Even before a tree is planted a number of factors will determine how safe the tree will be when it matures. The variety of the tree, the growing conditions at the nursery, the method of planting and early pruning all make the difference between a safe tree to enjoy for generations or a hazardous tree doomed to fail.
While any tree has the potential to fail; avoid trees that are structurally weak and prone to failure. Some trees are more likely to be brittle and drop branches, especially during storms with high winds. These include willows, eucalyptus, poplars, Monterey pines, Italian stone pines, douglas firs, and Monterey cypress trees.
Trees which often have trunk failure (splitting of the trunk) include several native trees and some ornamental species. Native trees subject to trunk failure include coastal live oaks, black oaks, firs, and to a lesser degree redwood trees. Introduced trees subject to trunk failure include eucalyptus, locusts, ash varieties, Bradford pears, poplars, liquidambers, and some maple varieties. While some of these trees should not be planted in urban areas many can be pruned or repaired to overcome structural defects.
Trees subject to root crown failure where the entire tree falls over include eucalyptus, Leyland cypress, Douglas firs, Monterey pines, bay laurels, and poplar trees. To a lesser degree native oaks may have root crown failure; especially where roots have been damaged by construction, compaction, or summer watering.
Early growing conditions and planting technique also play a large in the future safety of a young tree. Nursery grown trees are often “tipped back” from the top and lower branches are removed. This makes for uninform trees that are easy to transport, but it creates potential problems for the young tree. Cutting the top of the tree causes several branches to grow from a single area and these co-dominant leaders are forced to compete for stability and few will succeed. Often the trunk failure occurs years in the future when the tree is an integral part of the landscape. It’s important to choose a tree which has not been tipped back, but has a single central leader with good spacing of the branches growing along the trunk. The branches should be horizontally attached, extending perpendicular to the trunk, rather than a high “V” shape attachment. Branches that connect to the trunk with a “V” shape are prone to failure.
Lower branches growing on the trunk should not be pruned off because they add diameter to the trunk. A trunk with good caliper will be stronger than a tall thin trunk. Avoid trees with circling or girlding roots. If a tree is left too long in a container the roots begin to encircle the pot. When planted, the roots continue to circle around the base of the tree and fail to extend outward establishing the strong buttress roots that prevent failure. Poor root development is one of the major reasons for tree failure at the root crown.
To help a tree develop good root structure it should be planted in a hole twice the diameter of the growing container. In clay or poor soils the hole can be even bigger. The depth of the hole should match the container depth. Plant the base of the tree slightly above ground level. Trees planted too deeply will develop root decay.
The International Society of Arboriculture (ISA) recommends that trees should not be staked at planting time if possible. Research has shown that un-staked trees develop stronger trunks than trees that are planted with stakes. If the tree is unable to stand without staking it should be staked as low as possible from opposite sides with approved materials that will not girdle the trunk or damage the bark. For more information on planting and staking trees, a pamphlet is available from the UC Cooperative Extension Office in Santa Rosa.
Early pruning of the tree will also affect its safety. Regular pruning establishes good structure. Waiting until a tree is mature and deciding to reduce the canopy by topping or over-pruning is dangerous. Topping a tree creates decay and weakly attached branches when the tree re-grows. Topped trees also develop thick canopies that can cause the tree to blow over. Over-pruning may also cause stress and lead to decline or death. The ISA recommends contacting a Certified Arborist for pruning trees. The definitive guide to pruning including information on correctly training young trees is the “ISA Pruning Guidelines” available from the ISA.