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FAQs - Shrubs Spring/Summer

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My lilacs are not blooming very well this spring. I cut them back in the fall because they were blocking the view from my window. Did I do something wrong? And if so when do I prune them?

The leaves on my roses are starting to develop black spots on them, the leaves yellow and drop off the shrub. This happens every year, but this year it seems to be worse. What can I use to treat my roses?

Now that my forsythia and azaleas have finished blooming I need to know  when to prune them. They are growing too large. 

We have a hedge of mature Photinia (Photinia x fraseri). For the last two seasons spots appear on new leaves which cause them to wither and drop off. We started spraying with a fungicide. However, despite spraying several times this season the condition appears to be getting worse.  We would like to save them and would appreciate any advice you can provide.

My 'Knock Out' roses that are planted in well-drained loamy soil seem to be doing okay. But, the leaves have developed holes that begin as lacey whitish-tan spots. The older leaves eventually become completely eaten, leaving only veins. The flowers and stems appear OK. Is there an insect eating the leaves?  I have looked and do not see anything.

What is the latest on rose rosette disease?

My azaleas look really washed out, almost yellow. I also see tiny black spots on the undersides of the leaves. What is wrong with them and if I need to spray I want to use something that will not hurt pollinators.

My 25 year old boxwood have stems that slowly turn orangey-yellow, brown and then die. What is the cause and what should I do to stop this?

My lilac bush has this white coating on the leaves.  The leaves are curling and it looks very unhealthy.  Can you tell me what is going on with one of my favorite shrubs and what I can do to treat this problem?

My lilacs are not blooming very well this spring. I cut them back in the fall because they were blocking the view from my window. Did I do something wrong? And if so when do I prune them?

When you pruned the shrubs in the fall you pruned off the flower buds that would have bloomed this spring.  The proper time to prune lilacs is immediately after they have finished flowering.  As they age, the older woodier stems should be cut back to the ground, the newer growth can be thinned out and stems can be shortened by cutting them back to a healthy bud.

The leaves on my roses are starting to develop black spots on them, the leaves yellow and drop off the shrub.  This happens every year, but this year it seems to be worse.  What can I use to treat my roses?

Your roses have black spot, a common fungal disease. Roses get this disease every year, but due to the wet weather, the problem is worse this season. This disease is controlled by cultural methods and fungicides. In early spring prune to increase air circulation and remove crossing canes throughout the shrub, avoid overhead watering, pick off infected leaves from the shrub and clean-up leaves that fall on the ground.  Begin to treat your roses with a registered fungicide labeled for black spot as soon as the leaves begin to emerge in early spring.  It is recommended to alternate fungicides to avoid resistance. Also, rake up and replace the mulch around your roses in March when you prune.  Select disease resistant roses when planting new shrubs.

Now that my forsythia and azaleas have finished blooming I need to know when to prune them. They are growing too large. 

Pruning in the spring after blooms have faded is the proper time to prune both forsythia and azaleas.  If you wait too long or do it in the fall you will prune off next season’s flower buds. Many people use hedge clippers to shear both of these shrubs to reduce their size or into a hedge, but this type of pruning takes away from their natural, graceful growth habit. For forsythia, selectively thin out about one-third to one-quarter of the older, woodier stems by cutting them back to the ground. This allows the younger, more productive growth to develop.  Older, neglected forsythias can be rejuvenated by pruning them back to about 4 to 6inches to the ground.  The shrubs will grow back quickly and should begin blooming again in one or two years. Azaleas can be shaped and lightly pruned after bloom. If they are very overgrown and need to be severely pruned do this in late winter-early spring before they put out their new growth. You will sacrifice the flowers and it may take a couple of years for them to rebloom.

We have a hedge of mature Photinia (Photinia x fraseri). For the last two seasons spots appear on new leaves which cause them to wither and drop off. We started spraying with a fungicide. However, despite spraying several times this season the condition appears to be getting worse.  We would like to save them and would appreciate any advice you can provide.

Photinia are very susceptible to a serious disease called Entomosporium leaf spot.  This disease is not easy to control and is the primary reason we do not recommend this shrub.  Rake up the fallen leaves, especially in the fall and dispose of them. Early next spring spray them with a labeled fungicide as soon as the leaves begin to open. You will need to spray a few times, about 10-14 days apart. Fungicides prevent diseases, they do not cure them. Spraying now would not be beneficial. This will be an ongoing problem every year now that the disease is present on bark and soil debris around the plant.  A better option, in the long run, may be to remove the plants entirely and plant a hedge of several different plants. This eliminates a disease or insect ruining your entire hedge.

