Q&A: Why are lilac leaves brown and curling?

Irregular brown spots and blotches appear on lilac leaves, followed by leaf curling and defoliation in late summer. Photo: UME Ask Extension

Q: My lilacs look like death-warmed-over this time of year. Do you know what’s wrong, and is there anything I can do at this point?

A: Lilacs are sadly not very well-suited to our mid-Atlantic conditions. We’re at the southern edge of their heat tolerance, so while they weren’t among the best flowering shrub choices to begin with, climate change is only going to worsen their prognosis. Several types of leaf-spotting fungi and bacteria, plus general heat stress (which also increases their vulnerability to borers), results in foliage that looks quite beat-up by late summer. Brown spots, crispy leaf edges, and bare stems from premature leaf drop are all typical. You can explore lilac ailments and their management on our lilac diagnostic page.

No fungicide will reverse these symptoms once they appear, and while they might work as a preventative if applied before bud-break (and re-applied repeatedly well into the summer), it’s simpler to just grow something else if a plant is going to be that much of a hassle. This is especially true if the treatments don’t work and the plant still winds up looking horrible. Fungicides also carry the risk of harming other organisms.

For now, you can rake up and dispose of any fallen leaves, though this isn’t a foolproof way of removing a source of infectious spores. Cut down the oldest, thickest stems this winter (they tend not to bloom well at that age anyway) and open up the canopy by selectively removing some stems that contribute to foliage crowding. You can do this thinning after bloom next spring.

For anyone really wanting to grow lilac despite these challenges, try cultivars with above-average disease resistance and heat tolerance. While not immune to problems, they perform much better, even if they don’t look exactly the same or have blooms as large or heavily perfumed. ‘Miss Kim’ is a round, compact-growing cultivar with pale lavender-purple flowers that’s been around for decades. Other varieties are now available with pink or deeper purple blooms, some of which even rebloom a bit, sporadically producing flowers into summer and early autumn, though high heat could still hamper that.

dwarf lilac with lavender flowers
Dwarf lilac species and hybrids handle Maryland conditions much better than the traditional varieties. Some recent introductions will also re-bloom sporadically later in the summer. Photo: M. Talabac

All lilacs, but especially the traditional, classic “French” types, should be planted in a location with great air circulation (so, not up against a fence or wall) so wet leaves dry quickly after rain, dew, or irrigation. Wet foliage is more easily infected by pathogens.

The main perk of growing lilacs is fragrance, so if you want a scented replacement, consider: Winterhazel (Corylopsis), Koreanspice Viburnum (Viburnum carlesii) and its hybrids, Summersweet (Clethra), Seven-son Flower (Heptacodium), Carolina Allspice (Calycanthus), various deciduous Azaleas (Rhododendron viscosum and several others), Mockorange (Philadelphus), and Fragrant Abelia (Abelia mosanensis). Their scent characteristics, flower colors, mature sizes, and preferred growing conditions may differ from lilac, but nothing is going to be an exact substitute. Plus, several of these species will offer the additional bonus of showy autumn foliage or (for the native ones) better wildlife value. These are just some shrub ideas; there are also fragrant perennials and, if you have the room, several fragrant trees.

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

Have a plant or insect question? University of Maryland Extension has answers! Send your questions and photos to Ask Extension.

Q&A: Why do my hollies and other evergreens have brown and pale leaves?

holly with leaves showing sections of brown and pale color
Winterburn symptoms on holly. Photo: David L. Clement, University of Maryland Extension

Q: Several of our evergreens (different kinds) have brown or pale, bleached-looking leaves. Do they have a disease already, and can anything be done? Is it preventable in the future?

A: Most likely it’s winterburn, especially since most infectious diseases won’t cause symptoms this early and seldom impact several unrelated plants to the same degree. Winterburn is an abiotic disorder or injury – abiotic translates to without (a-) life (biotic) – meaning the condition has a non-living cause. Abiotic plant disorders are environmental, and causes include wind, water, temperature, and soil pH. In comparison, biotic factors would include insects, mites, fungi, or bacteria.

No treatment is recommended because the damage has been done, but winterburn is rarely a serious threat to a plant’s long-term health. As new growth resumes, the plant will eventually shed the damaged leaves. If it’s too much of an eyesore, you can selectively trim away the worst of it this month. Causes for winterburn typically involve a combination of cold temperatures, wind, and exposure to sun. Any autumn pruning that results in tender regrowth is priming a plant for winterburn, which is one reason it’s not recommended.

Why are cold-hardy evergreens damaged? Leaves “breathe” through tiny pores on their surface, and this gas exchange also allows water vapor to leave the leaf. Moisture leaves our bodies the same way – picture foggy breath on a cold day. Breezy days, especially in winter’s drier air, speeds-up this evaporation, as can the sun’s weak warmth. Meanwhile, during cold snaps, moisture in the surface layers of soil freezes, which prevents roots from absorbing it. Since the plant cannot replenish all of the moisture it’s losing, the leaf tissue starts to essentially freeze-dry. A thaw won’t reverse the damage because the cells have been injured, just like skin with frostbite. (Unlike our skin though, which can heal to an extent, leaf tissue can’t repair itself.)

