Answer: When a tree dies from the top downward, it usually indicates a root problem. Since you're losing them one at a time and they were all planted at the same time, I'd suspect poor drainage. Soggy soils will rob the roots of required oxygen; sandy soils drain very quickly and the roots may not be getting the moisture they require. Over-fertilization will cause burning of the roots and subsequent dropping of foliage.
Why not dig one of the trees up and inspect the roots? At that time you can check the soil drainage by filling the excavation with water and seeing how long it takes to drain. (8-10 hours is good). Healthy roots will be creamy white inside; rotted roots will be mushy and black, brown or rusty colored. If the roots look healthy and the hole drains well, replant the tree and water it well.
If the roots are dead or dying, cut away the most obviously affected roots and replant your tree after amending the soil with organic matter to help it drain quickly. Then replant the tree and water it well to help it get situated in its new home.
Best wishes with your podocarpus trees!
Q&A Library Searching Tips