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What is growing medium?
Do I have to use growing media?
What is the best growing media?
What kind of media is best for seed starting/cloning?
Is Rockwool organic?
Are there any organic grow medias?
I heard you have to prepare rockwool before using it. What is involved?
I'm using a NFT / Aeroponic growing system. What kind of medium should I use?
How do I sterilize my media between crops?
I have green, slimy growth appearing on my grow media. Should I be concerned?
I have gnats breeding in my grow media. Should I be concerned?
What is growing media?
Growing media helps ensure that plants get the proper ratio of water and oxygen - both which are critical to healthy plant growth. Growing media also provides support for plant roots and it protects roots from sunlight. You can use many different types of media, but all should provide the following:
  • Physical support for the plant.
  • Free passage of moisture (nutrient solution) to the root zone
  • Proper drainage of surplus moisture.
  • Adequate air circulation to roots
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Do I have to use growing media?
Growing media is not entirely necessary in hydroponics, but utilizing a grow medium maintains a reserve of nutrient solution in the root zone along with a percentage of air pore space. This can act as a buffer and save crops from failure. The two kinds of growing systems which rely on grow media are Drip Systems and Ebb & Flow Systems These systems are most frequently used by hobby and beginning growers as they are more forgiving and they provide excellent results.

There ARE two types of systems which require very little growing media, NFT and Aeroponics. Both of these usually rely on a very small amout of media in which to root a very young plant, and the rest of the roots are allowed to grow in the enclosed growing chamber of the system. In these types of systems, watering and feeding of plants depend solely upon the flow of nutrient solution past the plants' roots. If water flow is interrupted, even for a short duration, plants quickly die. At the other extreme, if the root zone is continually flooded, roots suffocate of oxygen deprivation and plants quickly die. A good grow medium prevents these extremes.

Rockwool is a great growing media, because it provides an optimum buffering reservoir of nutrient while maintaining the volume of air in the root zone. The reserve of nutrient solution is still available even when the irrigation system is off for periods of time, or optimally, irrigation systems can be "pulsed" on/off to provide plants with that happy medium of air and water.

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What is the best Growing Media?
Different media work better for different growing systems and different plant types. There are many factors to consider such as water and nutrient retention capabilities, what kind of growing system you will be using, and whether or not you wish to reuse the media for subsequent harvests. Here are a few of the most common choices :

Perlite & Vermiculite are good media that are often used together. They provide excellent aeration and water retention. This sort of grow medium is most often used in homemade Drip Systems. The water retention capabilities of these sort of mixes are closer to soil and can even be hand-watered. Some growers use a mixture of the two and supplement it with natural minerals and nutrients. We sell a great organic pre-mixed mixture called Ready Grow .

Grow Rocks are another medium which provides great aeration. Grow rocks are a repellant medium, which means that they hold little water and nutrient. Consequently, this medium will act only as a support system for your plant's roots, and provide very little water reserves. This is a great way to go if you're using an automated container garden on a watering timer. Grow rocks also work well in the net cups used in any type of tray system (i.e. Aeroponic/ NFT). They can be reused over and over again for many, many crops. In a well-maintained garden, they would be sterilized between each crop. The one downside to grow rocks is that they are very heavy (they are rocks after all!) and they are costly to ship.

Rockwool, a fiber spun from volcanic rock, is another good choice. It holds a tremendous amount of water and offers a buffer against drying in case of electrical outages or pump failures. Between ninety and ninety-five percent of the space between rockwool's fiber is filled with air, and it holds more nutrient solution and air than any other grow medium. This air space makes oxygen, water, and nutrient solution easily accessible to plant roots.

There is a new rockwool product that we love, called Grodan® Growcubes. These little cubes can be used in a container just like grow rocks, but they weigh almost nothing! They are reusable, and when the time comes, they are easy to dispose of. These just might be our new favorite grow medium.

Van Patten, G. F. and Bust, A.F. 1997. Gardening Indoors with Rockwool. Van Patten Publishing, 18 pp.

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What kind of media is best for seed starting and/or cloning?
You have several choices when it comes to seed starting. Basically what you are looking for is a medium which will keep seeds or clones at the right level of moisture and which promotes strong root growth.

Oasis® Horticubes are specifically engineered for optimal rooting. They are inert with a stable pH and need only be wet down before being planted. When young plants are large enough for transplanting, the entire block can be transplanted to the system of your choice.

Another choice is Grodan® Two-Inch Rockwool Blocks which fit easily into larger Grodan® Rockwool Blocks. Great for propagation, they stack neatly together in nursery trays and hold young plants upright and steady.

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Is Rockwool Organic?
Rockwool is not considered organic, although it does make excellent use of natural resources: one cubic yard of rock becomes 37 cubic feet of wool! Rockwool can be reused for many crops. When you are finished with it, you can break it up and add it to your outdoor garden soil and the added aeration will be beneficial to your outside plants. Rockwool is inert, so it does not add or take anything away from plants. It can be used with organic nutrients.
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Are there any organic growing mediums?
We sell a few organic growing mediums that are proven to work. You may also experiment on your own and try things such as coco fiber, wood chips, or gravel and sand. The most successful organic substrates are those that do not break down readily. A coarse open structure is better for maximum rooting.