Broadleaf evergreens are more vulnerable to winterburn than needled evergreens because the leaf surface area and evaporation potential is so much greater. Younger plants also have greater vulnerability because they are still establishing roots. This is the main reason it’s risky to plant evergreens late in the fall. Cherry laurel, boxwood, holly, rhododendron, camellia, and southern and sweetbay magnolias are common winterburn victims in our area. Plants kept in containers are also susceptible because their roots dry faster and experience more drastic temperature swings than they would in the ground.

The only actions you can take to minimize winterburn risk is to site evergreens out of the brunt of winter winds and to periodically monitor their root zones for moisture, irrigating when dry during a warm spell. Plants overwintering in pots can be sheltered a bit near a wall or windbreak, but don’t bring them inside as the interruption of dormancy may detriment their health.

Learn more and see additional photos on the Home & Garden Information Center website: Winter Damage on Landscape Plants.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun. 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.

How to plant a container-grown tree – Featured Video

Late-winter through mid-spring and early through mid-fall are the best times to plant woody ornamentals, so this is a good time of year to start planning your process if you are interested in adding shrubs or trees to your landscape this year.

Take a look at this video showing you how to plant your container-grown tree, and for more information, view the HGIC page on the planting process.

Q&A: What’s wrong with my cherry laurel shrubs?

cherry laurel
Cherry Laurel with Leaf Scorch Symptoms and Holes. Photo: University of Maryland Extension / Ask an Expert

Q: My cherry laurels do not look good. There are brown spots and holes on the leaves and white stuff on the trunk. What can I do?

A: First, there is a lot to like about cherry laurels (Prunus laurocerasus). They are popular evergreen screening and foundation plants, deer-resistant, and pretty tough once established. Cherry laurels make their best growth in moist, well-drained soil in full sun to partial shade. They even tolerate full shade.

We receive numerous questions about problems with these shrubs. They can have multiple issues that are attributed to environmental, climate, or site conditions. When the plants are stressed, they weaken and become susceptible to diseases and/or insect pests. It is helpful to be aware of these issues before planting them. Continue reading

The Nativar Dilemma: The Case of My Purple Ninebark & The Leaf Beetle

Physocarpus opulifolius 'Diabolo'
Ninebark ‘Diabolo’. Photo by F.D. Richards. Source: Flickr Creative Commons

I thought I was doing the right thing. When I moved into my house 11 years ago, I found a purple barberry shrub (Berberis thunbergii) planted in the backyard by the previous owner. I thought right away, it had to go. I knew Japanese barberries, so commonly planted in landscapes, were escaping into natural woodland areas and creating dense thickets to the exclusion of native plants. These thickets have been shown to make suitable habitat for Blacklegged Ticks. I wanted no part in contributing to that situation, so I donned my work gloves and removed that prickly beast of a shrub.

In the barberry’s place, I planted a “native” purple-leaf ninebark, Physocarpus opulifolius ‘Diabolo’. It had deep burgundy foliage that made a nice replacement for the burgundy-toned barberry. And, I was selecting a Maryland native plant. I thought it was a perfect choice.

I sang praises about this ninebark for years when people asked me for native plant recommendations. It has great spring blooms, beautiful foliage color, and I never had to prune it. And native plants support native insects so I was doing a good thing to help wildlife. I was doing the right thing!

Or so I thought.

It was just this year that I learned from my colleague, native plants specialist Dr. Sara Tangren, that this particular cultivar of the native ninebark was actually detrimental to a specific native insect, the Ninebark Leaf Beetle. My heart just sunk when I heard this. I am first and foremost a plant enthusiast, but I also appreciate insects—the essential roles they play in our world as well as their often stunning beauty. And when I looked up the Ninebark Leaf Beetle, I discovered that it is indeed a beauty. And then my heart sunk even further. My purple ‘Diabolo’ ninebark, it turned out, was no good for this native beetle. The alteration in the leaf color – changing the green of the native species to the burgundy of the cultivar – makes it distasteful to the beetle.

Ninebark Beetle
Ninebark beetle. Photo: Pennsylvania Department of Conservation and Natural Resources – Forestry, Bugwood.org

The effects of altered leaf color on plant-feeding insects was noted in a new study published in HortTechnology magazine last month. The authors (Baisden et al.) conducted experiments on several native woody plant cultivars compared to the straight natural species. They looked at whether six altered traits in the cultivated varieties – leaf color, variegation, fall color, growth habit, disease resistance, and fruit size – had any effect on insect feeding, development, and abundance.

