Plant Doctor Archive

Trimming alder trees

I have 5 evergreen Alder trees (Alnus jorullensis) approx. 3 years old and 3 m high x 2.5 m wide. I am unsure as to whether I should leave the trees to grow in their natural form, or if I should remove the lower limbs for a nice clean trunk. If I chose the latter, will this ruin the look of the tree, as I have not seen mature trees anywhere as yet and do not know how they will eventually look.

I passed your question on to Jim Antill, one of our landscaping experts. He said:

Most trees that have a definite top leader (exhibit "apical dominance", in trade jargon) can have their lower canopy removed progressively to create headroom, etc. We call it "siding-up". The rules are:

1. Work out WHY you want to cut the tree — what exactly you want to achieve.
   
   
2. Make "proper" cuts, i.e. DO NOT EVER cut the branch flush to the stem, thus damaging the branch collar (that slight swelling that often occurs at the base of a branch). If you do, you wreck the tree's natural defences against infection, that are present in the collar, and lay the tree open to the incursion of rotting fungi.
   
   
3. Never remove more than a third of the canopy in any one year period, preferably only 20%. This reduces the shock to the tree and allows it to retain its disease resistance. (Alders are pretty tough, however.)

I'm not familiar with A. jorullensis either, but it's fair to say that the more you side-up a tree, the more it will build canopy further up and thus get taller. If this is what you want and if the site is reasonably sheltered, fine. If not, go a bit easy!

Note: Alnus jorullensis is also known as A. acuminata.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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