Grow. Bloom. Harvest. Dump all that media. Buy a load more. And start all over again. It’s a cycle that many of us, as indoor gardeners, are all too familiar with. Whether you choose to grow in soil, coco coir, rockwool, clay balls or soilless mix, chances are, when the crop’s been chopped, invariably it’s back to your local grow store to reload with fresh media.
However, it turns out that lots of indoor growers (especially those growing short-cycle plants) choose a different route. In short, they reuse their growing media! And before you think this is just the preserve of penny-pinching hippies, it turns out that it’s not all about saving a few dollars – it could increase your yields too!
Aha, there we go … increased yields … that’s all we needed to say wasn’t it?
What is Coco Coir?
Grab some coco coir out of the bag and, at first glance, it looks like soil. But it isn’t.
It’s actually a bi-product comprised of the fibrous husk of … you guessed it … coconuts. From this husk three main horticultural coir products can be acquired; coir chips, coir fiber or coir pith/dust. The latter dust retains water well while the fibers and chips help with air space and drainage. Many mixes and grades of coco coir are on sale in grow stores, variations of coir media exist to suit the irrigation strategy or hydroponic system of the grower.
Coco coir dust is the major constituent of most coir products and is composed of millions of capillary micro-sponges that absorb and hold up to nine times its own weight in water. It has a natural pH of around 5.7 to 6.5, plus a good cation exchange capacity or ‘CEC’ (how easily it gives up nutrients to the plant’s roots), making it ideal for hydroponic cultivation.
Plants grown in coir can develop large roots, stems and blooms. Unlike regular potting soil, which can easily become compacted, coco coir has plenty of air spaces for plant roots, giving rise to a healthy aerobic rhizosphere—essential for favorable nutrient and water uptake. Coco coir has a naturally high lignin content which encourages favorable micro-organisms around the roots and discourages decomposition, making it an ideal growing media for reuse.
Buffered and Non-Buffered Coco Coir
Coco coir in its natural state contains a lot of sodium ions, which cling to the coco coir like a magnet on the cation exchange sites, and is also rich in potassium. In order to make coco coir suitable for use as a growing media it must be pretreated or ‘buffered’ before use. The buffering process involves pre-soaking the coir for 12-24 hours with a buffering solution high in calcium; this displaces the sodium and balances the naturally occurring potassium. After the soaking period the media is washed with water, this removes the displaced sodium, leaving the calcium in the coir. This buffering process prevents unwanted draw down or ‘lockout’ of calcium and magnesium, and avoids sodium toxicity issues. Luckily for us home growers, most pre-packaged coir products in grow stores are buffered at the point of manufacture and will be ready to use, however; it doesn’t hurt to check the packaging before use!
In the early days when coir was first introduced into the hydroponic market, the pretreatment process was overlooked. Young non-composted and non-buffered coir products were sold without instruction and many growers suffered major nutrient issues and lost crops. Although these days are behind us, this initial introduction to coir tarnished its reputation as a quality growing media for many years.
It’s worth pointing out that not all growing media is suitable for re-use. So first, here’s what you should factor into your evaluation:
1. Structure stability
2. Nutrient retention
Now let’s look at coco coir in these three terms. Coco coir, as a soilless growing media, is usually a mix dust and fiber, but some mixes can also contain larger coir chips.
Good quality, buffered coco coir, will keep most of its attributes throughout its useable life. However, the structure of steam sterilized coco coir will degrade faster than its un-cooked counterpart. For the purpose of this discussion we will assume that the coco coir is soft water washed, unpasteurized and chemically buffered; this represents the majority of coir available to hydroponic growers. Coco coir’s miniature “sponges” will become misshaped and smaller towards the end of their life, resulting less air space between particles and an overall higher water holding capacity. Even though this is a slow process some adaptation in watering may be required. After using coir for short cycle crops, amendments with 10-20% fresh coir or perlite may be required when being reused for the 3rd and 4th cycle. Care must be taken with the irrigation regime and nutrient program to insure coir has an adequate life span. Principally, good watering practices and monitoring run-off EC (keeping it within optimal range) will permit seamless re-use.
