Author:Miri – Maryland Grows

Q&A: Why Do My Boxwoods Look Off-Color?

October 24, 2025Maryland GrowsLeave a comment

Boxwood leaves that are yellowish and blistered due to Boxwood Leafminer feeding. — Yellowing of boxwood leaves caused by Boxwood Leafminer feeding. Photo credit: Miri Talabac

Q: I have a low hedge of boxwoods that are looking somewhat orange-tinted and less dense than usual. What might be causing this?

A: Boxwood Leafminer is my primary suspect, and I have recently seen boxwoods looking just like you describe, which had high populations of this common insect pest.

With only one generation per year, this species of fly is currently in its larval (juvenile) stage inside the leaves. Since larvae feed on tissues between the upper and lower leaf layers, you need to peel the leaf apart to find them to confirm their presence. Outwardly, affected leaves will become blistered-looking and have a blotchy yellow or orange-yellow color. Leafminer damage can also cause the shrubs to shed some leaves prematurely, so the branches will have more sparse growth towards the plant’s interior than a healthy shrub would have.

Boxwood Leafminer larvae are tiny orange-yellow legless maggots, only a few millimeters long. However, they are readily visible if you open a leaf and examine it with a hand lens, since the rest of the leaf interior will be light green. They will pause feeding as winter approaches and overwinter in a state of suspended development, resuming more voracious feeding in early spring. Larvae pupate and transform into orange gnat-like adults by late April or early May (depending on temperature trends).

Boxwood leaf tissue has been cut away to reveal Boxwood Leafminer larvae feeding between the upper and lower leaf surfaces. Photo credit: John Davidson, Ph.D., UMD

Some boxwood cultivars have greater resistance to leafminer (as well as to fungal diseases) than others. When the insects are too abundant for their damage to be tolerable, prune out the infested branch tips or, as a last resort, consider using an insecticide to reduce their numbers. Keep in mind, though, that insecticide use can have unintended consequences: it might facilitate an increase in boxwood spider mites (by killing mite predators); harm pollinators visiting the boxwood’s flowers; or harm insect or spider predators wandering its foliage that would be eating pests.

You can learn more about leafminer as well as other common boxwood ailments on Extension’s boxwood diagnostic web page (go.umd.edu/boxwood]).

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.

Have a plant or insect question? The University of Maryland Extension has answers! Send your questions and photos to Ask Extension. Our horticulturists are available to answer your questions online, year-round.

Q&A: What Damaged My Hydrangea?

April 24, 2025Maryland Grows2 Comments

New green hydrangea leaves with brown spots and edges caused by cold temperatures. — Cold night temperatures caused the edges and tips of these new hydrangea leaves to turn brown.

Q: The new leaves on my hydrangea shrub recently turned brownish on the tips, and they look a bit withered. What can I do to save it?

A: The shrub is probably fine and you shouldn’t need to do anything for now. It’s likely cold damage, from overnight temperatures below or near freezing. Bigleaf hydrangea (Hydrangea macrophylla) tend to be particularly eager to leaf-out as soon as the weather warms in spring, often too early to avoid frost damage in years with fluctuating temperatures. This inconveniently early emergence is encouraged when plants grow near warmth-retaining materials like a stone retaining wall, solid fence, or house wall.

Give the shrub time to produce more new growth to cover any eyesore leaves. You don’t need to prune damaged leaves off, but you can if they’re too bothersome, once the plant is fuller with more growth. Be aware that, once dormancy is broken, flower buds on the stems are also vulnerable to frost/freeze damage, and that might not be as evident as the singed edges or distorted-leaf symptoms on expanding foliage. If the shrub doesn’t flower this summer, it could be due to spring cold injury, or branches that were pruned back before determining if the bare tips were actually dead.

In the future, you can try using an old sheet or pillowcase (any fabric; avoid plastic) to cover the plant for the night if the forecast calls for temperatures close to freezing. Garden centers that have the ability to protect tender plants can tip pots over and cover them with a thick horticultural felt, which gives the plants a few degrees of protection due to the cover blocking frost and trapping some warmth radiating off the ground. (You can do this with potted plants that have broken dormancy too early, but in home settings it’s more practical to just move them temporarily closer to a sheltered wall instead, or put them in a shed or garage for the night.)

New green hydrangea leaves with brown spots caused by cold temperatures. — Brown leaf spots on new hydrangea leaves from cold weather

New green hydrangea leaves that are turning a dark purplish color and are wilting due to severe cold temperatures. — New hydrangea leaves collapsing from cold injury. Photo credit: Debra Ricigliano

Q&A: What Can I Grow in Place of Dogwood?

