Preserving the Cherry Trees
How does the National Park Service take care of the cherry trees? It's a big job that requires teamwork.
The National Mall and Memorial Parks tree crew is a team of professional arborists. Their technical prowess and dedicated professionalism are the most important factors in the survival of the flowering cherry trees. The team performs a wide variety of services to maintain the trees, keeping them healthy and helping them thrive in a harsh, urban environment.
Pruning, the removal of specific branches, vitally contributes to the overall health and appearance of the trees. Pruning is conducted 1-2 times annually. Most pruning occurs from January through early March.
Younger trees may be pruned to promote a particular growth habit. Established trees are pruned to remove dead wood, making the tree more safe for people around it, improving the appearance of the tree, and reducing the potential for disease and pests.
Trees need the right amount of water to survive. Compacted soils from foot traffic can interfere with a tree's ability to absorb water. When needed, the tree crew supplements what nature provides from the sky. Young trees are spot watered using water trucks. Established trees in large stands are more efficiently watered using pump irrigation and sprinklers that draw water from the Potomac River, Washington Channel, or Tidal Basin.
When repairing bark wounds, only damaged or loose bark is removed to minimize the disturbance to live tissue. Wounding paints are no longer used as they have not been shown to be effective in preventing or reducing decay or preventing insect and/or disease infestations.
Insect and Disease Control:
Like all plants, the cherry trees have their insect and disease associates. The trees are monitored for insects and diseases under an Integrated Pest Management Program and only sprayed when populations reach predetermined threshold levels.
Bacterial canker (sunken or elliptical lesions on the trunks or branches that ooze gum from the wounds), also known as bacterial gummosis, is common but is rarely a serious problem on established trees. Canker is controlled through pruning and/or wound treatments (bark tracing).
Fertilization is only conducted to correct or prevent nutrient deficiencies. A slow release fertilizer specifically formulated for trees is applied by soil injection.
In 1998, on a very limited basis, endo- and ecto-mycorrhizal fungi, root bio-stimulants, and growth promoting bacteria were added to the fertilizer solution to help the cherry trees overcome stress due to low soil fertility, drought, temperature extremes, and visitor use impacts.
This process began on a very limited scale in 1995 to try to mitigate the problems of soil compaction due to recurrent visitor use. When soils become compacted, virtually all of the favorable soil physical characteristics are adversely affected, specifically, structure, tilth, oxygen/carbon dioxide balance, soil microbiology, total and capillary spore space, water infiltration, and water holding capacity.
This procedure consists of using a highly focused supersonic air stream (approximately 1,300 mph) to penetrate soil and fracture soil it away from tree roots in trenches that resemble the spokes on a bicycle wheel. The trenches are then backfilled with a mix of 1/3 sandy loam topsoil, 1/3 lightweight aggregate of expanded shale or slate, and 1/3 composted leaf mold by volume.
However, these efforts are ineffective if visitor use patterns continue to compact the soil.
Like all living things, cherry trees eventually die. Old age, anaerobic soil conditions, impacts from recurrent visitor use, or insect and disease infestations eventually claim the trees. When trees die in the park, they are replaced with trees purchased from commercial nurseries during the next planting season in the fall or spring.