Flat, brownish or violet-grey shards with a duller sheen than slate, shale turns deep and rich the moment rain falls on it. It is a mineral mulch that smothers weeds, cuts evaporation and pulls acid soil back towards neutral; lay it about three inches thick and the same stone will still be there in decades.
| Availability | common |
|---|---|
| Cost | low |
| Ornamental value | brown or tan |
| Ornamental layer | 2 to 4 in (5-10 cm) |
| Renew rate | every 3-4 years |
| Soil enrichment | brings the soil to neutral |
Shale is a type of gravel related to slate. It is a sure fit to any prized location in gardens and pathways. A few sacks of shale mulch will last a very long time, especially if you sift soil and dirt out from the mulch every few years.
Shale has all the advantages of mineral mulch, with the added benefit of a neutral pH.
Thanks to all these advantages,
Brownish or violet-gray and very stable as years go by, this mulch is particularly well suited to all kinds of beds: trees, shrubs, green plants and flowered plants, both outdoors and indoors.
Color depends on how much iron oxide the rock contains.
Shale heavily influences the pH of soil, bringing it closer to neutral (pH 7). If you have very acidic soil, using shale will bring the pH back to normal within a couple years in the immediate vicinity. Great to change the color of your hydrangea from blue (acidic) to pink (neutral)!
The main difference between shale mulch and slate mulch is that shale isn’t as shiny or reflective.
Shale is spread at different depths depending on whether the bed is new, being topped up or being stripped and cleaned. The figures below gather every depth given in this article in one place.
| Stage | Depth of shale | What it does |
|---|---|---|
| First application on a new bed | About 3 in (8 cm) | Blocks weeds without compacting the soil |
| Topping up with fresh shale | 1 to 2 in (3 to 5 cm) | Restores the look; no need to match the first layer |
| Lifting old shale to clean it | Dig down 4 to 6 in (10 to 15 cm) | Lets you sift soil and dirt out and reuse the stone |
| Routine upkeep | Sift every few years | Makes a few sacks last a very long time |
Stay within these depths and the same stone will keep the bed looking new for decades.
A layer about 3 inches thick (8 cm) is perfect. Any thinner and weeds will get through. Any thicker will make the soil heavier and more compact.
To avoid breathing in rock dust, wear a dust mask.
Although shale is rather blunt, you should wear gloves to avoid wounds. Working with gravel takes a toll on skin and gloves make spreading easier.
Pile small mounds of shale gravel every foot or foot-and-a-half (30 to 45 cm). Rake the mounds flat afterwards.
Use a sturdy rake that won’t break with this heavy material. Also use a strong bucket with a thick handle.
Soil and dirt will naturally rise up between shale gravel, in time. To bring the flower bed back to its initial appearance, two options are available:
A layer of new shale mulch doesn’t need to be as thick as the first layer.
You can recycle your previous stone mulch relatively easily.
Recovering old shale mulch is a great and cheap way to get your garden look astounding again. Your past investment will last forever!
Shale is a wonderful material to work with. From building a rock wall to creating a “dry river”, it’s a great mineral to decorate your garden.
The colors pair well with themed gardens, such as blue and white gardens. It also has a timeless look that sublimes arbors and wrought iron garden ornaments.
Lay a permeable geotextile under a dry river bed or shale path so the shards do not sink into the soil and vanish.
Keep shale and other stone beds outside mowing range: blades striking loose rock can fling fragments toward people or windows, and repeated impacts dull or nick the cutting edge.
Edge shale areas with timber, brick or steel strip, because the flat shards slide sideways underfoot and spread into borders and turf.
It’s always possible to find shale mulch in horticulture centers.
Shale is also known for another fact: it contains a small portion of organic compounds called kerogen. Energy companies try to extract this oil from shale in a process called fracking. It’s a disaster when done in pristine environments.
Shale occasionally has fossils in it – keep an eye out for those reminders of the past!
This mulch has many advantages, but it doesn’t help the soil get any richer.
As an underlayer, you can spread cocoa hulls, wood chips or, better yet, ramial wood chips and you’ll have the best of both worlds!
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