Maryland Grows

Post Oaks in the Big City

Post oak

This post oak thrives despite being squeezed between a parking lot and a sidewalk. November 2013. Takoma Park, MD.

If you live in one of Maryland’s older towns, you probably have a lot of heritage trees – native trees inherited from the forests and fields that existed before your town was built out. It’s part of what gives old towns so much character. In my home town, Takoma Park, one of the heritage trees I admire the most is the post oak (Quercus stellata).

Post oaks inspire me. I see beauty in their shiny, cruciform leaves and their tiny, striped acorns. I also admire the species’ capacity to cope with adversity.  Many of the post oaks in Takoma Park are confined to little hell strips, those narrow grassy areas between slabs of asphalt or concrete. There they must cope with soil compaction, deicing salts, copious quantities of dog urine, and the urban heat island, to name a few.

Post oaks are one of the most common trees at Soldier’s Delight Natural Area, where the serpentinite bedrock gives rise to a soil so laden with heavy metals that it’s too toxic for most plants to grow in. Perhaps tolerance of metals helps the post oak perform well at urban sites,  even sites near train tracks and in industrial parks.

Post oak leaves have a distinctive, cruciform outline.

Post oaks are planted by squirrels, not people, but I think perhaps we should give it a try. And I’m in good company on that score. No less an authority than Michael Dirr, author of the Manual of Woody Landscape Plants, has also “Developed a fondness for the species,” as he puts it, “because of the great foliage and picturesque growth habit. Trees become artistic, sculptural entities in old age.”

Fall color of post oak varies from golden to red

This post oak is located in a strip of grass between two parking lots. Just two blocks from the metro in Takoma D.C., it is safe to say this tree copes with the urban heat island effect, as well as de-icing salts, soil compaction, and invasive plants, especially the English ivy vine (green). This fall the tree put out a bumper crop of acorns which will be used by students at the Institute for Applied Agriculture as part of a plant propagation exercise. If successful, we’ll have many little post oaks to give away! Takoma D.C., October 2019.

A close-up of the English ivy in Fig. 3. The roots of this invasive vine dig into and destroy the bark of its host tree.


By Sara Tangren, Ph. D., Sr. Agent Associate, Sustainable Horticulture and Native Plants, University of Maryland Extension, Home & Garden Information Center

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.

Here are some of the most common problems with cherry laurels in the landscape and what you can do.


  • Cherry shot hole disease – The leaves have a shot hole pattern that looks like it was caused by an insect. However, this is a foliar fungal disease favored by wet weather. The infected leaf tissue falls out and the holes are left behind. The damage is cosmetic and no chemical controls are recommended. Rake up any fallen foliage. The plant will recover.

    cherry shot hole

    Cherry Shot Hole Disease Symptoms. Photo: University of Maryland Extension / Ask an Expert


  • Peachtree borer – Feeding by this insect causes branch dieback and leaf browning. Look around the base of the stems for holes in the bark and frass (sawdust). This borer is attracted to excessive mulch and deep planting. Keep mulch no thicker than 2 inches and away from the base of the stems. When planting, set the shrub slightly higher than the existing soil grade in heavy clay soils. There are no chemical controls.
  • White prunicola scale – Feeding by heavy infestations of this insect can cause leaf yellowing and dieback. Look for white scale covers (white stuff) on the trunks and branches. This is an armored scale that sucks out cell contents. It is often found on weakened plants.
    white prunicola scale

    White Prunicola Scale on Cherry Laurel: Photo: University of Maryland / Ask an Expert

    Prune out any dead or dying branches. If the infestation is not heavy, use a soft brush to brush away the white scale covers from the branches. During the growing season, wrap a piece of double-sided tape around one of the branches. This is a test to monitor the active crawler (juvenile) stage of these insects. They are more susceptible when they come out in May/June depending upon temperature and there may be several generations per year. When you see crawlers sticking to the tape, that is a good time to apply horticultural or insecticidal soap according to label directions.

Abiotic Problems

  • Poor drainage – Leaves may show yellowing, browning, and dieback. Cherry laurels do not like a heavy clay soil that drains poorly. Excess soil moisture reduces oxygen in the soil, damages fine root hairs, and the root system is unable to absorb water. Be sure to check the soil drainage and make sure there are no downspouts dumping water in the root zone. Plant in raised beds and/or divert downspouts to another location.
  • Winter damage – This shows up as leaf browning and scorch when temperatures warm up in the spring. Broad-leaved evergreens are susceptible to drying winter winds, low temperatures, late frosts or freezes. Water deeply so there is enough moisture available to the roots before the ground freezes.

    winter injury of cherry laurel

    Winter Injury of Cherry Laurel. Photo: Elizabeth Bush, Virginia Polytechnic Institute and State University,

To establish cherry laurels successfully in your landscape, here are tips for new and established plants.

  • Check the soil drainage and make sure there are no downspouts dumping water in the site.
  • Before planting, if the roots are root-bound within the container, make several cuts along the outside of the root ball and tease the roots out so they can establish into the surrounding soil. (See Planting Process.)
  • Do not plant too deeply. Dig the planting hole deep enough to accommodate the plant with the top of the root ball level with or slightly above ground level.
  • Mulch should be no thicker than 2-3 inches. Keep it several inches away from the stems of the plants.
  • Check the soil moisture of new plants weekly and water deeply. Water established plants during dry periods.

