Poor cultural practices – the things you do when you garden – can be a hazard to your trees. People kill trees all the time, quite innocently, simply by saying: Honey, let’s put a cute little raised bed around the tree.
All the active tissue in a tree trunk exists very near the surface under its protective layer of bark: the phloem (which is the conduit for food and water between the leaves and roots) and the cambium (busily growing the tree trunk, ring by ring). Damage the bark, especially if you damage it all the way around, and you put these important tissues (just a few cells thick) in jeopardy. You will effectively starve and strangle the tree.
Circling an established tree with a new raised bed or planter can do this, causing rot where soil, moisture and soil organisms touch the base. It may take years, and some trees are tougher. Nevertheless, it’s a sure way to risk injury.
If the tree is planted when the raised bed is created, this warning doesn’t apply. It’s changing the level of the soil that’s the problem. If you add mulch around the base of a tree, for example, always taper it so that the layer is thinner when close to the tree.
A trunk should flare out gently at the very base. That – and the continued healthy life of your tree – is what you want to see. There’s lots of room for flowers elsewhere.