I am soliciting some advice from you other blogging gardeners. My yard has several areas where I would like to plant perennials, but invasive, wide-spreading feeder roots from existing older trees and shrubs make new planting difficult and rob the soil of moisture and nutrients. These are areas that are 5-10 feet or more from the main tree trunks or shrubs. Some areas are shady, some are sunny.
I have had some success in recent years with making a couple of raised flower beds in these areas, covering the original existing soil and the inside sides of the wood bed material with high-quality woven weed barrier before adding improved planter soil. In one, I doubled the layer of weed barrier before adding soil. The raised beds are 12-14 inches high. They are planted with perennials in the back and center, leaving the front edges open for annuals.
Since my weed barrier is quite permeable to water and oxygen, I believe I am doing the existing trees and shrubs little damage, since the beds are watered regularly and the bed soil is quite light and porous, similar to standard potting soil. Also, the beds do not cover a large portion of the tree/shrub root area. Water, even heavy rain, seems to drain easily through both the soil and weed barrier. So far (in 2-3 years), the perennials seem to thrive in this environment, and the original tree and shrub invasive root problems seem to be alleviated. Whether the weed barrier will stand up to them long-term remains to be seen.
What I am wondering is: In your experience, are there many common herbaceous perennials that require a root depth of over 12 inches to attain healthy maturity? I have “Googled” to some extent to determine this, but would welcome your experiences and knowledge in this area. Thanks!
Gardener Dave