February 20, 2025Maryland Grows1 Comment

Photo of a fringetree in full bloom with white flowers in spring. — Lovely Fringetree flowers in spring

Q: I have to remove a declining Flowering Dogwood, and would like to replace it with another white, spring-blooming small tree. Crabapple doesn’t appeal to me, and I already have a Magnolia. What would you recommend?

A: I think Fringetree (Chionanthus virginicus) is underused and quite showy. White is a common flower color for spring-blooming trees, native and otherwise, but for some reason this species is often overlooked. Growing wild essentially state-wide, it’s adaptable enough to handle home landscapes well if given good drainage and a half-day to full sun. Established plants have some drought tolerance, and grow at a similar pace overall to Flowering Dogwood (that is, slow to moderate, averaging a foot per year).

A member of the olive family, Fringetree is a cousin to Ash, and as such might be vulnerable to Emerald Ash Borer, the invasive insect that has decimated Ash trees region-wide over the past two decades. However, I think their susceptibility is low enough to give them a try, as I have not learned about any rampant problems with the beetle seriously damaging Fringetree, especially with the loss of most of the local Ash population’s mature trees.

Photo of a mature fringetree in a landscape. The tree is about 8 ft. tall with a spread of about 15 ft. — A mature Fringetree (*Chionanthus virginicus)* in a landscape.

This is a photo of a dwarf fringe tree that is less than 5 ft. in height. The variety is 'Little Leslie.' — ‘Little Leslie’ is a dwarf cultivar that grows to a maximum height of 5 ft.

The olive-like dark blue berries produced by female Fringetrees are decorative and appealing to birds, but sexing trees is difficult unless they were propagated as cuttings from a tree of known sex. A lone female tree may not fruit if there are no males within pollinating distance, and male trees can’t fruit. Occasionally, a female tree might self-pollinate, but this is not the norm. If you have room, plant two and take a chance of having a pairing for pollination, or just enjoy the flowers (showy on both sexes), which are also scented.

Closeup photo of long, slender Fringetree flowers. — Fringetree flowers

Photo of a cluster of small purple fringetree fruits. Each fruit is about 1/2-in. in diameter. — Fringetree fruits are eaten by many different bird species.

Other candidates for the criteria of white spring-flowering trees with a maturity smaller than a shade tree include: Serviceberry (Amelanchier species); Redbud (Cercis canadensis; there are a couple white-flowering cultivars); Hawthorn (Crataegus viridis ‘Winter King’ being widely available); American Plum (Prunus americana); Chickasaw Plum (Prunus angustifolia); Carolina Silverbell (Halesia species); and Snowbell (Styrax species).

Q&A: Are These Red Leaves Too Stressed?

December 20, 2024Maryland Grows2 Comments

Photo of several plants in a terrarium. The focus of the photo is a plant with oval leaves that are red with white veins. This plant is Dischidia ovata, commonly called watermelon dischidia or hoya. — *Photo credit:* Miri Talabac
*Photo caption:* *Dischidia ovata* in a terrarium under intense light.

Q: I have a Hoya whose leaves have turned quite red. While pretty, I want to make sure I’m not stressing the plant too much. How can I tell?

A: If growth is progressing normally and there aren’t any other concerning symptoms (like leaf drop beyond the typical shedding of old leaves, or stunted growth), then I would say you’re at a tolerable level of stress for the plant. Reddening foliage is sometimes referred to as “sun stress” by houseplant growers, and it’s the plant’s natural response to intense light by generating sunscreen pigments to protect the leaf tissues.

Some species or cultivars of Hoya and their Dischidia cousins can turn ruby-red or rose-blushed if grown in bright light. I’ve grown cuttings propagated from a single Watermelon Dischidia, Dischidia ovata, in different light levels, and some will stay green in lower light and others turn varying degrees of red in higher light. In one case, under a bright spotlight over a terrarium, the plant turned completely ruby on every surface exposed to light. Even across a single leaf, areas in shadow can remain green while the rest blush red, as they did when I peeked beneath one leaf that was partially covered by another.

I have a ‘Sunrise’ cultivar of Hoya in a pot hanging underneath the edge of LED grow lights. The half of the plant growing close to the lights is red, and the half receiving much less light on that outer half is green. If anything, I’d say that the redder side grows more vigorously.

Other houseplants can also redden if grown at the higher end of their light level preferences, such as certain begonias and bromeliads. In other cases, over-lit foliage bleaches to a paler or yellower color, so not every indoor plant can adapt to stronger light.

Sometimes you just might need to experiment to see how a plant reacts, and keep in mind that other factors (nutrient availability, soil moisture level, ambient temperature and humidity) which might be causing plant stress can complicate your assessment of how a plant is faring based on light level.

Q&A: What is a Good Substitute for Spruce?

