Navigating the Big World of Grow Media
The options for different types of available grow media out there can be dizzying. There are hundreds of products on the market with all manner of claims about their relative superiority to grow enviable plants. Which one to choose?
All grow media is designed to support healthy plant growth, but how they do it, what they are made of, and how they can affect the plants they are supporting, and nourishing runs the gamut.
No matter the type, growing media needs to provide the home, storage, stability, and the basics for the health and benefits to the plants that call their growing media home. Grow media should provide plants with both water retention and good drainage. It should provide the aeration needed for your plants’ root systems to grow unimpeded and get the air they need to thrive. Ideally, it will also be long lasting and not change its composition much over time. Finally, it should be able to hold nutrients that are introduced so plants can use them as needed.
Ultimately, the selection of grow media type will depend on many factors. The type of plants to be grown and their needs, the pH of the irrigation water, the type of system it is going in, preferences, and the budget of the individual grower will all play a role in media selection. Let’s look at some the pros and cons of the more common media types.
Peat moss is a moderately priced, acidic medium with a relatively short useful lifespan. Peat moss is comprised of partially decomposed plants. There are three commonly available types. You can find sphagnum (the most common), hypnum, and reed or sedge peat mosses.
Pros — Peat moss has a very high capacity to hold moisture; up to ten times its own weight. Conversely, it does not hold on to excess water. It drains well too. It is a great medium to use for acid-loving plants such as blueberries and plants in the rhododendron family.
Cons — Its high acidity needs to be countered for plants that do not like a highly acidic environment, so products such as lime often need to be added when it is used. Peat moss can host several soil-borne pathogens, so it needs to be carefully monitored. It is also not environmentally sustainable. The supply of currently available peat moss is finite. Yes, more will be made in time, but it takes centuries to form.
Coco coir (or just coir) is a medium made from the shredded husks of the coconut. It is a relatively inexpensive medium that has a neutral pH. Like peat moss, it does not have an exceedingly long useful life once put into use. It is sold under a variety of trade names and is available loose, in sheets, or in pre-formed bricks or briquettes. Coir will expand by up to six times its original size when fully saturated.
Pros — Like peat moss, coir holds moisture well, yet allows for good drainage of excess water and good root aeration. It is a good medium for rooting and cloning, with claims that the coir dust actually promotes root development in addition to supporting it. Coir is biodegradable, organic, and a fully renewable resource.
Cons — Coir needs to be treated prior to use. It is usually high in salt and chlorine, so coir must be washed and often treated with additional calcium and magnesium. The dust of coir can be a skin and lung irritant to some, and a mask and gloves should be worn when handling dry coir for the first time.
Perlite is an extremely lightweight, white, volcanic stone that is produced in grades ranging from less than 0.1mm all the way up to more than 3mm in size. It is a relatively inexpensive grow media that is pH neutral and is reusable as it does not biodegrade.
Pros — Perlite is very porous which lends to good aeration, high oxygen retention, and water-holding ability. It can hold up to four times its weight in water. It is naturally sterile, so it is often used for seed starting, and young seedlings are less likely to die from damping off disease or root rot when started in perlite. It can be sterilized between crops and reused indefinitely.
Cons — Its lightweight nature means it is prone to washing away when irrigated. It can also be easily blown away by fans when in its dry state. Perlite can also host algae growth which can lead to fungus gnat problems. Its dust is an eye, skin, and lung irritant, necessitating the use of gloves and a mask when handling dry perlite.
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Vermiculite is a media that is created when mica is heated at up to 2,000°F until it expands into little pebbles. It is then graded into various sizes, ranging from 0.1mm up to 8mm. Like perlite, it is a sterile medium and is fully reusable. It generally costs more than perlite, though.
Pros — Vermiculite holds onto nutrients very well, making them available to plants for later usage. It is porous, so it has a high water-holding capacity and allows for good aeration.
Cons — Vermiculite does not drain well. When it holds too much moisture, it can cause root rot or invite fungi to colonize. While it is reusable, it cannot be steam sterilized as it will disintegrate if heated; other sterilization methods must be used to reuse vermiculite. Vermiculite contains small amounts of asbestos and as such, it should be handled carefully.
Mineral wool, often known as stonewool or rockwool, is a fibrous material that is created from superheating and then melting certain types of rock, which is then spun into threads to create various media products such as blocks, sheets, cubes, or slabs. There is little chance of running out of the raw material needed to make it, and it is non-degradable.
Pros — Mineral wool easily absorbs water and also drains fairly well. It is comprised of up to 25 percent air, so it gives root systems good aeration when it is not fully submerged in water. It is a good medium for starting seeds, rooting cuttings, and for supporting larger plants like tomatoes. Slabs can be cut to any size desired. It is a long-lasting medium and can be reused if sterilized between crops.
Cons — Mineral wool has to be buffered by soaking in a lower pH solution before it can be used. It needs to also be rebuffered between crops. When mineral wool is fully saturated and under water such as in some hydroponic systems, it can restrict root growth. Because it is non-degradable, it takes up room in landfills once it has used up its useful life. Mineral wool is highly irritating to skin, eyes, and lungs. Masking and wearing gloves are required to handle it before it has been moistened.
Sand has been used as a growing medium for thousands of years. It is relatively inexpensive compared to other media options. Growers typically opt for coarser grades of sand to increase drainage and aeration.
Pros — Sand is one of the most readily available media all over the world. Different grades of sand offer different benefits. Finer sand particles allow for translocation of water. Sand is easily reused after being sterilized and does not break down.
Cons — Sand is not good at holding nutrients. Whatever the plants cannot use right away will be leached out. An exception to this is salt, which can build up to toxic levels if not periodically flushed. It is also not the most convenient medium because of its weight.
Floral foam is a very inexpensive medium used for seed starting and cuttings, often for quick-germinating crops.
Pros — Floral foam has a very high water-retention capacity and no pre-soaking is required to use it. It is versatile and can be used in almost any growing system, or even used within other growing media. It is available in any size needed.
Cons — It is not an ideal medium for growing beyond the young seedling stage. It does not allow for any translocation of nutrients and does not have any buffering capacity if nutrient levels are too high. Pieces of the foam can easily break off and wreak havoc on filters in hydroponic systems.
This is by no means an exhaustive list of available media, just the more common types. Most commercially available media mixes will be a blend of some or all of these components. Some growers prefer to make their own mixes by using some of these ingredients and custom-blending them with other nutrients, minerals, or other materials. Some other materials used as media include gravel, clay pellets, glass grow stones, rice hulls, pine bark, straw, sawdust, and lava rock or pumice. Others still just use water or field soil. Though it may take some trial and error, there is a blend out there that is right for you, your growing system, and your crops.
Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional
Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.