Expanded Shale: How To Use It In The Garden
Updated: Jan 6
Expanded shale is a soil amendment that is gaining popularity among gardeners in the United States, and for good reason. It can retain moisture, improve soil structure, deter ground-dwelling pests, and it never breaks down, unlike other organic additives.
What is Expanded Shale?
Expanded shale is sedimentary rock (shale), that has been heated at high temperatures—often up to 2,000° Fahrenheit, which causes the shale to expand. The heat chemically alters the makeup of the rock resulting in a lighter, more porous structure. Expanded shale was traditionally used in building materials to lighten concrete structures. Only recently has its many uses in gardening applications been realized.
How Can Expanded Shale Be Used?
As a Soil Amendment for Garden Beds
Add about 3 inches of expanded shale on top of a new garden bed and till it in to a depth of about 6 inches. For existing garden beds, work a 1-3" layer of expanded shale into the native soil at the same time as you apply compost in the early spring or fall.
Expanded shale is a porous material, and it lightens and loosens dense clay soils. The pores also act to capture water as it moves through the soil, improving water retention and absorption by plant roots. It is estimated that expanded shale can hold up to 40% of its weight in water. Dr. Steve George of Texas A&M Cooperative Extension says "Based on a two-year research study and six years of field trials, I feel that expanded shale will open up and aerate heavy, sticky clay soils faster than any material that I have ever tested. Due to its porous nature, it provides aeration from within the shale particles and, in poorly aerated clay soils, resulted in a more extensive and healthier root system than did other treatments being tested....Even though I dearly love finished, plant-based compost, if I were limited to only one application of one soil amendment with which to open up heavy clay soils, I would take expanded shale and never look back!"
In Potting Soils
Put a layer of expanded shale on the bottom of a potted arrangement to improve drainage and to mitigate root rot. Mix 1:1 expanded shale with a standard potting mix to lighten the potting mix for greater plant health and moisture retention.
As a Bonsai, Cactus, or Orchid Mix
Bonsai and many epiphytes like orchids require loose and airy growing mediums. Many aren’t even planted in soils, they’re planted in a mix of larger, chunkier aggregates. This loose structure allows delicate feeder roots to grow easily and rapidly, which in turn supports overall plant health. Expanded shale can be used aesthetically on the top of planters, and can also be mixed into the growing medium to allow for optimal oxygenation and moisture retention.
As a Barrier to Voles and Moles
Expanded shale can also be used as a humane vole and mole deterrent. Ground dwelling pests do not like to dig through the gravelly texture of expanded shale. The shale particles effectively create a barrier for voles and moles when used 1:1 with native soils. You can even dig a trench around your garden beds filled with expanded shale to inhibit ground dwellers.
In Aquatic Applications
Expanded shale is a popular growing medium for hydroponics and water gardens. The porous material naturally filters the water, and it also creates oxygen pockets for the roots of the plants to grow and thrive. It does not break down in water, and is a beautiful addition to a water garden or koi pond.
Discover the many uses of expanded shale with EarthMix® EnLighten™ expanded shale. Find EarthMix® at a retailer near you.