Why Are There Yellow Or Brown Leaves On Boxwood Shrubs
They make the perfect thick, luxurious hedge, but boxwoods aren’t all they’re cracked up to be. They’re plagued with a number of problems that can result in brown or yellowing boxwood shrubs. These boxwood problems range in trouble from very easy to cure to extremely damaging. Although boxwoods can be beautiful barriers when they’re healthy, they’ll need your help to deal with whatever is ailing them.
Brown or Yellowing Boxwood Shrubs
Here are some common causes of a boxwood turning yellow or brown:
Winter Damage. If you live in a place that experiences freezing temperatures in the winter, your boxwood may have been damaged by excessive snow, ice and cold – or even winter burn. The cold-nipped tissues can take many months to become obvious, so if the yellow leaves are appearing in the spring, try not to panic unless they continue to spread. Feed and water your bushes like normal to help them recover.
Root Rot. Sometimes, the root systems of boxwood shrubs get infected with fungal pathogens like Phytophthora. When root rot becomes serious, it’ll manifest as yellowing leaves that curl inward and turn up, and the plant will grow poorly. Really serious root rot may move into the crown, discoloring the wood near the plant’s base.
Treating root rot is all about increasing the drainage around the plant’s roots, so if it’s potted, make sure to reduce watering frequency. A landscape boxwood may have to be dug and the soil around it amended to give it a fighting chance. Unfortunately, there’s no chemical intervention available for root rot.
Nematodes. The tiny roundworms known as nematodes are no strangers to boxwoods. These microscopic pests feed from plant roots, causing symptoms of general decline. Plants will yellow and wilt or even die back if root damage is extensive. You can prolong the life of these infected plants by providing plenty of water and feeding them regularly, but they will eventually succumb to nematodes. When they do, consider replacing them with nematode-resistant American boxwoods, yaupon holly or Buford holly.
Macrophoma Leaf Spot. This common fungus looks alarming when a gardener first notices it, with the yellow or tan-color leaves sporting black fungal fruiting bodies. Fortunately, even though it looks horrible, it’s nothing to worry about. If your plant is completely covered in those black fruiting bodies, consider treating it with neem oil; otherwise, the disease will clear on its own.
Volutella Blight. When large portions of your boxwood’s new growth is turning from red to yellow at the beginning of the growing season, with salmon fruiting bodies following, you’ve got a bigger problem on your hands – closer inspection may reveal that your plants have loose bark and girdling on affected branches. Volutella blight can be difficult to control, but remember that the goal is to decrease favorable conditions for fungal growth.
Trimming the boxwood by up to 1/3 will help reduce the interior humidity and remove the infected branches, which are sources of fungal spores. Make sure to remove as much of the dead growth as possible before you begin a spray program. In the early spring, before new growth has started, spray your boxwood with a copper fungicide and continue to spray according to package directions until the new growth has hardened. You may need to spray again in the late summer or fall if your boxwood adds extra growth during particularly rainy periods.
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Why are the leaves on my boxwood brown?
|Winter damage on boxwood.|
Photo: Mary H. Meyer, UMN Extension
The 2018-19 winter took a toll on these plants. When it was cold in December and January, before snow covered these plants, their leaves were frozen and killed. This was not so much normal desiccation or drying out, as it was outright death by extreme low temperatures when the plants had snow cover. The damage is just showing up now. Record snow came later after leaves were exposed and open to killing temperatures.
I planted this boxwood knot (see above image) on the north side of my home in 2004, 15 years ago. This year it shows the most winter injury ever, because normally it is covered with snow and protected from extreme temperatures.
Hardy broadleaf evergreens
Tips for plant rejuvenation
What can we do now? For my boxwood, I am waiting to see how it regrows. If the branches are alive, new buds will grow and cover the brown leaves. Pruning off the brown leaves too soon will leave misshapen plants.
Stay tuned for how my boxwood will look in mid summer. I usually prune it once a year in late June after new growth has come. This year with the late spring, no sign of new growth so far. So my pruning may be later this year, but I always do it by July 15 to allow for any new growth to harden off before yet another winter.
Can I prune it now?
