· by Michael Blair, Nashville's Landscapers
Rock is usually chosen because it looks permanent and mulch looks like a recurring bill. In Middle Tennessee both parts of that reasoning tend not to survive the first couple of summers.
What each does to soil temperature
Organic mulch insulates. Two to three inches keeps root-zone soil measurably cooler through July and holds moisture underneath it, which is exactly what a shrub in Nashville clay needs.
Rock does the opposite. It absorbs heat all day and radiates it into the bed and against the foundation, and light-colored stone reflects it up into the plants. Beds against a south or west wall are where this shows most.
The weed argument does not hold
Rock does not stop weeds for long. Dust, leaf litter and organic debris fill the gaps between stones within a couple of seasons, and that becomes a seed bed sitting on top of the fabric.
Weeding rock is also considerably worse than weeding mulch, because roots wrap around stone and the fabric underneath tears when you pull.
Where rock is genuinely right
Drainage features. A dry creek bed, a downspout splash zone, a French drain outfall or a swale bottom all want stone, because organic mulch floats and washes out.
It also suits genuinely xeric plantings and areas where you want no organic accumulation at all, such as a strip directly against a foundation for termite reasons.
- Dry creek beds and swales
- Downspout and drain outfalls
- Foundation strips where termite pressure matters
- Paths and utility areas
- Succulent or gravel-garden plantings
Choosing mulch that lasts here
Shredded hardwood knits together and stays put on Nashville slopes better than nuggets, which float and travel in heavy rain. Pine bark is slower to break down but more prone to washing.
Two to three inches is the number. More than that suffocates roots and encourages the volcano mulching that girdles trees. Refresh annually rather than adding a full layer each year on top of the old.