The clay soil around my evergreen shrubs has become very compacted. I lost a Japanese yew and a cherry laurel to root rot. Can I aerate around the shrubs and fill with mulch? Or will that risk doing too much damage to the root systems? The attached pictures are of the roots that rooted. Thanks,
District of Columbia County District of Columbia
It could indeed damage more roots, as vertical mulching is often targeted to trees and the hole size can be substantial relative to the size of the root zone of these shrubs. Vertical mulching is most often used for trees needing soil aeration around the roots, and it is done in winter to minimize plant stress from the damage to fine feeder roots. While it may help here, it also may not solve the problem of compaction overall. If the rot set in only in the past year or two, the excessive rain the past two springs probably were the tipping point. If planting new shrubs in that site, you could use a soil amendment like compost to reduce compaction, though there are some situations where amendments can be problematic. A raised bed in that site may also help mitigate future problems. If the plants are scattered among others, however, this is understandably not practical.
If you are opting to replace the plants with alternatives, Japanese Plum Yew (Cephalotaxus) looks similar to Yew and is purported to be more tolerant of damp soils (which also can be lower on oxygen, as with compacted soils). Inkberry holly (Ilex glabra) isn't as close of a match to Cherrylaurel in appearance, but it does have better tolerance for wet soils as well. (It doesn't thrive in more shade, however, like Yews and Cherrylaurels can.) Many plants will struggle with some soil compaction, but using a bark or wood chip mulch in the bed can help amend the soil gradually and help mitigate compaction.
Organic matter helps keep our fine clay soil particles from being compressed as closely together; on a small scale, it glues together clumps of particles into larger sizes that allow for more air and water penetration into the soil, and larger particles can't compress as tightly as smaller ones. Other than keeping foot traffic off of that planting site, using organic matter to "fluff" up the soil is likely the most practical long-term approach. That said, be sure to amend with restraint as a little organic matter goes a long way, and using too much will create water permeability issues between the two soil types (amended and un-amended).
Thanks for all that information, Miri. Very helpful. Do you think aeration with those high powered air hoses would be helpful to the shrubs that have survived in the same hedge?
It's pretty difficult to change the soil profile in an area with established plants.
The use of an air spade is iffy. It could possibly end up making thing worse if a hard pack soil 'bowl' ends up holding water underneath.
It's hopeful that the conditions are better in the area where the surviving shrubs are located.
Unfortunately the two plants you have just don't tolerate wet soils.