Designing Downtowns Fit for Trees

Across America, downtown commercial districts struggle to compete with big box stores and online shopping. For small downtowns to flourish economically and socially, they need to attract shoppers and residents by creating attractive, safe, walkable settings that people want to visit and spend time out of their cars. Trees play a major role in creating that setting, and research has shown that shoppers have positive views about tree-lined business districts.

National research conducted by Dr. Kathleen Wolf at the University of Washington looked at consumer preference finding that shoppers preferred business districts with trees and that landscapes projected feelings of comfort, upkeep, product quality, and merchant caring. The studies also found that shoppers are willing to travel longer distances, visit more frequently, and stay longer in downtowns with trees. Consumers were even willing to pay more for products in tree-lined business districts. Details on this research can be found at Dr. Wolf's website, the Human Dimensions of Urban Forestry and Urban Greening.

Creating tree-lined downtown business districts that are appealing and become destinations requires thoughtful consideration to allow trees, businesses, and people to thrive in inviting spaces where shoppers and visitors spend more time and money.

Minimizing Conflicts

As we begin to plan for trees in a downtown, we need to consider how to minimize the conflicts arising from trees being messy (dropping stuff like flowers, fruit, and leaves), blocking shop signage, obstructing doorways and deliveries, low branches injuring pedestrians and vehicles, or roots lifting sidewalks as the trees grow larger. Most of the conflicts can be mitigated through proper planning, site preparation, species selection, and maintenance. If we don't plan for and properly install the correct trees in a downtown, the trees will soon be considered a nuisance by merchants who will demand the trees be removed because they are impacting businesses.

Species Considerations

Some of the conflicts can be mitigated by selecting the right species of tree. Downtown streetscapes are some of the harshest environments for trees, so selecting tough trees is critical if they are to survive, grow, and provide benefits to the community. Trees need to be drought and heat tolerant because of the reflected heat loads from buildings and paved surfaces. Salt tolerance is very important because they are growing along a street that will have de-icing salts spread heavily on both the roadway and sidewalk. The trees selected need to have a branching habit, along with proper pruning, that allows clearance for pedestrians, vehicles, delivery trucks, and shop signs. Typically, a medium to large maturing stature tree, like an oak, ginkgo, or maple, and preferably with an upright branching habit will work better than small statured trees, like flowering cherry, do not work well. These small trees will mature at a height that blocks shop signs and low branches obstructing views and pathways to storefronts that can’t be pruned up for clearance because it would leave too little tree canopy. Instead, we want to grow a canopy of medium and large-stature trees to grow up above storefronts and signs. Downtown trees should also be fruitless to minimize messiness for merchants. Municipalities and merchant groups will have to develop special maintenance programs to provide proper clearance pruning over time, and increased leaf clean-up as needed.

Site Preparation

Growing trees in pits with compacted soils (or sidewalk fill material), surrounded by concrete, increased heat loads, de-icing salt exposure, and utilities both underground and overhead makes for a very difficult site for a tree to survive in.

Trees need healthy soils and root space. Providing that in a downtown takes some planning and site preparation. If you are planning to just cut small openings in the concrete and plant trees, that might work for a few years. In the long term, though, the trees will struggle and will probably raise sidewalks as the roots grow into the gravel subbase of the concrete sidewalks because they can't venture into the compacted soils or fill below. 

When planning for a downtown streetscape, consider using CU (Cornell University) Structural Soils or a soil cell under the sidewalks. Both allow for deeper and better rooting of the trees, without impacting the sidewalks, but there are differences. Structural soils, developed by Cornell University’s Dr. Nina Bassuk, are comprised of 80% angular stone and 20% clay loam soil. This very rocky soil mix is installed to a depth of 24 inches and highly compacted so a sidewalk or road can be poured over it. Despite the heavy compaction, the void space between the rocky structural soil provides rooting space and promotes deeper rooting, thus protecting sidewalks from heaving in the future. Soil cells, such as Sliva Cell or Strata Cell are stackable cells that allow for un-compacted soils to be installed inside the cells, while the cell structure holds up the sidewalk (a floating sidewalk that does not require soil compaction). Both structural soils and soil cells are designed to be used to create large usable soil volumes under pavement and should not be used just in a tree pit or small areas surrounding the tree.

