The next 3 months are the optimum planting time for lots of trees and shrubs. Temperature start to drop, the rain hopefully returns but the soil remains warm to encourage fast root development. Planting now results in a lot less stress on the plants so they will establish faster, grow stronger and therefore be healthier and able to better resist attach from pests and diseases.
You can only plant once and those plants are an investment. The best way to ensure this investment in time and money is to make sure you lay the best foundation possible to ensure the greatest success. Soil preparation is not the most fun activity or sexiest thing to spend your garden allowance on, but is by far the most important aspect to new planting.
Our soil here is predominantly clay based and clay soils are generally poor in nutrient levels as well as poorly draining when wet and prone to solidifying & cracking in the heat. Clay is made of many small particles tightly packed together. Therefore the best way to increase the viability and health of clay soils is to add lots of organic matter such as compost and products such as blood and bone, animal manures and especially Gypsum. Gypsum works on clay by breaking it down at a chemical level rather than the others that physically break down clay.
Give your plants the best chance of not just surviving but thriving and growing strong by doing the groundwork first. Skipping this step often results in poor plant growth and performance, causing the plant to become stressed and weak, which is when pests and disease are most likely to attack.