A hot windless summer morning. A really loud crashing sound. The street behind our house is completely blocked by a huge oak branch that has just fallen, crushing a parked car. Neighbors and I walk up to the shattered branch to take a look. The limb is enormous – at least 20 feet long and 3 to 4 feet in diameter. The valley oak tree that the limb fell from is over 30 feet tall and appears to be healthy (green leaves, no branch or tip dieback). The branch had broken off about 5 feet from the trunk and it wasn’t a clean break. Both ends of the oak limb are jagged and torn. No signs of disease, insects or rot.
Sudden limb drop, sometimes called summer branch drop, is common in our climate zone. On hot windless days, large heavy branches from mature trees (especially oaks, Southern magnolias and eucalyptus) will suddenly crash to the ground with no apparent cause. The breaks typically are not clean – the branch splits unevenly, 3 to 12 feet from the trunk. Long horizontal branches drop most often.
No definitive cause for sudden limb drop has been discovered yet. Theories on possible causes include loss or movement of water within the tree and higher concentrations of ethylene gas.
Deep irrigation of mature trees that are more susceptible to sudden limb drop seems to have little effect (but it sure can’t hurt to deep irrigate our venerable landscape trees during the hot summer months). This tree had been poorly pruned (what’s often referred to as lions tale’s pruning) with main branches stripped of secondary branches. Tufts of leaves were left at the end of the branches, placing more weight there.
Repair of the damage to this tree and others that drop large limbs in summer is work for highly skilled, certified arborists. The loss of a major branch can unbalance the tree structure which must be correctly rebalanced and the broken limb must be trimmed so that the break heals properly.
It’s common in our hot dry summers for small interior branches on redwoods to die and fall, littering the ground underneath the trees with brown needles and dead wood. The amount of litter often concerns homeowners who assume the entire tree is dying. This process is normal during hot weather for coastal redwoods that are better suited to cooler climates; they’re shedding excess interior wood and needles to maintain a heavy outer canopy that creates a protective cooler microclimate around the trees.
Many redwoods in our area are subjected to thinning, a type of pruning which removes many of the main branches to create an open branch scaffolding. Thinning a redwood tree raises temperatures inside the canopy and exposes bark to scorching sunlight. Interior branches that have died during the summer heat can be cut off without stressing the tree, but major exterior branches should be left in place. Some homeowners don’t mind the look of the thick carpet of needle litter and leave it on the ground to act as a mulch. Good idea.
In summer, keep the soil under redwood canopies consistently moist using soaker hoses, small oscillating sprinklers or bubblers to slowly irrigate the shallow root system.
Send Elinor Teague plant questions at firstname.lastname@example.org.