Aside from the need for below-ground rooting space, there are lots of considerations that go into designing the downtown streetscape and selecting locations for trees. Develop a design team including a landscape architect, an engineer, an arborist, and municipal tree commissioners and have them all involved in the project as it is being designed. Once you have a design, take the paper plan into the field and figure out where the trees go based on where the utilities, street signs, street lights, and shop signs and storefronts are located. If the project is just designed on paper without examining field conditions, utilities, parking and shop signs, mistakes will be made, and the trees will create conflicts that upset merchants.

We need to have all the underground utilities located and marked in the field and decide what size opening the trees will be grown in. Above ground, avoid planting trees within 40 feet of a corner, or closer than 15 feet from a streetlight. Place planting sites between shop signs and businesses to avoid blocking signs and entrances. Avoid planting in delivery zones. If there are designated parking spaces, place the tree in line with the back quarter panel of a parked car to avoid having car doors obstructed by trees. There may be some places we just can't plant a tree. Trees in the downtown are designed to green up the street and accent the buildings. Do not try to plant a forest and squeeze trees into the streetscape with tight spacing. They will only grow into each other, creating maintenance issues as they shade out each other's branches, creating deadwood, or densely shaded streets that then block streetlighting in the evening. Work with the buildings, the signs, and utilities to select the best places to grow healthy large canopy trees.

When it comes to the size of the tree pit opening, the bigger, the better for the tree, but the space for the tree is competing with space with pedestrians. Tree pit openings should not be smaller than 4 by 4 feet, although a narrower 3-foot wide pit that extends at least 5 or 6 feet long might work where sidewalks are narrow and ADA standards need to be met. If there is room, consider designing longer tree pits, or interconnecting the tree pits underground with structural soils in a trench running parallel to the curb or utilize soil cells under the sidewalks. While the tree is young and small it is best to cover (or close) the tree pit to provide more pedestrian space and avoid pedestrians stepping in a muddy tree pit and then walking into a downtown business. Many designers use metal tree grates, but they are quite expensive, can cause tripping hazards, and trees can become girdled as they grow larger in diameter. Consider using brick pavers, cobblestones, slate, or Flexi-Pave to cover up some of the large tree pit openings for a few years. As the tree grows, it is quite easy to remove pavers, giving the tree more room to fill in the tree pit. Another option is to cover the tree pit with mulch, which is good for the tree, conserving soil moisture and reducing compaction. If mulch is used, beware of discarded cigarettes causing mulch to burn and smolder. Leaving behind stakes that were used initially to stabilize a newly planted tree can provide an additional barrier to protect an established tree while still small. Just be sure to remove the webbing or strapping attached around the tree to support it during establishment.

Solving Tree Sign Conflicts

The biggest complaint from merchants about trees is they block storefronts and signs and customers will not find them. If trees are blocking signs and the municipality is not working to solve the problem, merchants may take it upon themselves to begin pruning (usually topping the trees) or removing the trees when no one is looking. Or they will just lobby local elected officials to clear-cut the downtown of those messy trees they believe are destroying their businesses. 

It is best to prevent the tree/sign conflicts through proper design and siting of trees, selection of species that have upright branching habit and mature above the shop signs (don't plant small-stature trees), and proper clearance pruning as the trees grow. One of the biggest mistakes is that small-stature low-growing trees are planted, thinking they will not grow to block the signs or because they think larger trees will be too dangerous. Small maturing trees like flowering cherry, serviceberry, tree lilacs, and hedge maples grow to a  mature canopy height right in line with signs, creating a visibility issue that is almost impossible to solve with pruning. The other mistake is tree crews are hired to "top" or "round-over" the canopy of the medium and larger stature trees, making them bushier, leaving the canopy in line with signs, and causing decay on branches that were improperly pruned with large heading cuts.

