Choosing the right grow medium is an essential part of gardening. With so many varieties of grow media available to the modern indoor hydroponic gardener, it can be difficult to know which will work best for you.
You must choose one that suits both the needs of the plant and the needs of your system. Water retention, aeration and pH balance are all aspects that vary depending on the medium and must be considered when making the final choice. In this article, I will discuss the attributes of some of the most common grow mediums and which systems they are ideal for.
Expanded clay is a popular grow medium. It is exactly as described—small bits of clay (clay pebbles) that have been fired in a kiln and popped or expanded like popcorn into small, round, lightweight, porous balls.
Expanded clay is ideal for use in deep water culture, nutrient film technique and expandable drip systems because of its ability to wick up water to the roots without retaining too much water and allowing for the maximum amount of oxygen to be present in the root zone. Expanded clay is inert, generally pH neutral and is not an ideal place for root-borne insects to live and lay eggs. It can be reused if cleaned and sterilized between crops.
Since expanded clay does not retain much water for long, it is not ideal for flood and drain systems because it will dry out too fast, leaving roots vulnerable to damage. Expanded clay must be washed thoroughly before use to prevent clay dust from entering your system and clogging pumps or drippers.
Peat moss is a relatively inert organic grow medium. It retains a lot of moisture while allowing for plenty of oxygen to be present in the root zone. Peat moss is mainly used in passive hydroponic systems or in containers for plants such as orchids or Venus flytraps as these plants need a medium with no nutrient value. Peat moss is not ideal for any other type of hydroponic system, as it falls apart easily and needs to be replaced periodically.
Coco coir is an organic, inert grow medium derived from frayed and ground coconut husks. It is close to pH neutral and retains water while allowing for a good amount of oxygen for the roots.
Coco coir is mainly used in container gardening and in passive hydroponic systems. It is available as a loose potting material, as starter disks for seeds and cuttings, and larger cubes for placement in hydroponic systems or transferring to larger containers.
Coco coir is also a renewable and under-used source of grow medium. Coco coir can be used in expandable drip and flood and drain systems but is not ideal as it is dirty, can clog pumps and drippers and can contribute to the buildup of reservoir sludge.
Gravel can be used in all types of hydroponic systems. Gravel will not absorb and retain moisture—it acts simply as an anchor for the roots of the plant to cling to. This makes it more ideal for passive, expandable drip and nutrient film technique systems since they keep the roots of the plant in constant contact with water so the roots won’t have a chance to dry out. It is not ideal for flood and drain systems.
Gravel is sometimes used as a bottom layer for any potting mix in a pot or container. Having an inch or two of gravel at the bottom of your container will ensure the plant has better drainage. Proper drainage is essential to container gardening. Without proper drainage, salt from the nutrient solution can build up in the soil or potting mix.
These salts will eventually reach levels that are toxic to the plant, causing burnt roots and leaves. Nutrient toxicities lead to disorders and can lock out other essential nutrients. If nutrient toxicities persist, plant growth can be hindered and it can lead to the death of your plants. Some gravel can be jagged and may damage roots.
Use smoother gravel to prevent this. Gravel can sometimes have different minerals or salts that can leech out. Check with the manufacturer beforehand. Make sure to thoroughly wash the gravel before use. It can be reused if washed each time.
Rockwool, also commonly called stonewool, is one of the most popular grow mediums. It is made by heating and spinning certain silica-based rock into a material much like cotton candy. The end product is a firm material that when used will have the ideal ratio of water to oxygen for the plant’s roots to thrive in. Rockwool is also relatively pH neutral and helps resist root-based insects from growing and reproducing.
Rockwool comes in a variety of sizes and shapes and is most commonly used in cube form. One-inch starter cubes are great for starting seeds and rooting cuttings. The starter cubes can then be transplanted into larger cubes that have a 1-in. hole to perfectly fit the starter cube into.
Rockwool cubes are used in many hydroponic systems such as flood and drain. The smaller cubes are often used to start plants that are then transferred into other grow mediums such as expanded clay for drip, deep water culture and nutrient film technique systems. Many people use them to start clones that will be planted in outdoor gardens or indoor container gardens.
Rockwool also comes in lengthy 36-in. by 8-in. slabs that are mostly used for drip and occasionally flood and drain systems, and it can be found in a loose, chunky form for those gardening with containers indoors who don’t want to use messy soil or potting mix. The loose rockwool is ideal because it can be used for all container sizes.
Sand is often used as a grow medium. It can be used on its own or mixed into soil, vermiculite or some other grow medium. Sand can retain enough moisture while providing excellent drainage, but only use river sand, never ocean sand.
Perlite is made by heating glass or quartz sand. It is a soil or potting mix amendment, meaning that it is not to be used on its own, but rather to improve upon an existing grow medium. When added to a potting mix such as peat moss or coco coir, perlite can greatly improve the drainage and aeration of the mix.
This will prevent nutrient buildup and subsequent toxicity issues while providing an oxygen-rich environment for roots to thrive. Perlite comes in three grades: fine, medium and coarse to meet the various needs of gardeners. Perlite should never make up more than a third of your potting mix, otherwise it will float.
Vermiculite is mica expanded by heat. Like perlite, vermiculite is a soil and potting mix amendment. Vermiculite is used to improve the water retention of soils and potting mixes. It is also often used in combination with perlite for its wicking abilities in passive hydroponic systems. Vermiculite also comes in three grades: fine, medium and coarse.
Diatomite is formed by the skeletal remains of microscopic algae. Light and porous diatomite makes for a great grow medium. It will retain moisture while allowing the roots to grow in an oxygen-rich environment. It also helps discourage root-borne insects from laying eggs, which become harmful larvae and adults that feed upon the plant. Diatomite is a versatile medium. It can be used in deep water culture, nutrient film technique, flood and drain, expandable drip and passive hydroponic systems.
Recycled glass foam is a highly porous medium made from recycled glass beverage containers. This is a chemical and a toxin-free growth medium and a great soil aerator engineered to deliver the proper air and water balance in the roots without the danger of over watering. Recycled glass foam can be customized to fit a wide range of plants and climates as well as different growing systems.
There are so many grow mediums to choose from, each with their own unique characteristics. Choosing the right grow medium can make the difference between success and failure in the growroom. Here we have laid bare the basics of some of the most common grow mediums.