A limb comes down on a windless day
It happens across the North Shore every summer. No storm, no wind, thirty-five degrees and still, and a substantial limb drops out of an apparently healthy gum. Owners assume the tree must be diseased. Usually it is not.
What is going on
Summer branch drop is a recognised behaviour in eucalypts and has been documented for well over a century. The mechanism is not fully settled, but the contributing factors are reasonably well understood.
On very hot days a large limb loses water faster than the tree can move it, and the wood's internal moisture balance shifts. At the same time the limb heats unevenly along its length. Long horizontal branches carrying a lot of foliage and fruit at the far end are already under considerable bending stress, and small internal changes can be enough to tip a limb with existing internal defect past its margin.
It happens most on still days rather than windy ones, which is exactly why it catches people out. There is no gust to blame and no warning noise.
The North Shore species that do it
Blackbutt, Sydney blue gum and forest red gum on the shale ridge, and angophora on the sandstone country through Hornsby, Lane Cove and Castlecrag. Angophora in particular grows heavy, spreading, near-horizontal limbs over rock outcrops, and those limbs are frequently directly over the paved area or driveway the house was built around.
What actually reduces the risk
Three things, in order of usefulness.
Reducing end weight on over-extended limbs. Taking the outer portion of a long horizontal branch back to a suitable live lateral lowers the leverage at the attachment without disfiguring the tree. This is proper reduction pruning, not topping.
Removing deadwood and hung-up material. Dead limbs come down regardless of weather and they are the easiest thing to eliminate.
Moving what is underneath. If the outdoor setting, the trampoline or the car is under a spreading angophora limb, relocating them costs nothing and is more effective than any pruning.
What does not help
Topping the tree. Removing a large proportion of the live crown in one go stresses the tree, triggers a mass of weakly attached epicormic regrowth, and creates a worse hazard within five years. It is also an offence on a protected tree, which on the North Shore most large eucalypts are — see the permits article and the article on topping.
Worth an inspection before summer
The practical move is a look over the tree in late spring — deadwood out, over-extended limbs assessed, anything hung up from winter storms removed. Ring (02) 9090 4915. The tree loppers page covers the work and the blog has more.
Frequently asked questions
Which trees are most likely to drop limbs?
Large mature eucalypts with long horizontal limbs are the usual candidates — on the North Shore that means blackbutt, Sydney blue gum, forest red gum and angophora in particular. Older trees with over-extended branches carrying a lot of end weight are more prone than young, compact ones with well-proportioned crowns.
Does pruning stop summer limb drop entirely?
No, and any contractor claiming otherwise is overselling. Reducing end weight on over-extended limbs and removing deadwood lowers the odds and removes the material most likely to fall, but the behaviour is inherent to the species. Where the consequence matters — a pool, a driveway, an outdoor area — reduce the exposure as well as the tree.