How the Smithsonian Gardens Tree Collection Benefits the Environment and Our Well-Being

The Smithsonian Gardens Tree Collection currently consists of over 1,900 trees spread across the Smithsonian’s downtown Washington, D.C. campus as well as at the Institution’s support facilities in Maryland.  These trees add great aesthetic value to the grounds, and complement the flowers, shrubs, and other plantings in the gardens here.  However, and perhaps more importantly, these trees offer a myriad of environmental and health benefits.

Trees along Constitution Avenue at the National Museum of Natural History create a cool, green tunnel for pedestrians. Photo by Greg Huse.

During the summer of 2015, information on all of the collection trees was run through i-Tree, a software program developed by the U.S. Forest Service to calculate the benefits of trees.  The results were eye-opening, and it was encouraging to see how much our trees benefit our campus, city, and planet.

Poor air quality, especially in urban areas, can lead to decreased human health, poor visibility, and damage to plants.  Trees in urban forests help improve air quality by capturing air pollutants, reducing air temperatures, and decreasing energy consumption in buildings which reduces overall air pollutant amounts by lowering demand on power plants.  The SG Tree Collection was calculated to remove 2,700 pounds of air pollutants (carbon monoxide, ozone, nitrogen dioxide, sulfur dioxide, and particulate matter) annually with an associated monetary value of $31,185.  This is the equivalent of taking ten cars off the road for a year!

Trees leafing out in spring at the National Museum of Natural History. Photo by Eric Long.

One of the most important environmental challenges we currently face is global climate change.  Trees can help mitigate this problem by capturing atmospheric carbon and storing, or sequestering, it for many years.  As trees grow, they continually store carbon in the new wood they produce.  The trees in the SG Tree Collection store an estimated 37,877 pounds of carbon annually.  The large majestic oaks, elms, and other trees found at the National Museum of American History sequester twice as much carbon as the trees found in any other Smithsonian garden or landscape.

Surface runoff of stormwater is a cause for concern in urban areas because it can contribute to pollution in streams, rivers, lakes, wetlands, and oceans.  Precipitation that reaches the ground and does not infiltrate the soil becomes surface runoff, which carries pollutants into waterways.  Urban trees reduce this runoff by intercepting the water in the tree canopy and allowing it to evaporate over time.  Additionally, water that lands on trees may run along the branches and down the trunks which helps it reach the ground more slowly and infiltrate the soil.  Smithsonian trees intercept 383,705 gallons of rainfall annually, thereby preventing it from becoming surface runoff.

In addition to these environmental benefits, many recent studies show how trees, especially in urban areas, contribute to better health for people.  Areas that have many trees can lower blood pressure, have a calming effect on teens and adults with ADHD, improve breathing for those with asthma and other lung conditions, decrease healing times for sickness and injury, contribute to overall emotional and psychological health, and even improve birthweights of newborns!

An oak transformed by autumn color at the National Air and Space Museum. Photo by Greg Huse.

We appreciate trees for their beauty and grandeur, especially at this time of year when they come ablaze with autumn colors.  Beyond appreciating their beauty, it’s important to remember all of the environmental and health benefits they provide as well.  Planting and properly maintaining trees are important steps to take to continue to improve the world in which we live.  We here at Smithsonian Gardens are proud to do our part in contributing to that goal.

– Greg Huse, Smithsonian Gardens Arborist and Tree Collection Manager

The Smithsonian Gardens Tree Collection: Proactive Management

This American elm tree on the grounds of the National Museum of Natural History is over two hundred years old. Eric Long, photographer.

The Smithsonian Gardens Tree Collection currently consists of over 1,850 trees, approximately 1,400 of which are located on the downtown Washington, D.C. and Anacostia campuses.  These trees add beauty to our grounds, and they offer myriad environmental and health-related benefits.  Unfortunately, it seems that trees are constantly under attack by a host of problems, ranging from severe climate, to native and exotic pests and diseases, to damage from construction and development projects, to the tough urban environment in which they grow.  Once these plants become stressed, it’s more likely that they will suffer due to one or more of these issues.  In addition, as trees grow, certain structural defects can develop which may cause problems in the future, especially when severe weather events can exploit the inherent weaknesses in these defects.

For these reasons, we at Smithsonian Gardens take a proactive, rather than reactive, approach to caring for these green assets.  Oftentimes, defects, cultural stressors, or insect and disease infestations that have gone unnoticed for a time can be difficult, if not impossible, to reverse.  Therefore, a thorough health and structure assessment of these trees was completed at the end of 2014.  This assessment consisted of a top-to-bottom, 360 degree visual evaluation of each tree.  All defects and other potential issues were noted and assigned a rating based on the severity of the condition observed.

Smithsonian Gardens staff band trees.

What we had at the end of the evaluations was a complete list of trees, their problems (if they had any), and recommendations for correcting anything of concern.  Based on the ranking system, we now have an organized and detailed list of what maintenance and tree care work is needed, with a clear indication of where we need to start.  This has enabled Smithsonian Gardens to find and fix issues before they become more serious, and gives us the ability to be proactive with our tree management.  It also gives us a better idea of how to budget for upcoming maintenance needs.  Prevention is the best medicine, and any time we can find and correct an issue before it becomes serious it allows us to keep our trees happy and healthy for many years to come.

-Greg Huse, Smithsonian Gardens Arborist and Tree Collection Manager

 

Preparing Your Trees for Winter

Japanese coral bark maple (Acer ) in the Mary Livingston Ripley Garden, next to the Smithsonian Arts & Industries Building.

