Landscaping & Planting Trees
Objective: To reduce the negative impacts when improving the landscaping around established City Street Trees.
- Tree Protection Zone (TPZ) - The area specified by the City Arborist typically within or beyond the tree's drip line. The TPZ is critical to tree survival and lies in the upper 3 feet of the soil surface. This is a restricted area and the soil should not be disturbed unless otherwise approved
- Avoid root damage - Ninety percent of root damage occurs in the upper 18" of soil due to equipment and compaction. Rototiller, trenchers and hand tools cause damage to the surface feeder roots and this is the largest single factor responsible for a trees decline during landscape projects.
- Compaction Compression within the upper 18" of the soil structure may cause an impermeable layer where water and air cannot circulate and cause damage to the root and decline to the tree. Avoid heavy equipment under the TPZ or placing heavy stones such as river rock as ground cover. Do not use this area as storage site for material during the project.
- Ground cover /mulch/rocks - Ground cover, small shrubs and small stones may be used as covering underneath trees. Leave 1-2 feet clear around base of trunk to allow for proper air circulation. Avoid large heavy stone and do not place sod up around base of tree. Use permeable weed fabric and never use plastic as a weed barrier
- Water and air irrigate the TPZ prior to start of landscape project and continue to keep moist until the completion of the project. The soil should be kept wet but not soaking to a depth of 24 -30 inches. This will prevent the tree from becoming drought stressed. Keep the TPZ free from compaction, this will allow for air circulation around the root zone and prevent the roots from suffocating.
- Grade changes - Grade changes within the TPZ are not permitted unless otherwise approved by the City Arborist. Approved grade changes shall not allow for more than 4 inches of fill soil. To prevent decay, new soil shall not be placed around root flare.
Landscaping is a popular option for homeowners who want to improve the curb appeal of their home. If done incorrectly and without regard to the City’s street trees, may lead to the death of trees.
When improving the landscape, take into consideration the location, size and variety of the City street tree. At the request of the homeowner, Park staff will gladly meet with residents to discuss the health and care of a City tree before the project begins. Before starting a landscape project consider the following:
- Never change the grade around the base of the tree. Adding soil around the base of a tree may cause the tree to die by suffocating the root zone. Removing soil may damage feeder roots.
- Never place sod directly up to the base of the tree. Allow at least one foot for young tree or at least 2 feet for older trees of sod free soil around each tree. Placing sod directly to the base of trees will cause the base of the tree to stay moist which may encourage decay.
- Root barriers. Placing root barriers around newly planted trees will delay the roots from entering landscape and turf areas. For young trees, bury the root barriers as far away from trunk as possible. For established trees, roots can be cut and root barriers installed. Contact the Parks Division for specific instructions.
- Damaging surface roots. Many tree roots lie within the first 8” of soil absorbing as much water as possible from lawns and landscape areas. Care must be taken when installing new lawns or landscapes, not to damage or remove a significant amount of roots. Rototillers, sod cutters, hand tools and heavy equipment can all cause injury or death to existing tree. The weight of large rocks may cause compaction, and they are discouraged as ground cover under trees. Contact the Parks Division on how to handle roots in these situations before the project begins.
- Irrigation. The preferred method of irrigating new and established trees is by drip irrigation. This allows water to be absorbed into the soil with minimal loss by evaporation. It allows for water to penetrate deep into the soil and force root growth down and away from landscapes and sidewalks. It also conserves water. The drip system should be placed under the trees drip zone and moved outward yearly, as the tree grows.
Six steps to successful tree planting
- Dig the planting hole: Before digging locate all underground utilities and pipes such as water, gas and electrical. The planting hole needs to be only as deep as the container of the tree allowing for the root ball to sit 1 to 2 inches above the finished grade. The bottom of the hole should be compacted to ensure root ball will not settle. The hole should be at least twice the diameter of the container and the sides should be scored or sloped rather than vertical.
- Tree Preparation: The tree purchased should be of good quality. Inspect container before purchase for girdling, circling or twisted roots. Remove tree form container and prune and broken, circling or girdling roots. Roots matted along the sides and bottoms or a container can be cut and spread apart. Remove any dead limbs and correct any structural defects such as multiple leaders.
- Planting: Lightly compact bottom of the whole to avoid settling. Place tree in the hole and check depth to make sure that the final height of the root ball is 1 to 2 inches above grade. Check that the trunk is straight.
- Backfilling: Soil from the hole should be satisfactory for backfill. If it is of poor quality, amendments may be added. Amended soil has not shown any significant benefits from native soil. Place backfill evenly around root ball and lightly compact and add water to eliminate air pockets. Any excess soil can be used to form a berm around the edge of the hole to hold in water.
- Staking: Staking a newly planted tree protects the trunk, anchors the roots and supports the crown. Use a 2 inch round lodge pole stake (3 inch for 24 box size trees) and if possible, place it on the windward side of the tree for support. The stake should be place outside the root ball and 2 to 3 inch rubber ties should be installed with a twist and nailed back to the stake. Staking the tree too loosely will not support the tree; staking to tightly will not allow the tree to flex in the wind and develop a taper to support the tree. Staking is only a temporary treatment and the stake should be removed after 1-2 years.
- Mulching: Place 3-4 inches of organic mulch around the tree to retain moisture. Avoid piling on mulch against the trunk of the tree.