by Matthew Downs, Urban Forestry AmeriCorps Member
Trees along a street in Portland (photo by Julie Fukuda)
Join Urban Forestry and the Powelhurst-Gilbert Tree Team for a free workshop (presentation and treewalk) to learn about general tree care:
When: Saturday, May 7, 2016, 1-3pm
Where: Holgate Baptist Church (11242 SE Holgate Blvd)
Why do we need trees?
Trees help city-dwellers in multiple ways. They filter the air we breathe, calm our nerves, shade our streets, and cool our neighborhoods in the summer. Some studies have correlated trees with reduced crime and better birth outcomes. And in Portland’s stormy winters, trees help to mitigate flooding by intercepting rainfall. So yes, trees are necessary to our health in the cityscape – bringing us back to nature – but their management is essential before we receive any such benefits. In Portland, we’ve implemented Title 11 Trees to help do so.
Right Tree, Right Place
Two main things to keep in mind here: site width and overhead wires. Between our utility and communication lines, buildings, streets and sidewalks, there wouldn’t seem to be a lot of space left for trees. Yet, there is! And it becomes a matter of choosing the right species for the available planting site.
Here at Urban Forestry, we like to use the helpful motto Right tree, Right place. By this we mean that trees vary greatly across genera and species. Say you’ve got an empty 6 ft planting strip in front of your house without high-voltage wires overhead. How about an evergreen like a live oak to intercept that rainfall!
Leaves of an evergreen oak (photo by Nik Desai)
Before setting your heart on any one tree, visit Urban Forestry’s Approved Street Tree Planting page for species lists based on site width and overhead wire obstructions. These lists account for size and form, but you’ll have to dig deeper to ensure your selection is getting the proper sunlight or soil conditions. Visit Friends of Trees to browse tree options with helpful characteristics reports for each.
If you’ve got the ability to make your selection from nursery stock, ask yourself the following questions about form when actually selecting the tree:
Is there a proper central leader?
Nurseries often prune for presentation. Be wary of trees that have been topped to encourage lush lateral growth, but lack dominant central leaders. Keep in mind the majority, if not all of the branches on young trees are temporary.
An evergreen magnolia with a strong central leader (photo by Nik Desai)
Are the roots girdling?
Seedlings are grown in pots, transferred to bigger pots as they grow, and later bigger pots as needed. Sometimes that doesn’t happen as timely as we’d hope, and the roots can encircle and constrict their own optimal form. While root pruning is possible, some girdling roots make for an absolute lost cause. That is to say be choosy, because not all nursery stock is fit for planting.
This poor tree is in for some trouble due to girdling roots. Pay attention to girdling roots before you plant your tree! (photo by Gina Dake)
Obtain a permit before planting in the right-of-way. This is an essential part of Title 11 Trees that enforces the “Right Place” end of Right Tree, Right Place. An Urban Forestry Tree Inspector will mark your curb at the exact spot that the tree should be planted. This ensures the tree won’t interfere with drivers’ vision, utility lines, or create any other potential hazards.
The roots have many variables to consider. Are they wrapped in a ball with burlap and soil? Is the tree in a container? Are they roots completely exposed? If the tree is in a container or has a root ball, the roots will be tightly bound by soil. Use a knife or a spade and rough up the exterior of the soil wad to free some roots for lateral growth. Often times the root flare tree in a container will be covered by soil as well, so be sure to dig it out so the flare is visible. This will help with the planting depth.
Debra Kneeshaw, Portland Parks Horticulturalist, demonstrates proper planting (photo by Jim Gersbach).
Way too many trees die because they’ve been planted too low or too high. The root flare must be visible above the soil surface, but not so high that the roots are exposed. If the tree sits too low, add compacted soil under the roots to bring it higher. The proper depth is right at that Goldilocks point.
Fill the rest of the hole in with the soil you’ve dug out for the hole. Do not use fertilizer or potting soil. This will train the tree to expect nutrients in its immediate vicinity rather than growing vigorous, supportive roots via the search of water and nutrients.
Pull away competing plant roots, and use the excess soil to build a berm in the form of a ring around the tree. This will lock in water for the first stage of establishment.
(Diagram courtesy of Urban Forestry)
Cross section of a tree planting. Note the berm built up around the tree. The diameter of this circle should be twice the diameter of the root ball.
For questions or further information, please contact: Matthew.Downs@portlandoregon.gov