Ready Grow is a specialized blend of good things to help your plants grow, along with a nice helping of orgainc compost. For organic seed starting and cloning you can try Rapid Rooters, an organic plant starter from General Hydroponics.

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I heard that you need to prepare rockwool before using it. What is involved?
When Grodan is new it contains some residual lime from production. Rockwool should be soaked in pH 5.0 water for about 24 hours before use. This is done to dissolve the lime. The lime will make the pH value raise to 6.0. Immediately before use, flush the rockwool with your nutrient solution. When you flush, you also flush out the dissolved lime. From this point onwards rockwool does not change the pH in any way.

It is important that you don't condition your rockwool with water at a pH lower than 5.0. If you do this, you can damage the actual fibers of the rockwool. If you use pH 4.0 water, you will find that your pH jumps all the way to 7.0 . The lower the pH you use, the higher it jumps. If the fibers are damaged it can be difficult to re-establish a stable pH level, so never go below pH 5 with rockwool.

To soak cubes, put them in a bucket filled with water. To soak slabs, cut a hole in the plastic bag they come in and fill it with water until totally saturated. After 24 hours, cut drainage slits in the bottom.

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I'm using a NFT / Aeroponic growing system. What kind of medium should I use?
One of the brilliant aspects of these growing systems is that they don't need a large amount growing medium. The only media you may need is a small root starter cube which can be then transplanted into a second, larger cube or into a plastic net cup. The roots will grow freely down into the nutrient and oxygen rich environment.
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How do I sterilize my media between crops?
After you harvest your crop you can wash the media to remove all the old roots and then sterilize with a 5% bleach and water mix (apply one cup of bleach for each five gallons of water. ) Some people dislike using bleach because it can leave a slight residue. Plants can handle a certain amount of chlorine, and so long as it is very dilute, they should be fine. Flush or soak the media with the bleach solution for at least half and hour (an hour is best). Then rinse the media in clean water and keep rinsing until the telltale 'bleach' smell is gone.

You may also choose sterilized by using a mixture of Hydrogen Peroxide and water (use a glug of 35% food grade hydrogen peroxide per gallon of water). H202 breaks down quickly to leave only water and oxygen. For this reason, it does not require as much flushing. You should soak the medium for at least an hour and you should then r flush the solution out of the medium with clean water. H202 is good for plants (it adds extra oxygen to water) so if there is a small amount left in the media, your plants should be fine.

No matter which method you use, be careful! Both bleach and H202 will stain clothes and they should both be handled with care. H202 is especially corrosive and you should never let it get on your hands. Wear gloves!

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I have green, slimy growth appearing on my rockwool. Should I be concerned?
Algae flourishes in wet, well-lit locations, and since rockwool stays moist, it can become an excellent algae breeding ground. Although algae is really ugly, it doesn't directly harm your plants because it does not compete with the plants for nutrients. Algae CAN become detrimental to plants if it starts to die and decay. (it will be black and slimy) In this case it can attract fungus gnats, invite bacteria and viruses, and it can compete with your plants for oxygen necessary for nutrient uptake.

The best way to handle algae is to prevent it from occurring. Algae needs light to grow, so cover the rockwool with a dark plastic to prevent light from reaching it. The authors of the book, Gardening Indoors with Rockwool suggest that a thin layer of gravel or expanded clay over the top of rockwool will help prevent algae growth and gnat breeding. You should also take care to prevent algae growth in the nutrient tank. Shading the tanks, input and output pipes, and other "wet" equipment will inhibit algae growth.

Another option is to cover the rockwool with black/white 6 ml plastic or any other type of plastic covering. We offer Root Guards in our pots section. Even better are Sunleaves' Block Covers. Black on one side, white on the other, these covers do a great job of protecting plants and promoting growth.

Algaecides are not recommended for use on food crops. If you choose to use a chemical product, use very little and take great care to not expose yourself, your pets or other people to toxic products.

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I have gnats breeding in my grow media. Should I be concerned?
Those little black critters are known as fungus gnats. The term refers to a large group of insects, most of which have not been extensively studied. They reproduce in moist, shaded areas in decaying organic matter like leaves and algae. The life cycle is about four weeks, with continuous reproduction when warm temperatures are maintained. Larvae not only feed on fungi and decaying organic matter, but on living plant tissue, particularly root hairs and small feeder roots. Usually, there are very few ill effects from these flies, but control is advised. After the roots have been injured, root rot may attack the plant. Entire crops have been lost in this manner. The plant symptoms may appear as sudden wilting, loss of vigor, poor growth, yellowing, and foliage loss.

Fungus gnats can be easily controlled with a pyrethrin spray. They can also be physically captured with yellow sticky cards.

Perhaps the most important weapon you have against fungus gnats (and all pests) is good grow room sanitation. Don’t allow decaying plant material to buildup. Always remove fallen leaves, algae, or any sort of organic material that collects around the base of plants. This material is a breeding ground for pests and diseases.

For more information on controlling these pests, check our Pests and Disease FAQ.

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