Tree and shrub roots that become buried too deeply by mulch and/or soil can "suffocate" because they don't have enough access to oxygen. The depth of mulch that is too much depends on the kind of mulch used. Bark mulches should be laid around three inches thick or less, but wood chip mulches can sometimes be laid deeper without problems. Deep mulch layers might also interfere with how well rain or irrigation water can percolate into the root zone, potentially keeping them too dry. Any tree or shrub, regardless of its age or species, should not have the base of its main trunk/stems covered in mulch. Instead, they should get good air circulation by keeping mulch several inches away (at least) from the bark and never piling mulch up against it.

If you are referring to a variety of Japanese Holly (botanical name Ilex crenata 'Convexa'), root rot tends to occur for other reasons with this species. There is a type of root infection called Thielaviopsis that can kill this and other holly species, and tends to be a problem in sites with over-watering (or poor soil drainage) and/or soil that is not acidic enough (the pH is not low enough). We see this often in locations like near a house foundation (could concrete be leaching and raising the pH?) or where a roof downspout outlet empties too close to shrub roots. This is a pathogen of opportunity; it can be present in soils but harmless if the plant is vigorous and healthy, but when conditions stress the plant, it can infect roots and kill the shrub from there. There is no curative treatment, and prevention focuses on giving plants the conditions they need to thrive. If one plant type, like hollies, don't work well in a certain location, another shrub species might do better without having to try to alter the site conditions.

Laboratory soil testing only screens for nutrient content, acidity (pH), and organic matter content. They do not have the capability of testing for root rot pathogens or other plant diseases. Still, its information can be valuable. Did a soil test you had performed indicate what the pH was for the soil in that planting bed? (Was it a lab test or at-home test kit? The latter are less accurate and sometimes hard to gauge.)

Other causes for root rot or shrub dieback include planting too deeply (the soil depth itself being too high compared to the original root ball, mulch depth aside), not loosening-up the roots before planting, and watering post-planting being too often or too infrequent. We don't know how long ago those hollies had been planted, but rarely do serious diseases afflict shrubs in their first year or two in the ground. More often, it is the environmental/site conditions that cause plants too much stress to resist the normal amount of disease pressure above- or below-ground that ordinarily would not bother them.

You mention that other plants in that area died. Can you tell us what they were, and what symptoms they exhibited as they declined? What time of year did the symptoms start? We realize you can't share photos for diagnosis now, if the hollies died awhile ago, but if you notice any more symptoms this year, feel free to send us photos so we can try to narrow-down possible causes.

Miri