In all three experiments they conducted, the researchers found that the cultivars with leaves that were altered from green to red, blue, or purple deterred insect feeding. Results were not consistent for the other cultivar traits they tested.

ninebark leaf variations
Ninebark cultivars, ‘Lady in Red’, ‘Dart’s Gold’, ‘Diabolo’. Photo: Leonora (Ellie) Enking, Flickr Creative Commons

There are a couple hypotheses as to how leaf color affects insect feeding. Most plant pigments are compounds that do not contribute to the growth of a plant. They may instead provide a defense mechanism. Anthocyanin pigments make a red coloration that may warn insects that the plant has defensive, distasteful chemicals – and they stay away.

The question of whether cultivars of native plants – nativars – have positive or negative effects on native wildlife is an active and ongoing area of research. Results in past studies have been mixed. (For more on this topic, see Mt. Cuba Center research).

Physocarpus opulifolius
The native species of ninebark, Physocarpus opulifolius, has green foliage. Photo: John Ruter, University of Georgia, Bugwood.org

I know how appealing it is to choose plants with special characteristics – the colored foliage, bigger flowers, the more compact form or general appearance that suits my personal taste. But my choices may not, in some cases, be to the taste of other things in our environment and the things that depend on them for food. It is a dilemma. I do like specific non-native plants (I’ll never give up my dahlias), but I also adore the lilting songs of chickadees in the springtime. Many birds like Carolina chickadees need caterpillars to feed their young and many caterpillars can only feed on wild, native plants. See New Smithsonian Study Links Declines in Suburban Backyard Birds to Presence of Nonnative Plants. I should note that a different study (Craves) found that native birds, including chickadees, were able to find insect food on non-native, invasive Amur honeysuckles. (Which makes the issue even more confusing, right?)

“When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” – John Muir

For an ecologically-minded gardener, it feels complicated to sort this all out and do the right thing. I felt disappointed that my ‘Diabolo’ ninebark was a missed opportunity to support a particular native insect, but, as Dr. Trangren explained to me, it becomes more of a problem when cultivated nativars cross-pollinate with the wild species and change the genetics of the native populations, making them less capable of supporting insects on a broader scale. For this reason, she recommends choosing cultivars that are sterile.

It was a lesson learned and one that makes me more thoughtful about my plant choices and their broader impacts.

By Christa K. Carignan, Maryland Certified Professional Horticulturist; Coordinator, University of Maryland Extension Home & Garden Information Center.

References:

Craves, Julie A. 2017. Native birds exploit leaf-mining moth larvae using a new North American host, non-native Lonicera maackii. Écoscience, 24:3-4, 81-90.

Baisden, Emily C., Douglas W. Tallamy, Desirée L. Narango, and Eileen Boyle. 2018. Do cultivars of native plants support insect herbivores? HortTechnology 28(5) 596-606.

Narango, Desirée L., Douglas W. Tallamy, and Peter P. Marra. Desirée. 2018. Nonnative plants reduce population growth of an insectivorous bird. Proceedings of the National Academy of Sciences of the United States of America.

Learn more about native plants on the University of Maryland Extension Home & Garden Information Center website.

Q&A: Is Burning Bush an Invasive Plant?

euonymus alatus
Winged burning bush (Euonymus alatus) is a Tier 2 invasive plant in Maryland. Photo: C. Carignan

Q: A friend has offered me a sapling of a burning bush. I am a little concerned about it being invasive. Could you please tell me if this is a true concern? Thanks.

winged burning bush berries
Berries of winged burning bush. Photo: C. Carignan

A: Yes, the burning bush shrub, also called winged burning bush (Euonymus alatus) is considered invasive in Maryland (and many other places) and deserves concern. In fact, this particular species is now regulated by the Maryland Department of Agriculture (MDA) as a Tier 2 invasive plant. This classification means that retail stores that offer this plant for sale must display a required sign indicating that it is an invasive plant. Landscapers may not supply burning bushes unless they provide the customer with a list of Tier 2 invasive plants.

Continue reading

The Elusive Fig

Figs
Container of fresh-picked figs. Eat at once as they only hold up for 12-24 hours on the kitchen counter. Do not refrigerate!

Fig questions have been pouring into the Home & Garden Information Center’s Ask an Expert service this fall. One reason for this, I think, is that most of Maryland’s fig trees were not “killed to the ground” during the relatively mild winter of 2016-2017, and were, therefore, able to produce good crops on the new shoots that grew from trunks and branches.

Fig Bush
Multi-stem fig bush 8-10 ft. in height

We were not so fortunate during the winters of 2014-2015 and 2015-2016. Prolonged freezing weather during those winters killed most of the above-ground plant parts. (Fig wood is damaged when temperatures drop below 20⁰ F.) Gardeners were left to cut and remove the dead trunks and branches.

The good news is that fig root systems almost always survive even the coldest winters. The bad news is that the new shoots growing from the roots the following spring rarely produce a crop. Frequently you’ll see many small green figs develop in summer that do not fully enlarge and ripen before the first frost.

Some common fig questions this year: Continue reading