Ask a Consulting Arborist Q&A: Tree Planting Tips and Species Recommendations

Q: Are there any trees that will grow more than 5 feet high in just 5 inches of soil?

Monday, January 04, 2016  
We recently repaired a slope failure on a steep slope. We used crushed rock to fill in the voids and then put top soil over that. The trees we planted all died, I suspect because there's only 5 inches of dirt over compacted rock. Is there anything that will grow more than 5 feet high in that amount of soil? This is in San Ramon, California, where it doesn't rain between March and November and gets really hot, although this site is located along a stream bank.

A: You are likely correct in your assumption that 5 inches of soil is insufficient to establish a sizable tree. However, that may not be the cause of the demise of your trees. Newly planted trees are typically at risk and require great attention to planting technique and aftercare (e.g., irrigation). The long-term success of larger trees depends on the volume of soil versus gravel and the specific location of the trees relative to gravel.  With the information provided, it may be best to stick with smaller, slow-growing, drought-tolerant, preferably native trees or shrubs.  Examples might be California buckeye (Aesculus californica), Western redbud (Cercis occidentalis ), Manzanita spp. (tree forms; Arbutus spp.), Ceanothus spp., etc.  I have no information as to irrigation or aesthetic requirements of the site, but these ideas might put you in the right direction.  Regardless of species, if the gravel is distributed throughout the planting site, there may be insufficient soil to retain enough moisture for most plants to survive.  In that case, regular irrigation may be unavoidable.

Responder: Torrey Young, RCA #282, Castro Valley, CA