If the right species and cultivar are planted with upright branching and mature size that extends above signs, then all that is needed is proper clearance pruning. Completing this clearance pruning every two to three years will allow the tree canopy to be raised with time to a point that it is not blocking the shop signs. If municipal crews will be pruning, then training is critical so they make proper cuts and do not harm the trees. If you are contracting out the pruning of lower limbs, use an ISA Certified Arborist and require in the contract that they are on-site during pruning. ISA Certified arborists are trained and have passed an exam demonstrating they have the knowledge to properly care for trees. All it takes is one improper pruning or "topping" to destroy the investment of time and money planting and growing downtown trees.

It is also helpful if the downtown merchants work with the trees and install signage at human scale and not thirty feet in the air on a pole. Remember we are trying to create walkable downtowns, so signage at 10-12 feet works best and can even be seen by drivers through a windshield. 

If you select appropriate sites, species, cultivars, and maintenance program, the downtown trees will not conflict with shop signs or historic, period streetlights that are situated at 12-14 feet above the sidewalk.

Selecting Tough Species to Survive Downtown Conditions

As mentioned earlier, downtown site conditions can be very inhospitable for trees. Often, large caliper trees (3” diameter) are planted at great costs, only to survive a few years because either an inappropriate species was selected that could not survive the site conditions or little to no site preparation was accomplished before trees were planted in sidewalk cuttings.  

Several species of trees that perform well in downtowns are:

• Honey locust (Gleditsia triacanthos var. inermis) – 'Skyline' and 'StreetKeeper' are two of the cultivars that hold a central leader and have upright branching.
• Ginkgo (Gingko biloba) – Plant males only to avoid fruit that has an undesirable odor. Cultivars for downtowns include 'Princeton Sentry', and 'Golden Colonnade'.
• Zelkova (Zelkova serrata) – 'Halka', 'Village Green', 'Musashino', 'Wireless', and 'Green Vase' are cultivars with varying forms.
• Hybrid Elms – New hybrid elm cultivars have very good disease resistance and good form. 'Accolade' is a very good cultivar. 'Frontier' is a small elm tree with purple red fall color. Other cultivars to consider are 'Homestead', 'Urban', or 'Patriot'.
• Chinese or Lacebark Elm (Ulmus parvifolia) – A medium sized elm. Cultivars for streets include 'Alee' or 'Bosque'.
• English Oak (Quercus robur) – Upright cultivars include ‘Fastigiata’ and 'Skyrocket'.
• Hardy Rubber Tree (Eucommia ulmoides) – A tough drought and heat tolerant tree native to China.
• Upright or Columnar Red Maples (Acer rubrum) - Cultivars for streets include 'Armstrong', 'Bowhall', or 'Columnare’.
• London Planetree (Platanus x acerifolia) – Cultivars include 'Bloodgood' and 'Exclamation'.
• European Hornbeam – Cultivars include 'Fastigiata' or 'Columnaris'. 

Creating a walkable, attractive downtown fit for trees while reducing potential conflicts with shop signs, traffic, sidewalks, and pedestrians can be a challenge, but it is not impossible. Start by developing a team of designers. Consider involving the merchants in the design process to ensure their concerns are addressed and they are on-board for planting downtown trees. Once you have a design and species selected, make sure to ground truth it in the field and adjust the planting locations as needed to avoid conflicts. Creating rooting space through good site preparation, can mean the difference between a healthy tree that provides benefits for many years to come and a tree that slowly dies a few years after planting. 

For more information, view the webinar Creating a Downtown Fit for Trees presented by the Penn State Extension Urban Forestry Program.