Mike Hogan
| Special to The Columbus Dispatch
Container gardening tips for beginners
Container gardening is great for decks and patios.
Problem Solved
Perlite, a volcanic rock, is a common ingredient in potting mixes.It improves drainage, aeration and water retention in soil.Perlite is lightweight and can be added to in-ground gardens, but it’s best incorporated with a tiller on a windless day.
After answering gardening questions for almost 39 years now, it is rare that I receive a question from a gardener that I haven’t gotten at some time before. But recently, I received a question from a gardener that I had not received before now.
The question came from an avid container gardener, and she wanted to know why small white Styrofoam pellets are contained in potting mixes.
The caller was tired of these small white pellets blowing out of her pots onto her patio, and also questioned the environmental effects of adding Styrofoam to potting mixes. The answer was simple: The white pellets are not Styrofoam, they are perlite.
This led to the next question I had never received before: What is perlite?
Perlite is volcanic rock
Whenever you purchase seed-starting mixes or potting-soil mixes, you will see the small white particles that are perlite, which is a volcanic mineral. This mineral is almost always added to potting mixes as it improves drainage, aeration and water retention.
While drainage, water retention and aeration are important soil characteristics when growing plants in native in-ground soils, they become even more critical components of root health in the small ecosystems of containers.
Perlite is a naturally occurring volcanic rock which is mined in the western United States and several other countries. Perlite is formed when molten volcanic lava cools. Perlite contains aluminum oxide, silicon oxide and other minerals, but is added to soil mixes not for its nutrients, but primarily for its physical properties. After being finely ground and super-heated, perlite forms small, lightweight, white or gray kernels.
Using perlite
It is difficult to find commercial potting mixtures which do not contain perlite. Pure perlite can be purchased from garden supply retailers and added to heavy clay soils in raised beds and in-ground garden beds to improve drainage.
Perlite is sterile, lightweight, inorganic and does not affect the pH of the soil. Because it is lightweight, it can be blown easily by the wind and can be difficult to evenly incorporate in outdoor garden beds, particularly in heavy clay soils. The best method for adding perlite to outdoor garden beds is to incorporate it into the soil with a tiller on a day when there is no wind!
Perlite can also be used as a growing medium in hydroponic gardening to aid in root development and plant stability.
Benefits of perlite
The presence of perlite in potting mixes provides several benefits, including:
Moisture retention: The small particles of perlite have tiny pockets that trap water and provide for moisture retention, which is incredibly important when growing plants in any type of container.Drainage: Soil drainage is critical to plant health and prevents root rots and soil-borne fungal diseases. The presence of perlite kernels in potting mixes provides pore spaces, which allow water to drain through the soil.Air circulation: Those same pore spaces provided by the presence of perlite allow for oxygen to move freely through the root system. Oxygen is a critical component for healthy root systems, and compacted soils without adequate pore space can prevent adequate movement of oxygen throughout the root zone.What else is in potting mixes?
Most potting mixes also contain vermiculite, which is a naturally occurring mineral similar to perlite. While perlite holds water in its tiny crevices, vermiculite absorbs water like a sponge does, allowing higher levels of soil moisture to be maintained.
The base ingredient of most potting mixes is sphagnum moss, which is dead, dried organic matter harvested from a bog. Like perlite and vermiculite, sphagnum moss has outstanding moisture-retention ability, making it perfect for houseplants and outdoor plants grown in containers.
Sphagnum moss is different than peat moss, which can be too acidic for nonacid-loving plants and is less sustainable than sphagnum moss.
Mike Hogan is Extension Educator, Agriculture and Natural Resources and associate professor with Ohio State University Extension.
hogan.1@osu.edu