If you are inpatient and not too concerned about the overall shape of your brown evergreens, you can start to prune your plants at the tips by removing brown foliage. Look for green stems and signs of life. Some evergreens you might be able to shear off the brown needles or leaves, without sacrificing the overall shape or appearance of the plants.
After mid summer and some pruning, we will all decide if the plants look too strange in shape and need to be replaced, or if we can live with the shape as they grow back to normal.
Can I prevent this from happening again?
|Photo: Mary H. Meyer, UMN Extension|
It is not realistic for me to cover my boxwood knot. Hopefully the snow will come normally and prevent this from happening in the future.
For more information see this Extension article on how to protect your trees and shrubs in the winter.
Author: Mary H. Meyer, Extension Horticulturist and Professor
YELLOWED BOXWOOD LEAVES A SIGN OF ROOT ROT FUNGUS
The boxwood sitting next to my desk had a depressingly familiar look about it. Healthy green for the most part, but with yellow patches appearing at random. I was grateful for the size of the sample, since the roots were included, though my appreciation decreased as I dragged it outdoors to examine it.
Sure enough, it suffered the bane of boxwoods, root rot. Root rot is caused by a fungus that lives in the soil and can attack over 100 different types of ornamental plants. The fungus may be carried in the roots of infected plants, or in infested soil or water. It may exist in your soil for years without doing any harm, then the right set of factors combines to initiate growth.
The fungus enters the roots and works upward, blocking the flow of water and nutrients. The first sign is leaves turning off-green or yellowish on one or more branches. At this stage, it's often blamed on the neighbor's dog. The leaves gradually turn dull yellow and die, but remain attached to the shrub. Oddly, surrounding branches may still be a bright, healthy green. Other branches yellow and die as the roots supplying them become infected, and eventually the entire shrub may die.
If this describes something happening in your yard, check for root rot by slicing into the bark near the base of the stem at ground level. Healthy wood is white diseased wood may be dark, or white streaked with brown.
Examination of the roots will tell the whole story. Dig into the root ball and take a good look. Healthy roots are white, firm and resilient. Diseased young roots are brown and decaying, and easily slough or pull apart.
Root rot may occur under good growing conditions, but there are some factors that favor the disease. Heavy, poorly drained soils and overwatering make boxwoods more susceptible. Mulches deeper than 3 inches can also add to the problem.
Research at Virginia Tech even revealed a relationship between root rot and cold weather. One group of young boxwood was inoculated with the fungus causing root rot, then kept at moderate temperatures. A second group of healthy boxwood were simply subjected to 23-degree F. temperature overnight, but not inoculated with the disease.
All of the plants exposed to the low temperature developed signs of root rot infection, but only 10 percent of the inoculated plants kept at moderate temperatures were invaded by the fungus. So following a really cold winter, your boxwood may have a higher likelihood of developing root rot.
Once root rot gets into boxwood, there is no cure and no chemical control. The fungus remains in the soil indefinitely, so replacing the dead boxwood with a new one is not practical, unless you replace the soil. Try a variety of small leaf Japanese holly instead, if you like the boxwood look.
To prevent boxwood root rot, select healthy transplants with firm, white roots.
Plant them in semi-shade in well-drained soils with a pH between 5.5 and 7.5. Avoid sites near downspouts, roof drip lines, or other wet areas. Coarse sand or perlite can also be worked into the soil to improve the drainage.
Do not plant the shrubs too deeply. The soil should come no higher than even with the original soil line when in the container.
Mulch with 2 inches of an organic material to conserve water and moderate soil temperatures. Transplants will need frequent watering, but established boxwood only need a thorough watering about every 10 days.
This should be continued until freezing weather. Prevent stress by feeding with cottonseed meal, bone meal or 10-10-10 in early spring, then again in June.
A very common problem of boxwood that causes poor growth is feeding damage from a mouse-like animal called a vole. Voles will kill stems and roots by stripping the bark near the base of the plant. Vole damage can be diagnosed by gnawing injury on stems and roots, as well as the evidence of runways, and small holes in the surrounding areas near plantings. Voles can be trapped with mouse traps set at right angles to the runways baited with fresh fruit pieces or peanut butter.
Boxwood prefers a neutral soil pH (6.2-7.5) and requires adequate drainage with ample amounts of organic matter.