Rockwool

Samples of rockwool products, from left to right: croutons which can be used in nets and pots; small cubes which can start small seeds such as lettuce; same-size cubes which can start larger seeds such as squash; and slabs which can be used for plants with a matting or clumping habit, such as oregano and thyme, or to encourage new rooting behavior along vines, such as tomato.

Rockwool is a manmade material, composed of stone and mineral fibers, spun at high speeds into a loosely woven material much like fiberglass. And like fiberglass it is very efficient at trapping both air and water within its fibers. It can be formed into different shapes, with cubes and slabs as the two most common forms. It is fire resistant but will melt and burn at extremely high temperatures. It is water resistant and will not dissolve, but it will support algal growth if used for long periods of time in moist environments. In addition to hydroponic usage, it is used as insulation.

Advantages and Disadvantages

For hydroponics usage, rockwool is generally used in three forms:
1) in small cubes that are about the size of croutons, which are poured into small containers or planting beds to support the plant roots.

2) in starter cubes or round plugs of various sizes (for instance, 2" cube, 3" round plug, 4" cube), intended to be used to start large seedlings such as tomato, pepper and squash. These cubes or plugs are often started in netpots, which are plastic pots with regular grid openings to allow for the free movement of water in and out of the pot and growing media within.  As the seedlings grow, they are eventually transplanted into a larger growing container.  This larger container typically features more rockwool.

3) in large slabs intended for mature growth. These large slabs are of various dimensions but one example would be 18" wide, 4" deep and 4' long. Seeds like lettuce could be started right on the surface of the slab, or starter cubes or plugs could be placed on top of a larger slab.  In that instance, the roots would simply grow through the cube or plug and then into the slab. 

Whichever product is used, the advantages are many. First, it is as lightweight as any other material, which of course makes it very easy to work with. The multiple forms are very handy because they can accommodate both small and large seeds without changing planting material. This is a huge advantage compared with larger growing media, where small seeds can fall down into the planting bed and never be seen or heard from again. Secondly, the material is both soft and crushable, yet they return to their form after compression. So it is amenable to being put into whatever larger container you care to use. The slabs are more rigid but they can be cut down to size and the leftover converted into cubes. Third, they are chemically inert so they do not affect pH balancing. Fourth, rockwool has an almost ideal mix of water retention and drainage characteristics such that the cubes and slabs maintain decent humidity yet allow free movement of water and slow air filtration. The extremely small fibers also wick moisture extremely well, so cubes placed in reservoirs will keep the plant watered as long as the reservoir has nutrient solution in it. And finally, these products are readily available through either local retail or wholesale outlets, and/or via online suppliers. Its light weight and compressability makes it easier to ship long distances if needed without a huge shipping bill. 

With all these advantages you'd think rockwool was the answer to every hydroponic prayer. Sadly that is not the case. It has some big disadvantages. First, it is irritating to the skin, eyes and lungs much like regular fiberglass. If you work with only a little of it at any given time that can be a minor nuisance. But work with it all day long, day in and day out, and it can become a moderate to serious health issue. Truth be told that's why I stopped working with it. Secondly, it cannot be recycled because it builds up algae so easily, along with old roots, and there's really not a satisfactory way to clean that up. So it must be disposed of and replaced annually. That leads to the third issue. It cannot easily be burned and it won't dissolve in water. Many growers dispose of it with their garbage service, which can be either a financial and/or philosophical issue depending on your viewpoint and trash service. Ironically, I have heard of folks shredding it and adding it to compost. Since it is chemically inert, it can actually add some nice drainage and aeration features to the soil. That may actually be the best disposal option there is. If you are considering rockwool usage, be sure to consider your disposal options before committing large financial resources to this method.

Where to Find Rockwool

These products are not yet common enough to be regularly available at hardware stores or home improvement centers. Such stores may offer them seasonally, and/or be able to special order whatever you need, depending on how much gardening, landscaping or nursery business they do.  So that would be a good place to start. 

A second possibility is a specialy hydroponics supply house, or a commercial nursery supplier. If you live anywhere near a metropolitan area, you will probably have at least one or two such businesses within that metro area. They may typically deal with larger growers but they often work with smaller growers as well. 

A third very good option is online retailers. These products are lightweight enough that they can often be shipped without a great deal of additional cost. Or, you might be near an online retailer and not even know it.  Be advised that many retailers only offer a small fraction of the wide variety of rockwool products.  SimplyHydro is one of the few online retailers I've ever found which offers a full range of rockwool loose cubes, grow cubes and plugs, and slabs in multiple sizes.