Ask the Expert

By Miri Talabac

Good Choices for Native Street Trees

Q: Can you suggest a few underused native species for use as either a street tree or a smaller accent tree? They don’t have to have showy features, just be tolerant of minimal care.

A: I think several of our locally native large shrub or small tree species deserve to be grown in home landscapes, but for some reason, they’re hard to find. Maybe they’re harder to propagate, or maybe it’s just an issue of low demand because they’re not as flamboyantly showy or well-known.

American Bladdernut (Staphylea trifolia) grows wild in much of the state, and in habit is more of a large, multistemmed shrub than a tree, though it could be cultivated as either. On average, its white blooms appear from late April to early May, and develop into interesting inflated three-sided seed pods by autumn. It is adaptable once established to drier or periodically wet soils, in full sun to understory shade, and a near-neutral soil pH. Thriving plants might sucker a bit.

Blackhaw Viburnum (Viburnum prunifolium) tends to be sold as a shrub but can mature into a small tree if you let it. Found statewide, it adapts to many soil types and tolerates drought. It blooms about the same time as Bladdernut and the autumn berries are a colorful medley of mint-green changing to pink to dark blue-black as they ripen. Foliage may turn burgundymaroon in autumn.

Hophornbeam (Ostrya virginiana) could be suitable for street tree use (as well as an in-yard specimen) due to its tolerance of rocky soils that skew dry, although moist (not wet), acidic soils are ideal. Slow-growing, it is an understory species, but can do well in full sun in cultivation. Named for the interesting seed pods that resemble hops, it is related to American Hornbeam (Carpinus caroliniana), because both are members of the birch family, but where Hophornbeam prefers upland soils, Hornbeam prefers floodplains.

We think of oaks as large shade trees (most are), but a more obscure native that is almost shrublike in stature is Dwarf Chinquapin Oak (Quercus prinoides). Once established, it has good drought-tolerance, and it prefers acidic soils. If too short for use as a specimen tree (genetics and soil traits will influence its mature size), a group of them could serve as a thicket to block a view, or a minimally pruned or unpruned hedge. Despite being an oak in a much smaller package, its foliage and acorns still support lots of wildlife.

Identifying Fungal Twig Galls

Q: My young serviceberry tree has lumpy, dark swellings on the twigs. When I checked my flowering plum tree, it looked worse, with the same symptoms. Is this a spreading disease, and how do I control it?

A: They are diseases, but two different infections that happen to look similar. This is a good example of how knowing the plant ID helps to narrow down a diagnosis when symptoms overlap greatly. The serviceberry (Amelanchier sp.) has a rust gall, and the plum has black knot disease. Both are caused by fungi, but they are not spreading between these two trees; serviceberries don’t contract black knot and plums don’t contract rust.

Gardeners who have encountered Cedar-Apple Rust on their serviceberry foliage or fruits may be surprised to learn that this twig gall symptom is caused by the same group of fungi (Cedar-Quince Rust in this case). Instead of the familiar bright, rustyorange spores on infected fruits and leaves, the fungus creates galls (abnormal tissue swelling) on the twigs, where spores emerge.

Black knot is aptly named, because it causes knobby-looking black galls on the plum twigs, which spread the fungal spores. It does not create leaf symptoms.

Both diseases will kill the affected twig beyond the gall, so when found, they have to be trimmed off and the debris thrown away or hot-composted. Neither disease can be cured with fungicides. Wild plum and cherry (Prunus americana, P. serotina, and a few others) are sources for spore spread for black knot, and several other native plants are sources for spores of rust, which alternates between host plants to complete its life cycle. (Part of the cycle takes place in juniper, and part in rose-family trees and shrubs. Eastern Redcedar is the wild juniper in our region, and serviceberry is a member of the rose family.)

Our native Prunus, Juniperus, and Amelanchier have great wildlife value and are worth having in the landscape, and vulnerability to these two diseases is just a fact of life for them. Variations in weather and individual plant genetics influence how rampant or limited each disease may be from year to year in your garden. o

Sporulating (above) and dried (below) rust galls on serviceberry twigs. Photos by Miri Talabac.

Miri Talabac is a Certified Professional Horticulturist who joined the University of Maryland Extension Home and Garden Information Center in 2019 as a horticulture consultant. She is a graduate of UMCP with a focus in entomology. To ask a gardening or pest question, go to http:// extension.umd.edu/hgic and select “Ask Extension.” Digital photos can be attached.

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The National Garden Bureau is a nonprofit organization that exists to educate, inspire, and motivate people to increase the use of garden seed, plants, and products in homes, gardens, and workplaces by being the marketing arm of the gardening industry. Our members are experts in the field of horticulture and our information comes directly from these sources. Find out more at https://ngb.org/

Prizes

Washington Gardener Magazine presents the 20th Annual Washington Gardener Seed Exchanges on Saturday, January 25, 2025, 12:30–4:00PM National Seed Swap Day! at Brookside Gardens in Wheaton, MD

Registration is now open at https://www.eventbrite.com/e/20th-annual-washington-gardener-seed-exchangeon-saturday-jan-25-2025-tickets-1117021353939?aff=oddtdtcreator and on Saturday, February 1, 2025, 12:30–4:00PM at Green Spring Gardens in Alexandria, VA

Registration is now open at https://www.eventbrite.com/e/wg-seed-exchange-212025-invirginia-tickets-1117024784199?aff=oddtdtcreator

Overview

Washington Gardener magazine, the publication for DC-area gardening enthusiasts, is hosting the 20th annual Washington Gardener Seed Exchange at Brookside Gardens and Green Spring Gardens. These seed swaps are in person and face to face. You bring your extra seeds and swap them with other gardeners. Everyone will leave with a bag full of seeds, new garden friends, and expert planting advice.

When

Two Seed Exchanges will be held: Saturday, January 25, 2025, in MD and Saturday, February 1, 2025, in VA from 12:30–4:00PM (Foul weather that day? Call 240-603-1461, for updates about possible snow/ice delay.)

Where

We are holding a duo of Seed Exchanges one week apart on opposite sides of the Washington Beltway. We urge you to attend the one closest to you.

One exchange will be held at Brookside Gardens, 1800 Glenallan Ave., Wheaton, MD. The other will be at Green Spring Gardens, 4603 Green Spring Road, Alexandria, VA.

How to Register

You can easily register online at Eventbrite.com. Be sure to select the correct date and location for the Seed Exchange you are going to be attending. You can also fill out the form on the opposite page. Send the form, along with payment, to Washington Gardener, 826 Philadelphia Ave., Silver Spring, MD 20910, Attn: SE Registration. Please make checks out to “Washington Gardener.” Registration fee is $20 per person. Friends of Brookside members, Friends of Green Spring members, and current Washington Gardener subscribers receive a discount rate of $15 per person. We strongly urge you to register in advance. There is a limit of 100 participants at each location!

We are GREEN!!!

Garden Book and Seed Catalog Exchange

Seed Exchange attendees are encouraged to bring their used or new garden books and seed catalogs to swap and share at this year’s event. We also ask you to bring your own water bottle or reusable mug and a home-made nametag. We will have a “best nametag” contest, so get crafty!

Hashtags #GardenDC and #SeedSwapDay