All of the soilless grow media options out there, including rockwool, coco coir, clay pebbles and perlite, have advantages and disadvantages. To make the best choice, do your research and select the one that most closely fits your application, style of gardening and budget.

A good medium has key beneficial characteristics. It should:

  • Be inert or close to it
  • Be relatively clean
  • Have good air and water retention
  • Be easy to work with
  • Be lightweight without becoming buoyant
  • Be sustainable and/or reusable

Since it can be difficult if not impossible to change media once you’ve begun a grow cycle, it’s best to start with the most compatible media you can afford that meets a majority of these standards. Save the fancier stuff for experimental side projects until you know how they’ll perform. Let’s take a closer look at some of your options:

Rockwool

Rockwool, also known as stonewool and mineral wool, is made of basalt, a volcanic rock that’s melted with chalk and sand and spun into an intertwining mat of fibrous threads. This mat is then processed into granules, cubes, blocks or large planks or slabs that can be used to root and anchor plants in a hydroponic system.

Although it has become popular for both commercial and hobbyist growers, mineral wool was originally developed in the late 19th century as an insulating material, somewhat like fiberglass, and is still used extensively today for thermal insulation and soundproofing.

Rockwool has some impressive advantages in an indoor garden. It works with most nutrient blends, is lightweight and has good water retention. Some rockwool products are also conveniently scalable. Once a plant start has a good root system going, just pop that cube into the preformed opening of a larger cube, and so on as the plant outgrows its accommodations. Think of it as modular indoor gardening. This approach is a favorite for growing tomatoes.

Rockwool can be used in a number of different hydroponic growing situations, including aquaponics, deep water culture, ebb and flow systems and nutrient film technique. Seedlings and clones grown in rockwool also transplant easily into soil, coconut coir and other loose media.

As nice as all this sounds, there are some things you need to consider. Rockwool can be a pricey medium, largely because it can only be reused a couple of times, if that. It isn’t a sustainable material, either, but is somewhat biodegradable in that it can be reintroduced to nature. The benefit of this is that it does tend to retain its form for a long time instead of breaking down readily into small particles.

Over time, you might start to wonder if a reusable and sustainable material might not be worth a try, especially as your operation expands. If this does happen, you can mix and match, starting with rockwool for seeds and seedlings, and changing over to another option like coco coir at transplant time.

You can transplant a cultured rockwool cube into a second medium and let hungry plant roots take it from there. Just be sure to keep mixed media uniformly moist. This is a flexible and cost-effective approach that’s easily accomplished when using rockwool.

When handling, rockwool can be dusty, so proper handling is important, especially if you or a family member has a history of respiratory problems. Some manufacturers recommend the use of a respirator during initial set-up, especially when working in confined spaces. With all types of media, always read and follow the manufacturer’s safety recommendations.

Although it’s a convenient gardening medium, rockwool needs some special preparation before use. It’s on the alkaline side, with a pH of around 8, and requires pre-treating with a pH lowering solution and adequate time to drain. Depending on the product and size, this can take up to a day.

Introduce a nutrient mixture only after treating for pH. Even though these two steps are sometime combined, the pH treatment can be more effective if completed first. Rockwool becomes spongy when wet, but squeezing it too hard can destroy its excellent water retentive and air-trapping characteristics, so take it easy. Once planted out, monitor rockwool regularly for pH shifts and adjust accordingly.

Expanded Clay

Expanded clay, also known as grow rocks and clay pellets, is a little like rock popcorn, but instead of popping open when exposed to high heat like popcorn does, steam and pressure buildup during kiln firing causes clay pellets to swell and create small, abundant cavities. These cavities do a good job of maintaining a favorable ratio of air to water in a recirculating hydroponic system.

Expanded clay is pH stable, meaning it doesn’t require much prep or pH monitoring. It is also surprisingly lightweight for its sturdy appearance, and its irregular surface structure allows roots to get a good grip without sustaining damage. Even though the initial cost may be high, expanded clay is reusable after clean-up and sterilization in a simple bleach and water bath. Depending on your long-term goals, this can make it a budget-friendly option. Expanded clay is not a sustainable product, but it does have a long useful life.

