Having just attended a very busy two days of the Qld Garden Expo in Nambour, I should share some of the things I learnt from talks attended there, before they disappear from the memory. Peter Young of Birdwood Nurseries related that most commercial growers of fruit trees are now restricting the height of their trees to around 3 metres. This done mainly because of WPHS issues for their pickers, all fruit can now be picked from the ground. The growers have pruned the trees so that they spread out more sideways and the harvest has not been affected as much as they expected. Where possible they have also interplanted. This has further enhanced their harvest. When the trees get their tops taken off, some of the root mass dies back. This knowledge has implications for people like me who grow some of their trees in pots. Obviously it is wise to not allow trees to get too big in pots for many reasons. One of the main worries that I was concerned about was that where I have a tree like a Dwarf Avocado getting taller, I can cut the trees back each year to control top growth (height), but does the root growth keep on going until the plant is pot bound? I was envisaging having to take these large trees out of their pots and trimming the roots back every few years. Well now I know that I don't have to. If I prune back the top, a corresponding amount of root will die back. So In theory I don't have to repot into ever larger pots, I can control the tree size all round. This means that once the tree has attained a good size that will allow me to get a harvest size that I am happy with, I can leave the tree in that size pot.
Other info relating to growing trees in pots is that the growing mix should be around 30% sand, 60% of a good (four ticks) potting mix, and 10% coir or peat moss. The slow release fertiliser that comes in the potting mix will only provide nutrition for a few months, so you will need to top this up around 4 times a year. I always put well rotted horse manure in with my mix, down the bottom of the pot so that the roots can grow into it. I then top up the pot with manure a few times each year, and mulch with sugar cane.
If you just use organic materials in your mix, the mix can slump too much, the sand provides some body to hold everything together.
Where a fertiliser bag states " provides food for up to 3 months" (or however long). The nutritional release is like a bell curve, The initial amount is small and gradually builds to maximum after a month and a half. By the time the three months is up the amount is back to almost zero. So to keep food up to you plants, when using the above type of fertiliser you would need to add more after a month and a half, etc. etc.