September 18, 2024September 17, 2024Maryland Grows2 Comments

blue atlas cedars in front of a building — Blue Atlas Cedars on the American University campus in Washington, D.C.
Photo: M. Talabac

Q: I have always loved the look of blue spruces, and I see many Maryland nurseries stocking them. I’ve heard that I should consider alternatives, though. Why?

A: Although every plant has something it can be vulnerable to (pests, diseases, environmental stress) and nothing is risk-free, Colorado spruce (Picea pungens), the species with the popular blue-needled cultivars, struggles in Maryland growing conditions. As such, it is at risk for several problems that cause dieback and significant aesthetic damage. Overall, spruces as a group are not well-adapted to high summer heat and humidity, or compacted or clay-dominated soils common to urban or suburban landscaping.

No spruces are native to the state except for the tip of western Maryland, where the locally rare red spruce grows in the mountains. (Maryland Biodiversity Project notes that prior logging and wildfires reduced this already limited population.) You can see from the spruce species range maps on the Biota of North America Program website that no other spruce species grows wild anywhere near Maryland.

Due to the particularly damaging nature of the issues spruces can develop here, primarily from fungal infections — drastic needle browning and shedding, plus branch dieback — I suggest using alternatives if you want a large-statured, pyramidal, and/or blue-leaved evergreen specimen tree. This spruce decline issue is prevalent enough that it has its own University of Maryland Extension web page, “What’s Wrong with my Colorado Blue Spruce Tree?”

A substitute plant won’t necessarily look the same (it might have broader or looser branching, or won’t have blue foliage), but it can have a better chance of thriving. Your options will depend on the site conditions, including how much space you have for the plant to mature, though your best range of choices will be for a location in full sun (in summer) with good drainage and no deer browsing pressures.

Few locally native species are evergreen and large-statured, but they include American holly (Ilex opaca), Eastern redcedar (Juniperus virginiana), and Atlantic whitecedar (Chamaecyparis thyoides), the latter of which is rare, unfortunately hard to find for purchase, and limited to the coastal plain in the wild. Several species of pine grow wild throughout Maryland, but their mature habit is quite different from spruce.

Eastern redcedar tree — *Left: Eastern redcedar (Juniperus virginiana), Right: American holly (Ilex opaca). Photos: M. Talabac*

American holly tree — *Left: Eastern redcedar (Juniperus virginiana), Right: American holly (Ilex opaca). Photos: M. Talabac*

Non-native candidates include other holly species or hybrids, Japanese cedar (Cryptomeria japonica), true cedars (Atlas Cedar, Cedrus atlantica, and Deodar cedar, Cedrus deodara), and Western arborvitae (Thuja plicata). Arizona cypress (Cupressus glabra) and atlas cedar come in silvery-blue forms if you want a foliage color similar to blue spruce. While leyland cypress (Cupressocyparis leylandii) is commonly planted, it suffers from some serious shortcomings that can make it short-lived and prone to fungal branch dieback.

Arizona cypress tree — *Smooth Arizona cypress (Cupressus arizonica glabra). Photo: M. Talabac*

Whichever plants you try, use a mix of species if you’re growing them in a group, like a screen or living fence, rather than mass-planting only one species or variety. This helps the planting to resist future pest, disease, or weather stress problems, since different species have different tolerances and vulnerabilities. That way, a future issue won’t be likely to impact them all equally severely, like a fungal needle cast infection wiping-out a row of blue spruce.

Q&A: What Causes Patchy Wilting?

August 15, 2024August 13, 2024Maryland Grows1 Comment

*Sometimes individual stems wilt on otherwise healthy perennials. Photo: C. Carignan, University of Maryland Extension*

Q: Why would my perennial Lobelia be only partially wilting? I keep an eye on watering so it doesn’t get too dry.

A: This can often be the work of an infection or, less commonly, stem-boring insects. Since stem-boring insects in perennials are rarely a cause for concern, I’ll focus on the pathogens.

Sometimes it’s hard to diagnose the cause of wilting with certainty since distinctive symptoms identifying the culprit aren’t always visible; there’s a lot of symptom overlap. During summer, heat-loving fungi like Southern Blight can cause wilting because the pathogen kills the stem tissue close to the soil line, cutting the top growth off from its water supply and causing it to collapse. A wide variety of fungi and fungus-like “water molds” can cause above- or below-ground infections throughout the year, with some prospering in cool conditions while others need heat. Wilting when the soil isn’t dry can be a telltale sign of infection or physical stem or root damage.

To reduce vulnerability to diseases like Southern Blight, plant crowns need to get good airflow. (The crown is where stems emerge from the root system.) Avoid over-mulching, which covers up or crowds the crown and stem bases; nudge it a few inches away so it’s not piled against the stems. High humidity and dew are sometimes enough moisture for fungi to survive and begin infections, but over-watering plants by irrigating too often can make fungal outbreaks more likely. (This includes lawn grasses, if sprinklers are run frequently.)

