Planting in clay soils is one of those oft-debated horticultural practices.
Essentially, research showed that trees and shrubs planted in heavy clay had two responses:
If the soil was amended and made very rich within a planting hole – the woody plant would establish and grow very quickly but when the roots hit the clay soils on the edge of the planting hole, they would tend to turn back towards the good soil.
Plants stalled out and became ‘root bound” within the planting hole. It would take a few years before the roots would venture out into the native clay.
The plant during this stalling out phase was more vulnerable to flooding with heavy rain or drought conditions.
Response #2 (The Preferred One)
If the soil was simply turned over but not amended – the woody plant was slower to establish itself but once it started growing, it grew consistently and wasn’t as stress prone as the more pampered plants.
The lesson learned within the hort trade then has been to not amend heavy clay soils when planting.
And that’s the other lesson – only use appropriate plants.
Trying to force fit a plant into a soil for which it is not genetically adapted is a waste of money and good garden plants.
Your choice as gardeners then with trees/shrubs and perennials is to find plants that deal with clay or totally amend the garden so you can grow the plants you want.
This means that growing something like lavender in a heavy clay soil is going to be tricky unless you modify the entire area that influences this plant’s growth.
The lesson here? Pick your plants for your soil. Because anything else is just a waste of time, money and good plants.