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.
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 →
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.
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.
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).
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.
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.
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.
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.
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.