My 'Knock Out' roses that are planted in well-drained loamy soil seem to be doing okay. But, the leaves have developed holes that begin as lacey whitish-tan spots. The older leaves eventually become completely eaten, leaving only veins. The flowers and stems appear OK. Is there an insect eating the leaves?  I have looked and do not see anything.

The culprit is an insect called a rose slug which is the larvae or immature stage of a sawfly.  Sawflies are non-stinging members of the wasp family. Young rose slugs feed on the upper or lower surfaces of leaves between veins, leaving a piece of translucent tissue that turns brown and eventually drops out. Look on the undersides of the leaves for the small, slug-like, greenish-yellow larvae.  If the infestation is light, pick off and destroy the larvae. To control heavy infestations, use horticultural oil or a residual insecticide labeled for sawflies.  Keep tabs on your roses because feeding damage can progress quickly and can occur throughout the growing season.

What is the latest on rose rosette disease?

Rose rosette disease (RRD) does continue to escalate. Although this is not a new disease; it was first detected in the Rockies in the 1940’s, there is still much that is not known about it.  Only recently has it been confirmed that RRD is a virus. In addition to studying the disease and how it infects and spreads throughout a rose, research is being done on the eriophyid mite that transmits the disease. Researchers are also studying whether pruning can aid in the control of RRD.  Control measures include removing multiflora rose from the vicinity of cultivated roses, providing ample spacing when planting roses and monitoring for the disease. Infected roses, including the roots, should be uprooted and removed promptly. Bag the shrub and dispose of it in the landfill. Unfortunately, there is no treatment for the disease and spraying to control the mites is not practical. 

My azaleas look really washed out, almost yellow. I also see tiny black spots on the undersides of the leaves. What is wrong with them and if I need to spray I want to use something that will not hurt pollinators.

Your azaleas are infested with azalea lace bugs. They are tiny sucking insects that pierce leaves with a long, slender mouth part which they use to suck out cell contents. Most lace bugs live on the lower surface of leaves where they deposit their black eggs and excrement. To preserve pollinators, use horticulture oil or insecticidal soap. Spray the undersides of the leaves. There are several generations of lace bugs that hatch up to late summer into the fall. Begin to monitor in early spring by looking on the undersides of the leaves for the insect itself or tiny black specks. Do not spray when temperatures go above 85° F and according to the label directions. Azaleas planted in full sun, over-fertilized and drought stressed shrubs experience the most damage.

My 25 year old boxwood have stems that slowly turn orangey-yellow, brown and then die. What is the cause and what should I do to stop this?

There are several possible reasons why this is happening. Boxwood decline is a term used when the problem is caused by a number of issues that are prompted by improper cultural practices like over mulching or constant shearing. Macrophoma and Volutella are two diseases that appear most often on boxwood that are sheared year-after-year. To reduce the severity of the diseases alternate shearing with thinning out the shrubs. Open them up to air circulation to reduce constant moisture by reaching into the center of the plant and selectively pruning out some of the stems. You want to have some small voids for air and sunshine to reach the center of the shrub. Do this pruning in the late fall/winter. Vole feeding can also cause boxwood stems to die. Look for gnawing at the base of the shrubs. See (PDF) HG 52 IPM Series: Boxwood for additional information.

My lilac bush has this white coating on the leaves.  The leaves are curling and it looks very unhealthy.  Can you tell me what is going on with one of my favorite shrubs and what I can do to treat this problem?

Your lilac is suffering from a common disease called powdery mildew.  It is not a lethal disease, but causes the shrub to look unsightly.  Usually it occurs in late summer or fall on lilacs.  By this time, the plant will have stored sufficient energy to flower and leaf out the following spring. Applying a fungicide now is not necessary or effective.  To avoid powdery mildew, plant resistant lilac species such as Syringa meyeri, S. x persica and S. emodi, do not crowd plants together, and prune surrounding trees and shrubs to increase air circulation around the shrubs. 

Please send us a question at Ask the Experts if you have a shrub question you would like answered. Digitial photos can be attached to your question.

FAQs - Shrubs Fall/Winter

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