Mycelium, which is the “body” of a fungus, is sometimes visible as white webbing or threads covering the infected part of the plant, but it’s not always prominent. (It is usually noticeable with Southern Blight.) Any wilting or collapsed stems need to be removed, since they will not recover. Prune them off and dispose of the debris; don’t compost it.

One or two wilted stems doesn’t necessarily doom the entire plant patch or garden bed, but keep an eye out for worsening symptoms like more rampant dieback. No fungicides can reverse this damage or eliminate spores from the soil permanently. Nor would you want to attempt chemical soil sterilization, as plenty of beneficial fungi, bacteria, and other organisms inhabit the soil. Many of them can out-compete, directly kill or consume, or otherwise stifle the development of pathogens like these. Fungicide impacts are not limited to the pathogens, and some could even have harmful consequences for pollinators.

If Southern Blight is responsible, you can dig out and dispose of the affected plant, plus remove a few inches of the surface soil with it. This is not because the plant’s roots are going to also be infected, but because plant regrowth could be infected again by the spores remaining on the soil surface, and you don’t want to give the fungus more fuel for expansion. If you dig out soil but don’t want to throw it out (soil is a precious resource, after all), just bury the layers with potential spore contamination to a depth of 6 inches or so, where the spores should not survive.

Southern Blight, as with many similar blight and rot diseases, have an extensive range of plants they are capable of infecting. Why, then, don’t we see rampant die-offs in cultivated and wild plants for such common and destructive fungi? Interactions between organisms are complex, and as with insect diversity, there are microbe species which are beneficial, especially in healthy soil that is not too compacted, oxygen-deprived, wet or poorly drained, or low in organic matter (all conditions that favor common pathogens).

Q&A: Reduce the Height of an Arborvitae?

July 16, 2024July 16, 2024Maryland GrowsLeave a comment

Arborvitae (*Thuja* sp.). Photo: Jason Sharman, Vitalitree, Bugwood.org

Q: I need to reduce the height of an arborvitae…. I might take around 5 feet off the top because it’s too big. When should I prune?

A: While late winter or early spring is generally a good time to prune conifers (needled evergreens), in this case timing won’t matter much, because the amount you want to remove is more than the plant can handle. Instead, it may be better to replace the plant with a smaller-growing option.

Most conifers, including arborvitae, do not regrow foliage when it’s lost due to heavy pruning, deer browsing, or crowding from being planted too close together or too close to a wall or fence. Unlike broadleaf evergreens (boxwood, holly, euonymus, etc.), they don’t have dormant buds along the older stems, lying in wait to grow if the branch or foliage beyond them is removed.

Once the foliage is gone and bare wood is visible, it’s not coming back on that part of the plant. This is the reason why deer-browsed arborvitae are easy to spot, developing a shape sort of like a stemmed flute glass, because the parts the deer can reach become stripped of foliage and never fill back in again, even as the out-of-reach tops get wider.

As conifers age, it’s perfectly normal for the innermost branches to become quite bare, as those older leaves shed over time. They are deliberately jettisoned by the plant because they are being progressively shaded by the outer shell of live growth, so they cost the plant more to keep alive than the meager photosynthesis energy they get back. This will be exacerbated if the plant is sheared, where the foliage tips are lightly trimmed to give the plant a more manicured look, because that makes the layer of foliage casting shade on the plant’s interior even denser.

Pruning cuts that take off that outer layer or shell of younger growth on the branch tips will result in permanent bare areas. Once the foliage is gone from that inner wood, it will not regrow, even if sunlight now reaches the interior due to pruning cuts. This also applies to the main leaders. Reducing the height of a tall arborvitae will stunt the top growth and give it a permanent gap or flat-looking top. If this isn’t really visible from where you typically view the plant, then it’s not necessarily a problem (assuming the pruning cuts seal-over well and don’t develop wood decay). Otherwise, nothing will give the plant its former shape back.

Fortunately, there are lots of compact and dwarf conifer varieties on the market these days. A couple conifer types, like yews, will be able to rejuvenate after heavy pruning. Even so, it’s still best practice to select a plant that should fit in a given space in the yard without relying on pruning to make it fit.

I acknowledge that there are many gardeners that inherit poorly-chosen plants in their new yards, but if or when it comes time to replace them, research your options to make sure you won’t run into the same problem down the road. Plants never really stop growing, though older specimens can slow down. Due to a reduced growth rate, dwarf and miniature cultivars will stay much smaller over the same amount of time as their full-size counterparts, even though some dwarf cultivars can also get larger than you’d expect a few decades after planting.