Sunday, 5 July 2015

Large street tree maintenance/pruning

Something we know that generates a lot of heat amongst local residents is the arrival of the chainsaw gangs that are contracted to maintain some of the larger trees along our streets. In the autumn of 2014 work was done on the Silver Maple trees in Harefield Road and Manor Avenue. Neither street had had extensive work done on these tree species for some time, and the effects looked alarming, which was partly what was concerning to so many. 

Harefield Road - November 2014 - pruned late summer
Already these trees were replacing foliage even though they were approaching their dormancy period
In this post we will try to cover the main points of contention. 

Why do this work? 

All trees need to be maintained. Without attention, defective branches or malformed boughs can introduce stresses that eventually cause boughs to shear or break in high winds and storms, causing damage to property and risk to life. Failing boughs can also allow water and pathogens into the trunk, eventually causing the death of the tree itself. Judicious pruning and maintenance can radically improve the long-term health of the tree and of course adds to the overall aesthetic value of the tree in the local environment. 

Mature deciduous trees take a great deal of water out of the ground during the growing season which they use to create new foliage, flowers and wood. In removing branches and foliage the tree takes less water with the result that more stays in the ground. After pruning the tree limits the growth of its root system, which of course is how the tree gets the water out of the ground in the first place! This is of course relevant to those residents who are concerned about the effect of trees undermining foundations or changing the water table and hence contributing to subsidence. 

There is also a conservation consideration for carrying out a regular maintenance programme on these trees: if the council can show that the tree is part of a regular pruning cycle it puts them in a much stronger position when defending against felling orders brought by insurance companies when trees are claimed to be causing subsidence. 

Thinning or "crown reduction"?

This is a complex question. Generally, thinning involves the removal of specific boughs and 'crown reduction' the removal of a good deal of wood and foliage around the periphery of the tree canopy. Each tree is generally considered on its own merits, and work is done usually according to age, condition, shape, etc. 

There is a very good account of these procedures on the website of the Urban Tree Foundation (in California) for those who are interested in reading more. 

When to do the work? 

Trees are never worked on when there are nesting birds in the canopy (this is illegal and can result in considerable fines for the contractor or owner). 

Most trees can be worked on at any point in the year, with a few exceptions, which are:
  • Maples, sycamore & birch - these trees are prone to 'bleeding' (i.e. loss of sap, which can be severe in certain tree species) so not pruned until in full leaf to prevent this
  • Walnut - extensive bleeding occurs unless pruned in full leaf, preferably in August
  • Cherries and plums - early summer pruning (April to July) reduces the risk of the trees developing 'silver leaf' (a fungal diseaseChondrostereum purpureum that enters the cuts made during pruning)
Summer pruning has a number of advantages:
  • Wounds start to heal immediately 
  • Pruning of deciduous trees to admit light or improve views can be more accurately judged   
  • Dead wood can be readily seen in deciduous trees 
  • Working conditions are generally safe on larger trees        

Response to pruning - broadleaf trees

The following table provides general guidance on the tolerance of varying broadleaf tree species to substantial pruning. Some respond with an immediate mass of new growth, others respond moderately well, whilst some (notably Beech and Birch) can be killed by injudicious pruning. 

Virtually all broadleaf trees will tolerate heavy pruning during their formative years.




Horse Chestnut




Harefield Road, July 2015 - the canopy on these Silver Maples has regrown

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