Magnolias have wet feet
I have bought 2 x 3 meter high Teddy Bear Magnolias which I recently planted in Melbourne. I dug a hole 1. 5 x the root ball, with extra depth for 200mm for scoria and then 300m of 5 way mix soil under the root ball. There appears to be a lot of runoff water from the recent heavy rainfall that is causing ponding of water above the bottom of the root ball. This is causing the leaves to turn yellow and lots of leaves are now turning brown and falling off. However, the new leaf buds are coming through.
I have dug a deep hole down the side of the tree to the clay, which is approx 300-400mm below the base of the root ball. Hence, the reason for the scoria for drainage. Unfortunately, all my storm water drains are too high to discharge into. I was thinking of possibly filling this hole with scoria only and inserting a pipe down to the clay level that can be connected to a small pump and occasionally after the rain turn the pump on. This all seems a little crazy.
Can you recommend any other physical contraption that may be more effective? Also, the trees are quite large - do the leaves normally fall of after shock or would it be just definitely the water ponding at the root ball (wet feet)? Any responses would be much appreciated.
Certified GKH Gardening Expert
Magnolias normally can handle wet feet, but with being newly planted, yes, it could be the shock that is causing the leaves to drop and the excess rain could be making the shock worse.
I would look into a temporary solution as for getting the water, as once the trees are established, they will like the extra water on occasion. Normally, for someone in your situation, I would recommend building a french drain, but it sounds like that it already along the lines of what you have done. This article may also help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/yard-drainage-problems.htm