As we gaze around at the beautiful autumn colors that our trees are showing us, we’re trying not to think about the arrival of the cold and snowy weather of winter.  However, arrive it will, and now is the time to prepare your trees for those coming winter months.  Although all trees are potentially susceptible to winter injury, young and/or thin-barked, and broadleaf evergreen trees require the most preparation.

Excessively cold temperatures, wind, and quick temperature changes can cause drying, browning, and death of evergreen foliage.  This problem is most prevalent on broadleaf evergreens such as rhododendrons, laurels, boxwoods, and hollies.  To help prevent this damage, construct a barrier of heavy burlap, like a fence, to block drying winds from their prevailing direction.  If the entirety of the plant is exposed, loosely wrap it in burlap.  In either case, be sure to leave the top of the plant exposed so light and air penetration can still occur.  In addition, it is important to keep watering your trees up until the time of the first hard frost.  A 4-6 inch layer of mulch over the root zone will also help the soil retain warmth and moisture.  (Remember not to pile the mulch up against the trunk of the tree.)

The boxwoods in the Kean Hall Garden in Livingston, New Jersey wrapped up in burlap for the winter, 1955. Smithsonian Institution, Archives of American Gardens, Garden Club of American Collection.

Another issue of concern, which is also caused by rapidly fluctuating temperatures, is sunscald.  This occurs when the sun has warmed the trunk of the tree, and then that trunk is rapidly cooled upon sudden shading from a cloud, etc.  This condition results in elongated, dried and cracked areas of dead bark.  This can be prevented by wrapping the trunk with commercial tree wrap (available at most home and garden centers) or other light-colored material.  This will reflect sunlight and keep the bark at a more consistent temperature.  The wrap should be placed on the tree in the fall and removed in the spring, after the last frost.

Broadleaf evergreen foliage damage and frost cracks are influenced by many factors, including plant species, location, drainage, natural protection, and how well established a plant is in the landscape.  There is no specific temperature at which damage occurs, but if the forecast calls for temperatures below the average seasonal low (29-33°F for Washington, D.C.), it is best to utilize the protection methods outlined above.

Tree branches can be prone to breakage from heaving snow and ice loads and by strong winds.  Weakly attached, overextended and broken limbs should be pruned.  Trees with an upright form, such as juniper, arborvitae, and clump birch, can be wrapped in burlap, or held together by wrapping the branches collectively with twine or rope.  Any wrapping material should be removed in the spring.

When natural food sources grow scarce in the winter, rodents may feed on the young bark and cambial tissue of trees.  Plastic tree guards or a cylinder of ¼” wire mesh placed around the trunks of young trees will help prevent this damage.  Be sure to remove these guards once the spring has come so the tree does not wind up growing into them.

Trees possess an extraordinary ability to withstand severe winter weather, with some being more hearty than others.  However, with proper care and attention, your trees should come through the winter ready to show off their new flowers and foliage for spring.

-Greg Huse, Smithsonian Gardens Arborist & Tree Collection Manager

Hope for the American Elm

The American elm at the intersection of Constitution Ave. and 9th St. on the grounds of the National Museum of Natural History. Eric Long, photographer.

Smithsonian Gardens manages the health and maintenance of 1,873  trees in the Washington, D.C. area. As you walk around the Smithsonian gardens and museums you may notice a common theme: many of these trees are mature specimens with historical context and connection to the museums they surround. This is extremely evident as you walk the grounds of the National Museum of Natural History, where the extensive American elm plantings bring us back to a time when Ulmus americana was the predominant street tree in America. In fact, the large specimen on the corner of 9th Street and Constitution Avenue predates the museum, which celebrated its centennial in 2010.

As Dutch Elm Disease (DED) has wiped out the majority of stately American elms throughout the U.S., we at Smithsonian Gardens work diligently to monitor and manage our trees in order to prevent the spread of this lethal disease.

Center Market vendors outside of the National Museum of Natural History in 1909. Dutch Elm Disease wiped out many of the majestic elms that predate the museum. Image courtesy of the Smithsonian Institution Archives.

It is with this management strategy in mind that we carefully select replacements when elm trees at the Smithsonian need to be removed. When one of the younger elms on the north lawn of the National Museum of Natural History was critically damaged during a storm, we once again debated and discussed which “resistant” elm to replace it with. One of the best choices for a true Ulmus americana replacement is the ‘Jefferson’ Elm (Ulmus americana ‘Jefferson’), selected through the collaborative efforts of the National Park Service and the USDA’s Agricultural Research Service (ARS).

This tree was selected for its excellent DED resistance and the fact that as a true species, it exhibits the classic American elm form, unlike some of the resistant hybrids.

‘Jefferson’ elms leaf out earlier in the spring and maintain their green color better in the summer than other U. americana specimens. We were very lucky to receive this tree from the National Park Service as it is just becoming available in the commercial trade at this time, and can be difficult to find. The National Park Service propagates ‘Jefferson’ by cuttings from the original tree, located on the National Mall, and grows the seedlings for six years at its National Capital Region Nursery. It is a long process and a difficult one, as only about 5% of the cuttings live to become mature trees. Hopefully this selection will become more common in the nursery trade so that we can once again plant these majestic trees with confidence. Until that time, we are very thankful for the ongoing collaboration between the National Park Service and Smithsonian Gardens to ensure that the American elm still graces the Washington, D.C. landscape.

–Jonathan Kavalier, Smithsonian Gardens Supervisory Horticulturist