Raised Beds Under Trees
To avoid the problems associated with planting beneath trees, a frequently adopted approach is to build a raised bed around the trunk and plant into this, rather than directly into the ground. A bed that is constructed too tightly around the tree looks out of proportion, and soil heaped against the tree trunk can harm it. However, it can be successful if the level of the ground is raised no more than 16 inches (40 cm) and soil is kept away from the trunk. Beds of this type look best when the diameter is at least 6 feet (2 m) if built under a young tree, and more if it is under a mature specimen.
Plants growing in a raised bed will be given a good start, and the roots will gradually make their way down into the soil beneath. Regular watering is essential to establish the plants and will always be required when the tree is in leaf.
A shallow raised bed, constructed of rustic logs or landscape timbers and planted with low, evergreen groundcover subjects will provide a green ‘pool’ around the base of the tree. It can also become the setting for an ornament or feature that will create a focal point in the shade of the tree.
Do Not Attempt the Impossible
Be realistic in your expectations. Large mature deciduous and coniferous trees are not hospitable to new plantings around them. Establishing new planting under their canopy will not be easy and may be impossible. It is best to experiment with a few tough plants before investing in a full-scale planting scheme and finding that it will not succeed. Autumn planting is always the best option: Plants have the winter to establish their root systems before competition from the trees increase in spring.