pruning trees and shrubs -wk 14

Today we meet the two professional arborists who work here at the gardens - Jerry and Alister. They will be taking us for our introductory session to pruning trees and shrubs.

Jerry shows us an example of a correctly pruned tree.

After a time inside running through notes which outline how trees grow, the root structure, main reasons for pruning and the correct and incorrect ways to do this - it's time to head outside to look at some trees and apply our new found knowledge. I'm super interested in this unit and trees in general so I'm literally soaking up everything Jerry and Alister say. 

The picture above is a good example of the branch bark ridge line that forms when the two different angles of bark meet. This line is what is used to correctly determine the best position to 'target' prune the branch if required. It is important not to cut too close or too far out from the branch collar. The bark ridge line gives you an angle (when you add an imaginary one vertical to the ground) that you then double in the other direction to find the best point to cut. Sometimes the collar around the branch is evident but in many cases it is not, so using this method will help to determine the best cut point. Below is an example of a bad cut from a tree on my property (don't worry we didn't do this!) where the cut has been made too far out and not able to heal over. Water has then sat in the cut and slowly started to rot away and this may have in turn introduced fungal disease into the trunk.

A poorly pruned branch has been unable to heal over
and now shows clear signs of rot.

One of the things we learn today is that the strongest, safest growth for a tree is one with a dominant apical trunk, that is one main leader all the way up. Trees with two or more leaders will always collect water and litter in the joins, which can lead to rot and an entry point for fungal diseases. Of course there are many trees that don't have this type of growth as nature intended - like multi-trunked maples and beautiful spreading oaks. But Jerry stresses that it's not about changing or forcing natures way to make all trees look the same shape (typical of us humans who like to have everything safe, easy and controllable!) but rather about having the knowledge to assist and manage trees where possible and if required. There are many times he says when it is far better to leave things as nature intended and not ruin the overall appearance of the tree or threaten it's life if it's clearly not necessary. He stresses the best way you can prevent problems later on however is to start is when the tree is young. Try to encourage a dominant leader and not make too many cuts (or wounds as he calls them) in a young tree in the early stages so it can develop a good strong trunk.

Alister showing us an example of a healed over cut.

We learn also of a good rule of thumb when pruning, remove no more than 30% of foliage from a young tree, 20% from a mature tree and 10% from a very mature tree. Also, always think before you cut as all pruning causes wounds and potential for infection by pathogens  - is there another option that might work like spreading or tying down? What season you prune in can also make a difference to the rate of healing and reduced vigour in the shrub or tree.

The stunningly beautiful multi-trunked paperbark Melaleuca. 

One of the trees we look at next in the gardens is a beautiful and very old Melaleuca. It has multiple leaders and has required the arborists to install a network of supportive cables in the canopy to stabilise the branches and also prevent them from causing too much damage to the rest of the tree, should they decide to crack and fall in high winds for example. Quite a few large older trees at the gardens have this method of support in place, as Jerry points out - it's about retaining the tree safely for a long as possible in a historic public garden such as this. Each tree has it's own significance and history and is worth the extra effort for extending it's natural life and preserving it that little bit longer if possible.

This paperbark tree has had some pruning to open up the centre of it's crown.

Later back at home I am thinking about my Virgilia tree (Virgilia capensis I think?) outside our bedroom and whether we should go ahead with building the kids treehouse in it or not. I decide to take a closer look with my recently acquired tree knowledge and unfortunately it looks as if the cubby will have to go somewhere else because the tree is a classic example of just about every problem in the book!

Odd galls like this can be found all over the tree.

This poor tree has had a dodgy pruning job by the previous owners at some stage and the result is wounds that haven't been able to heal over nicely and have started to rot. There is also a lot of crossing branches that have appeared to have grown together but are surrounded by swellings in the bark.

Crossing branches along with lots of swellings and die back.
Are these the indicators for rot and disease within the trunk?

At the base of the tree the multiple trunks show inclusion. We have learnt that bark inclusion is where the join and bark starts growing into the tree, weakening the structure of the trunk and making an entry point for disease and moisture to hide which can ultimately affect the tree later on.

A large bark inclusion on one of the lower main branches.

I read in my book that the lifespan for this fast growing South African member of the Fabaceae family is around 15 years. We have lived here for 8yrs and this tree was much the same when we arrived so my guess is it is probably much more than 15yrs old. Looking also at the tree in general, it has plenty of die back of the internal limbs although there is still some vigour and growth as well as flowers on the outer tips and branches. Perhaps time for this tree to be safely removed? Definitely not a candidate for my kids to play in that's for sure. I will show the pictures to Jerry and Alister next time to see what course of action they would recommend and whether the symptoms and decay are a natural part of the tree reaching the end of it's life span or because of disease? Stay tuned.

Time to go?