2. Nutrient Retention
Buffered coco remains relatively chemically stable throughout its life, particularly when used with a coco coir specific nutrient; formulated to complement the unique cation exchange properties of the media. Coco coir can be easily EC controlled by monitoring the EC of the run-off. Watering with a low EC nutrient solution will reduce its EC without rinsing off the famous buffer. If plain water is used in excess, the coir may be rendered chemically imbalanced and may create problems in subsequent culture. On the other hand, coir reacts quickly to low EC rinse and thus it requires much less run-off than peat mixes. Unpasteurized coco coir is also a very hospitable substrate for its natural beneficial fungal inhabitant trichoderma, making it a very disease resistant and root protective growing medium.
The widely held belief in the gardening industry is that coir may be used for up to one year, or three to four crops for short cycle plants, without any compromise on crop quality. Others growers claim that they have successfully re-used coir for years. The amount of times that coir cam be utilized ultimatly depends on the initial quality of the coir and the steps taken to prepare the media between each use. The first thing that must be done is the removal of dead cellulose e.g. root matter left over from previous plants. To remove the large roots the coco coir can be broken up, passed through a ¼ inch soil sieve and larger roots can be discarded. Enzyme products (e.g. CANNAZym, Hygrozyme / Grozyme, or Multi Zen) when applied in the latter stages of the crops cycle will do a sufficient job of cleaning up the residual decaying material. It is not advisable to have lots of dead roots remaining in the media because they can contribute to an anaerobic (low oxygen) environment. Once the root material has been disposed of, coco coir should be flushed with water or a low EC nutrient solution to bring the nutrient levels back down to an acceptable range.
Preparations and Considerations
Once the media is ‘clean’, the addition of beneficial microbes is highly recommended. Coco is an ideal environment for beneficial bacteria and fungi, in particular, the introduction of trichoderma and mycorrhizal fungi will help maintain good growth and disease resistance. Inoculating coco coir with beneficial fungi also has great benefits when re-using the media, as fungal colonies improve with time.
The key component to how long coco may be reused all has to do with how far it has decomposed naturally. Coir does decompose when wet over a period of time. Unfortunately, fungus gnats thrive in the presence of decaying organic matter, and coir is a perfect environment for them. Making sure that all the dead roots are removed, the coir isn’t over-watered and a using dry mulch of coir chips or clay pebbles on the surface of the media will all help to prevent fungus gnats. Utilizing certain “drench” products definitely controls gnat populations, along with sticky traps and the natural predator Hypoaspis miles.
IMPORTANT: You should not reuse coco coir if you encountered any pathogenic or root insect issues during a cycle.
Coco can often produce better results on the second or third use; this can be due to a number of factors. Coir can get better after the first successful crop because there will be a stable balance of ions on the cation exchange sites, leading to subsequent crops starting life with an improved root environment. Another reason growers often see improved results is due to the beneficial microbes present in media, these take time to develop and flourish; particularly mycorrhizae and Trichoderma, which can reach much higher potentials for growth improvement over longer time frames. However, an alternate explanation might be that the original coir coco was too young in decomposition first time round and may have degraded in a favorable way after reuse.
Crops that went through a flush period at the end of a crop cycle fair better because salt levels are reduced during the leaching process. Salts are constantly given off by coco coir as it decomposes, mainly potassium and sodium. Since the medium decomposes throughout its life, this process is continuous. Washing the media out well before planting should mitigate any negative effects. Reusing coir that has not been flushed often results in nutrient imbalance and over-fertilization issues. However, it should be noted that the presence of some of these ions is what buffers the coco. If you lose this buffer you return to calcium and magnesium lockout and pH issues of untreated coco.
In summary, coco has great potential as a reusable media, but to what extent is dependent on the motivation of the gardener. Should a gardener decide that the preceding is too much trouble, coco may always be reused as a soil improver for outdoor plants as well as a ‘brown’ high carbon addition to compost piles.
About the authors:
Robert Hunt is the owner of Rocky Mountain Hydroponics in Golden and Edwards, CO and of Evergreen Garden Center in Portland, ME. Zac Ricciardi is the products trainer for Rocky Mountain Hydroponics in CO.