Multiple pellet (or small rock) sizes are available, and can be used alone to cultivate seedlings or clones, or grow plants to maturity. Expanded clay also plays well with others. It is often combined with media like soil or peat moss to increase the amount of available oxygen or enhance drainage.

Pebbles look clean even while they are concealing a lot of dust and grit in their many surface cavities. Some expanded clay products can be dusty enough to cause blockages if they aren’t thoroughly and repeatedly rinsed before first use. The trapped air inside clay pellets can actually cause a few of them to float. This happens infrequently, but it still happens. Clay increases the drainage of your system when combined with other ingredients, but may drain too quickly when used alone. The positive and negative aspects of this medium’s ability to shed water should be factored into your growing strategy.

Coconut Coir

Coconut is a popular global crop, and coir is an abundant by-product of coconut processing. Coir (also known as coir, coco coir or coco peat) is made from the shredded, cubed or otherwise treated coconut husk, and the outer husk and shell of the coconut constitute from 30 to 60 per cent of its total weight. Demand for coconut products like oil and milk are increasing globally in the food and cosmetics industries, so this useful product will be around for a long time.

A sustainable and reusable growing medium that’s easy to work with, coconut coir is sold loosely bagged like peat moss, or in preformed blocks or chips. It is a cost-effective option for beginners and seasoned growers, and a particularly good choice for drip and ebb and flow hydroponic systems. Since coir is one of the most soil-like of the hydroponic media options, it’s also a good transitional medium for seasoned outdoor gardeners that want to expand their expertise to include year-round greenhouse or other indoor growing opportunities.

Although coir is sometimes marketed as an inert medium, this isn’t always true. Coconut coir absorbs calcium and magnesium from standard nutrient blends, and discharges them some time later in the growing process. It typically requires either nutrient designed especially for it, or the addition of calcium and magnesium to conventional nutrient or other types of special formulations. Since most hydroponic coir products react in a predictable manner, maintaining the right nutrient concentrations isn’t as challenging as it sounds, but does require monitoring.

Coconut coir is used for hydroponic, backyard gardening and ornamental applications. Products not intended for use in hydroponic systems may contain elevated levels of sodium chloride not appropriate for soilless growing. Choose a coir product specifically manufactured for use in hydroponic systems. It will have been repeatedly rinsed or otherwise treated to remove mineral salts.

With a natural pH range of between 4.5 and 6.8, coir requires the addition of a stabilizing ingredient like calcium nitrate to fix it at a pH of 6. For best results, look for products sold as pH stable or pH buffered. Because it does such a good job of retaining moisture—up to 10 times its weight by some accounts—coir is often paired with perlite to improve aeration and drainage. The ratios will vary depending on the application, but could go as high as a 50/50 mix. You can customize your blend or purchase a premixed product.

Perlite

Each of the grow mediums already mentioned here can be paired with other ingredients to help them perform better. Perlite is a common additive because it increases drainage and aeration. It is sometimes used alone for cloning and rooting seeds. Although not sustainable, it is reusable, relatively inexpensive and pH neutral.

Perlite is a heat-expanded product somewhat like clay pellets, but produced from obsidian, a volcanic glass, rather than clay. It’s interesting to note that other hydroponic and general gardening products are also derived through heat-treating to maximize air and water-retentive features. Vermiculite, from hydrated laminar minerals, and recycled glass (silica), are two you may recognize.

Perlite is not suitable for all applications. Wayward granules can become troublesome flotsam, which may cause problems in some systems, and layered perlite may also compact over time.

Additional Options

Along with these popular types of growing media, there are others to choose from. One large group includes options that may be inexpensive and easy to acquire, but challenging to use for one reason or another. They include sand, gravel, sawdust, wood chips and packing peanuts or other polystyrene shapes.

No Media Option

We can’t leave the topic of media without mentioning the minimalist approach of using no growing medium at all. This can be accomplished effectively with specific types of hydroponic systems, like aeroponics, fogponics and deep water culture.

Whatever grow medium you choose, we’re rooting for you, because a good indoor garden starts with strong, healthy roots.