Midwest Boxwood Complex of 2023

What happened to boxwoods in the Midwest last year?! Well, it was complicated…

What a mess for boxwoods this past year. The prior winter and spring were a perfect storm to create what I would call a complex, a mix of environmental damage, fungal disease, and insect infestations. Symptoms are very hit or miss; one landscape looks terrible, and a neighboring property may look fine. Why?! I wish I knew a simple answer, but there just isn’t one. As many of you may know, that has been the hardest part of diagnostics over this past year on boxwoods. Unfortunately, we will continue to see the damaging symptoms of last year throughout this year as boxwoods continue to recover.

I was asked to look at 82 different boxwood emails, text messages, and on-site visits in 2023. The majority (87%) of boxwoods I saw on site had a mix of winter injury, Volutella leaf and stem blight, and leafminer injury. The remaining boxwood percentage had the addition of psyllid damage and anthracnose. The majority of my recommendations included total plant removal and replacement or shear, prune (clean up the shrubs a best as possible) and spray a combination of chlorothalonil and bifenthrin (depending on the time of year) followed by at least two rounds of root drenching with Foliar-Pak Grow-In.

There were many factors at play:

• 2022 Fall Pruning/Shearing/Fertilizer – If boxwoods were pruned or fed too early in the fall/winter season and new foliage had a chance to emerge, it was all damaged from the drying winter winds of late December through February.
• Winter Injury – There was little to no snow cover from December 2022 to March 2023, and winds in central Ohio in late December 2022 reached -15°F. Snow often protects small shrubs and perennials from winter winds. Even some mature dense foliage could not handle the cold temps and high wind combo.
• Disease – With so many leaves and stems damaged during the winter, diseases such as Volutella and anthracnose quickly infected the damaged tissue.
• Susceptible Cultivars/Varieties – The small leaf boxwood cultivars showed more winter injury than the larger leaf varieties.
• Earlier Than Normal Insect Activity – Leafminer adult females were mating and laying eggs almost two weeks earlier than normal and boxwood psyllid egg hatch occurred almost two weeks earlier, also. Many bifenthrin sprays missed the window. Even some of the dinotefuran (Safari) and imidacloprid drenches were not taken up quickly enough to kill the first round of nymphs and larvae.

What to Do Now?

Prune, shear, spray, drench, protect! Build the root system and foliage back! 

• Pruning/shearing should be done in mid-to-late spring to shape hedges and individual shrubs. Prune out branches with brown and yellow leaves. They indicate disease or winter damage. If sheared in late May or early June, you will also trim away early-season leafminer larvae and/or psyllid nymph damage.
• Replacement – Be aware that replacing damaged boxwoods with the same cultivar may lead to this exact situation in the future. In my experience, American/English boxwoods (Buxus sempervirens) were the most affected. Tougher cultivars include Korean boxwood (Buxus sinicia var. insularis) or littleleaf boxwood (Buxus microphylla).
• Winter protection can be accomplished by lightly wrapping shrubs in burlap or spraying an antidesiccant such as Aqualock in late December.
• Fertility – Deep root fertilizer feeding or fertilizer drenches can be done in spring and/or late fall. If you drench in the spring you can tank mix with your systemic insecticides. Stimulating new root growth will be key to boxwood recovery. I recommend drenching/deep root feeding with Foliar-Pak Grow-In, Colonise LTO, Bio 12-6-6, or a combination of these products, depending on the severity of damage. Granular fertilizer can be used but will have a much slower effect on generating new top growth and roots.
• Treat and protect from insects and disease. Insect drenching should be done in the spring followed by bifenthrin and fungicide sprays through June. If boxwood blight or box tree moth is in the area, then continue the fungicide and insecticide spray program through September/October (depending on your climate). Please see our Adavanced Turf Ornamental Disease Guide and Ornamental Insecticide Guide for more details on products to use.

List of Boxwood Issues to Watch For

Winter Injury – Since boxwoods are a broadleaf evergreen, their leaves can be injured by cold temperatures and dry winter winds. This is very common in the Midwest and may cause a bronzing leaf color, which should return to green by mid-spring. If not, prune it out.

Insect Issues

*Keep an eye on growing degree days for your area to help with preventative treatment timing.*

Boxwood Leafminer – Very common damage caused by larva inside the leaves. Soil drench with imidacloprid in early to mid-April or dinotefuran by early May.

Boxwood Psyllid – Nymphs feed in early May, causing new foliage to cup upwards. Damage can be pruned/sheared away in late May/ early June, followed by a bifenthrin or horticultural oil spray to reduce adult populations.

Box Tree Moth – Moving up from the South and defoliates in one season. Contact your state’s Department of Agriculture ASAP!

Boxwood Mites – Most damage happens in the spring but is most notable in late summer/early fall. Mites feed on upper and lower leaf surface. The damage is typically minimal. Use horticultural oil in early May or as needed.

Nematodes – Typically an issue in warmer (southern) soils. Nematodes stunt top growth and cause branch dieback and clubbed roots.

Disease Issues

*Most disease issues can be prevented by spraying recommended fungicides at regular intervals during spring and early summer. Once the disease has infected boxwoods, shearing, branch pruning, and improved air circulation are top priorities.*

Volutella Leaf and Stem Blight – Common fungal disease, bronze/yellow foliage with branch dieback and loose bark, typically from the inside-out infection. Spores on the underside of leaves may be orange or pink in wet conditions.

Macrophoma – Minor leaf spot disease, yellow/pale leaves with small black spotting.

Boxwood Blight – Symptoms can include brown leaf botches typically starting at the bottom and moving upward on branches, black streaking on stems, and white spores on the underside of leaves. The disease defoliates plants in one season. Removal of infected plants is the only treatment. Replace with resistant cultivars and use a proactive fungicide spray program in the spring and summer.

Boxwood Dieback – A disease complex of Volutella blight and anthracnose, random twig/branch dieback with bronzed or yellow leaves. 

Boxwood Decline – This is considered a complex of multiple diseases and environmental factors combined with root stress from nematodes and phytophthora.

Bridget Bergey
Sales Representative