, which can be found on surfaces beneath the plant. This honeydew can also promote the growth of sooty mold on leaves and stems, giving them a dark, dirty appearance.

In Nebraska, there is one generation per year. The overwintering stage is the immature nymph, which remains on stems until new crawlers emerge in late July through early August. Like other scale insects, they are most vulnerable to contact insecticides during the crawler stage—usually from mid-August through September.

More information on Magnolia Scale can be found in this article. 

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7. Mimosa Webworm

Mimosa webworms, Homadaula anisocentra, are the larvae of a steel-grey moth. These webworms will feed on a few different trees including the mimosa tree and honey locust. They have two generations a year and I believe this is the first generation that we are seeing. The second generation usually starts in August and September. The second generation will overwinter inside of cocoons in the webbing or other protected areas. 

Damage can be moderate to severe depending on the health of the tree. The webbing is waterproof making it challenging to spray a product to kill the caterpillars. Natural predators help to a degree. Systemic products might provide some control. 

More information on Mimosa Webworm can be found in this Nebraska Forest Service publication.

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8. Winter Annual Weed Control in Turf

Use Fall-applied preemergence herbicide to kill seedlings. Preemergence herbicide should be applied by early September for control of winter annuals like annual bluegrass, downy brome, henbit, little barley and speedwell. Winter annuals will soon be germinating and preemergence herbicides should be in place before germination begins as they kill seedlings after 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 weed control includes maintaining a healthy and vigorou growing lawn or landscape bed plant cover to crowd out weed invaders. A layer of organic mulch 2-3 inches thick will also minimize weed germination.

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9. Commercial/Non-commercial pesticide applicators

If you have a pesticide applicators license which expired in April 2025 or you need to get a new license, testing options are listed below.

Testing-only Options

• Closed-book exams are given by the Nebraska Department of Agriculture (NDA). Preregistration is not required and 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.)

2025 Commercial/non-commercial training was held from January through April. Training schedules are available at https://pested.unl.edu/. Help your employees be successful at getting a license by purchasing study materials. 

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10. Digital Diagnostic Network - Need help with diagnostics?

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