Pruning citrus gets almost as many hits on social media as pruning roses. Unlike roses, citrus doesn't need annual pruning to produce good quality fruit in quantity.
So, why prune a citrus tree?
To shape a new tree
To make harvesting ladder-free
To rejuvenate an old unthrifty tree
To reduce pest and disease problems for existing trees
The best time is immediately after cropping before the onset of hot weather. One hint when exposing branches to direct sunlight you can unwittingly open up the bark to sunburn, as the trees are quite susceptible. Paint the newly exposed stems with a white acrylic outdoor paint. This reflects heat and prevents burning which also has the effect of encouraging disease and insect attack.
Rejuvenating old unproductive trees
Old lemon trees are the usual suspects here and the treatment is brutal. Fortunately, the tree can be completely reinvigorated and brought back into strong cropping. Tools needed include chainsaw or bow saw as well as branch loppers. Cut back main trunks and branches to around chest height. It's likely that you will be left with 4 or 5 bare trunks so yes it's brutal. Painting the exposed trunks is even more critical here to prevent sunburn. New suckering growth will explode around the cut. It's a good practice to allow only 2 to 4 shoots per branch to remain and go on to grow into the new canopy trunks of the tree. This level of pruning only needs to be repeated every 8 to 10 years.
Controlling pest with pruning
Perth has copped a few new pests from the east including citrus leaf minor and more recently citrus gall wasp. Both problems can be treated by pruning. Citrus leaf minor attacks the youngest foliage and once the grub hatches it eats out the cells between the top and bottom surfaces leaving a squiggly silvery mark within the leaf.
The Gall wasp lays eggs in young stems, which swell up in response. These swellings, which are tapered at both ends, need to be pruned out completely. Once again it's best to either solarize or dispose of these swollen stems through your rubbish system so as to prevent the young wasps emerging and infesting your or neighbours citrus trees.
Fruit fly can be a serious problem, particularly for mandarin and tangelo. My preferred control consists of covering the tree with shade cloth when fruits are about half their mature size. In order to easily accomplish this exclusion netting approach, I have found it necessary to reduce the height and spread of my trees, in order to easily pull the cover over the canopy. A quick hint I tie off the shade cloth around the trunk of the tree so the pesky fruit flies can get under the cover to do their egg-laying work.