The tree basin is the square – or rectangle – that is cut out of the sidewalk. Cover it with mulch, which retains moisture and helps prevent soil erosion.
Soil / mulch should be level with the sidewalk.
Mulch should be about 3″ deep but should be kept away from the trunk.
If you need to add soil, it’s best to add natural soil from the immediate area rather than to buy soil. (see the soil section to find out why)
Don’t raise the soil around your tree trunk! It can cause rot and kill the tree. If you must build a raised box around the tree, keep the tree trunk at a separate level from the raised part. Do this with a large tube or barrier around the trunk, with plenty of airspace, inside the box. Make sure there is room for the tree to grow!
Note that city code requires that you apply for a minor encroachment permit for anything that sticks up around the border of a tree basin. This includes benches, bollards (large concrete poles), fences, planter boxes, etc. The edging treatment must be at least 6 inches and no more than 15 inches high from the grade of the sidewalk and allow water to flow around or through into the landscape (tree basin). It may be made of wood, stone, decorative metal or other material, subject to staff review. For more information call the DPW Bureau of Urban Forestry, 415-641-2676.
Don’t add other plants to the basin during the first year after planting; they’ll compete with the tree for water and nutrients. On the other hand, bare dirt isn’t good either, because it doesn’t hold moisture well. If you do have other plants in the basin, keep them short and away from the trunk. See our tips on companion plants.
We recommend against covering the basin with bricks or stones; they won’t help control roots, and they may deprive the tree of water and oxygen. If you do add bricks or stones, leave spaces between them, and do not fill the spaces with mortar. Consider adding only a border of flat bricks, rather than covering the basin.
If the soil is sandy, the tree’s roots are less likely to crack the sidewalk as the tree grows, because roots travel easily through sand, and they’ll tend to travel downward to chase the water that penetrates quickly through the sand. If the soil is clay-like, however, roots are more likely to grow close to the surface and to crack the sidewalk. To avoid this, you can do preventive “root pruning” when the tree is young by taking a shovel and cutting around the border of the tree basin every 6 months or so. Using a “rootguard” for larger tree species or species with invasive roots may prevent or delay future sidewalk disruption, but works better in sandy soils.
Help! My sidewalk is cracking! What do I do?
If a tree is already cracking your sidewalk, the best solution is to expand the tree basin to accommodate the roots. The Americans with Disabilities Act requires that the sidewalk be at least 4 feet wide to allow the passage of wheelchairs, so there’s a limit to how wide your basin can be, but there’s often plenty of room to expand it lengthwise. An expanded basin can be turned into a beautiful sidewalk garden.
Expansion of the tree basin may require a minor encroachment permit. Please check with the DPW Bureau of Urban Forestry, 415-641-2676.
If you can’t expand the tree basin, follow these three steps:
- Remove the affected concrete.
- Have the tree root pruned by a certified arborist. This is not a job for a novice. The idea is to prune the roots without adversely affecting the stability and health of the tree. See our arborist referral page.
- Replace the concrete.