By Dan Pskowski
Several residents have inquired about the recently planted oak trees along Crocker Drive why they look like the leaning Towers of Pisa. There are at least a dozen of the these cork oaks (Quercus suber) that are leaning, and one has already been cut down, most likely because it blew over during one of the several recent storms.
There are approximately 50 cork oaks trees lining the street that were planted in early 2014. If you look closely at their trunks, you’ll see the outer bark is made up of cork. This is the same cork used to seal wine in a bottle. The cork is harvested from the tree just like wool from a sheep. Therefore, the tree isn’t cut down, and it continues to produce cork.
But why are these cork oaks leaning? Is it a shallow rooted species? No. Cork oaks are a deep rooting tree native to the Western Mediterranean and North Africa whose climates are similar to Sacramento’s hot dry summers. Drought tolerant tree species develop a deep root system. What’s happening here is these trees were not properly rooted into the surrounding soil to sufficiently anchor them.
There are several reasons why this happened. The tree could be pot bound. The tree was kept in the nursery container too long and the roots grew in a circular pattern conforming to the inside of the container. If a tree is pot bound before it is planted, the roots need to be cut and spread out. This can be done easily if the roots are ½ to 1 inch in diameter.
However, if the roots are 2 to 3 inches in diameter, it is difficult to correct these circling roots, which arborists refer to as girdling roots. This rooting pattern causes the tree to fall over during a storm event. Trees with severe girdling roots should never be planted.
Another cause for lack of rooting could be compacted soil. The construction of the road and adjacent sidewalk require 90 percent soil compaction. The soil in the park strip wasn’t properly prepared for planting trees. Good soil drainage is crucial for root growth. Roots won’t grow in a soil that stays wet and doesn’t have a chance to dry out. The turf which was installed up to the tree’s base exacerbates the problem. The constant watering that the newly installed turf requires doesn’t allow the tree to form deep roots.
What should be done to resolve the leaning oak problem?
Unfortunately, all these cork oaks, not just the ones leaning, should be removed and replaced with a more appropriate tree species. The cork oaks were planted in 4 ½ ft. to 5 ½ ft. wide planters, which are too small a planting space for a large canopy shade tree. U.C. Master Gardeners recommend not planting cork oaks in a turf area. Drought tolerant trees aren’t compatible with summer watering, which can predispose the tree to certain root diseases.
SCNA Board President Eric Johnson sent a letter to the City’s Urban Forester in July 2014 expressing concern over the planting of theses cork oaks. SCNA was troubled to learn that Urban Forestry’s own tree planting guidelines recommend at least a 7 ½ ft. wide planter for large canopy trees. Furthermore, the City should have required Petrovich Development to construct a 7 ½ ft. wide planter for these cork oaks. Why, because they were allowed to remove over 250 trees on site and, therefore, did not have to design roads and other infrastructure around existing trees. The Urban Forester wrote back defending the selection of the cork oaks.
SCNA sent a letter in June 2016 to the City’s Public Works Director expressing concerns about the fill soil around Portola alley oaks and reiterated the concern about the cork oaks along Crocker Drive. The City again refused to acknowledge the poor tree selection and take any action to replace the cork oaks.
Hopefully, when the cork oaks with poor root development have to be replaced, a tree more compatible to the site will be replanted.