If you notice problems with your shrubs during the growing season, we are happy to help diagnose what you are dealing with. Please send your questions and clear photos to the Home & Garden Information Center’s Ask an Expert service.

By Marian Hengemihle, Maryland Certified Professional Horticulturist, Consultant, University of Maryland Extension Home & Garden Information Center

Resisting Mosaic Viruses in Cucurbits

I grow most of my vegetables in a community garden plot: sunny, cheerful, with soil I’ve been improving with free compost for years now. It’s a good situation, but while community gardeners can share labor, tools, plants and seeds, they also end up sharing pests and diseases. Our garden has been suffering from a problem I never encountered at home or in the Derwood demo garden: mosaic virus on cucumbers and squash. Mostly I have dealt with this problem by the highly scientific method designated Not Even Trying, but this year I really want to succeed. So, some research!

Figure 1. Pumpkin plant infected with watermelon mosaic virus (WMV)

Photo by Dr. Gerald Brust of watermelon mosaic virus on pumpkin. Wait, what?

Read More

Q&A: What causes orchid leaves to turn yellow and shrivel?

orchid with yellowing leaves

Q: Why are my orchid leaves turning yellow and drying up? The plants are located in the bathtub where they get sun daily from the south and west window.

A: While it is normal for the oldest leaves of moth orchids (Phalaenopsis) to turn yellow and dry up as they age, when there is uniform yellowing and shriveling of newer leaves, it is a sign of distress. The shriveling suggests there is a lack of water reaching the leaves. Check the root system of your plant. If the roots are in poor condition, they cannot take up water. Overwatering can cause roots to rot. If you haven’t repotted your orchid in a couple years, the potting medium may have broken down and become too dense to allow for good drainage. Bacterial rot also can occur if water is allowed to sit around the center shoot or in the leaf sheaths for a long period of time. Water only in the morning so that your plants can dry out by nightfall. Never let them stand in water and keep the plants in a location where they can get good air circulation, indirect light, and a warm daytime temperature above 75F. Watering instructions can be found in our orchid care video.

See additional information on Phalaenopsis orchid care on the Home & Garden Information Center website.

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

Record Rainfall and Resilient Vegetable Gardens

More than 63 inches of rain has fallen so far this year on the Baltimore/Washington region, breaking a 129 year record. Gardeners are more often in the habit of hoping and praying for rain during hot, dry spells. This year we shook our heads in wonder as buckets of rain repeatedly pelted our gardens.

Climate change has already begun to increase yearly rainfall in Maryland. The NOAA State Climate Report (data through 2014) shows that “annual precipitation has been above average for the last two decades. The annual number of extreme precipitation events (days with more than 2 inches) averaged 2.5 days per year during 2005-2014 compared to 1.8 days per year during 1950-2004.” Scientists expect a 5-10% increase in Maryland’s annual precipitation by 2050.

Saturated Soil

Vegetable crops may recover from a24-48 hr. period in saturated soil
(where water replaces air in pore spaces). Photo credit: Wisconsin Horticulture

Resiliency is defined as “the capacity to recover quickly from difficulties; toughness.” Farmers, gardeners, and researchers are looking for practices and strategies that reduce greenhouse gas emissions and help us adapt food production to the rapidly changing environment. What makes this so challenging is that in addition to rising average temperature and precipitation we will have more unusual weather and extreme events, including drought! So what can gardeners do to improve garden resiliency in the face of excess rainfall?

  1. Move your garden location if most or all of your garden consistently has standing water in it eight hours after a heavy rain. Saturated soil deprives plant roots of oxygen and causes denitrification- the loss of soil nitrogen as nitrous oxide.
  2. Keep garden beds perfectly level and covered with plants, mulch, or cover crops to reduce soil erosion and nutrient run-off.
  3. Loosen compacted areas that stay wet with a garden fork (drive the fork into the soil and rock it back and forth).
  4. Is water flowing into your garden from the land above it? If yes, is there anything you can do to re-direct the water around your garden? Many neighbors had difficult conversations this year about the yard-to-yard movement of water, and the effects on landscapes and gardens at the bottom of the hill, or swale, or alley. We’re quickly learning that even minor land management actions can affect our neighbors.
  5. Keep soggy footpaths mulched and plant in deep raised beds filled with well-drained soil and compost. Add compost regularly to improve soil fertility and drainage. (Of course, raised beds and containers will require close attention and more frequent watering during dry periods.)
  6. Don’t fertilize all at once before or at planting. Instead, follow the 4Rs of fertilizing: right fertilizer, right amount, right time, and right place.
  7. Excessive rainfall and cloudy days can also encourage plant diseases, limit pollinator activity, and reduce produce quality and yields- even if your soil drains perfectly. Select disease-resistant cultivars, and space and prune plants to allow leaves to dry as quickly as possible.
  8. Keep planting seeds and transplants! Pull out stressed and dying plants and replace them quickly.


Zucchini in 15-gallon bags

Zucchini in 15-gallon fabric bags growing in a well-drained growing medium. Raised beds, bags, and containers are great tools for increasing rooting depth and protecting plants from excessively wet soils.  Photo credit: Connie Bowers


Pot gardening

Become a pot gardener. These herb and pepper plants (Thai and Scotch Bonnet) are
growing abundantly against a northwest facing brick wall. Dump out water from saucers
during extended wet periods and make sure the drainage holes don’t clog. Photo credit: Jon Traunfeld

By Jon Traunfeld, Extension Specialist



HGIC’s Climate Change and Gardening page has information and resources, including a Cornell University website (click “How is Climate Change Affecting Your County?”) that shows temperature and precipitation changes over the past 63 years.

  1. Flooding, Waterlogged Soils, and Effects on Vegetable Crops blog post. University of Delaware.
  2. Safely Using Produce from Flooded Gardens. Wisconsin Horticulture. UWE.


How to Choose a Seed Catalog

Like holiday decorations, seed catalogs seem to arrive earlier every year. They bring a bit of color and freshness into a cold and often dreary season, and winter gives us the perfect chance to sit down and plan next year’s garden. It’s SEED TIME!

seed catalogs

Seed catalogs. Photo: Erica Smith

But NO, I hear you say. I’m not ready yet! Well… maybe we don’t need to get organized about ordering seeds until sometime in the new year. As the catalogs slide into your mailbox, though, it’s a good time to review them and make some preliminary decisions about where you’ll get those seeds from. Even those of us who’ve been gardening for a while like to switch it up between vendors sometimes, and I’m sure many Maryland Grows readers who are newer gardeners have reached the point where they’re getting seed catalogs they never asked for, but which look tantalizing. But no one wants to pay shipping costs on orders of one or two seed packets from each company. How do you narrow the choice down? Read on for some criteria to make the selection easier. Read More

Survival of Baby Chickadees Declines in Yards with Less Than 70% Native Plants

Want to support nesting songbirds? Shoot for a minimum of 70% native plant cover in your landscaping, and 94% would be better, according to a recently published study by the University of Delaware’s Desirée Narango and others. For their study, the scientists chose Carolina chickadees as a representative suburban songbird. Homeowners across the southeastern U.S. delight to see them in our yards and at our bird feeders. Like most songbirds, chickadees provision their nestlings with insects from the landscape around their nest.

Caption: Landscapes with more than 94% native plants were excellent habitat for plant-eating caterpillars. These habitats provided enough caterpillars that Carolina chickadee parents could feed their young. Landscapes with 70 to 94% native plants may or may not support enough caterpillars. Nestlings in landscapes with less than 70% native plants were food-limited and had low survival rates. Image of chickadee courtesy of the National Zoo.

Caption: Landscapes with more than 94% native plants were excellent habitat for plant-eating caterpillars. These habitats provided enough caterpillars that Carolina chickadee parents could feed their young. Landscapes with 70 to 94% native plants may or may not support enough caterpillars. Nestlings in landscapes with less than 70% native plants were food-limited and had low survival rates. Image of chickadee courtesy of the National Zoo.

Study results show that baby chickadees reared in landscapes with less native vegetation are food-limited and much less likely to survive. So much so that the authors termed landscapes with less than 70% native vegetation as “food deserts” and “habitat sinks”. A habitat sink is a place with habitat sufficient to attract animals but insufficient to support their survival or the survival of their young. Habitat sinks are bad for a species because breeding pairs do not produce enough young birds to replace their parent’s generation. It would actually be better for the chickadees not to have that habitat available at all.

How much of the landscape in your neighborhood supports more than 70% native plants? Image: Hyattsville, Maryland, Google Maps.

How much of the landscape in your neighborhood supports more than 70% native plants? Image: Hyattsville, Maryland, Google Maps.

The study’s message is clear, if suburban residents want to continue to enjoy the company of native songbirds, they must support plenty of high-quality native landscape. Habitats with greater than 94% native vegetation were shown to be sources, in other words, places where the chickadees produced enough young to replace their parent’s generation.

To collect the data, the scientists worked with the Smithsonian Institution’s Neighborhood Nestwatch, a citizen science program. 159 homeowners in suburban Washington, D.C., installed and monitored chickadee nest boxes. The scientists measured native plant cover, insect abundance, bird diet, site occupancy, and reproductive success at each site. Many readers will recognize one of the co-authors, Doug Tallamy, from his now classic book, Bringing Nature Home. The study appeared in the August 2018 edition of Proceedings of the National Academy of Sciences, one of the world’s most respected publications.

By Sara Tangren, Ph. D., Sr. Agent Associate, Sustainable Horticulture and Native Plants, University of Maryland Extension, Home & Garden Information Center


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.

Carignan, C. (2018, Nov. 12). The Nativar Dilemma: The Case of My Purple Ninebark & The Leaf Beetle [Maryland Grows Blog]. 

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.

Neighborhood Nestwatch 

Tallamy, Doug. 2009. Bringing Nature Home. Timber Press. 288pp.