The Advantages of RTW/DTW Coir Growing over Recycling Systems
There are a few reasons that I have long used and promoted RTW/DTW coir/perlite growing to novice indoor gardeners.
Firstly, some organic substrates such as coir tend to offer similar root zone security to growing in soils; however, you get the higher growth rates and faster finishing times associated to hydroponic growing technologies. Even better, when discussing growth rates in coir, several studies have shown that coir produces higher yields than substrates such as rockwool, peat or perlite. To generalize somewhat, researchers attribute the higher yields achieved in coir to a more ideal root zone water, oxygen relationship that coir provides when compared to some other hydroponic/soilless substrates. What this boils down to is that coir is an ideal substrate for both beginner and expert alike because it tends to be very forgiving while also promoting a root zone environment that promotes optimum yields.
Other benefits of growing in coir are:
- Coir is rich in hormones and bio-stimulants that aid root and shoot development. For example, Lokesha et al (1988) postulated that the release of phenolic compounds by coir may have contributed to the increased rooting observed in bougainvillea.  Increased rooting has also been observed in other studies.
- Coir, due to its unique chemical properties, tends to help buffer pH -somewhere between 2 and 6.5 dependent on source. Commercially sold products available through the hydroponics sector, however, should ideally buffer between 5.5 and 6.0. For example, we independently tested four random commercial coir samples that are available through the Australian ‘hydro’ retail sector and came up with this: Canna Coco = pH 5.60, Amgrow Coco brick = pH 6.32, Sri Lanka Coco = pH 5.98, and ACE Hydro Coconut Coir = pH 5.84.
- Some organic substrates such as peat and coir offer a high degree of plant security when compared to recycling substrates (e.g. perlite or expanded clay). For example, coir can hold up to 8-9 times its own weight in water. What this means is that if a pump or pump timer failure occurs this gives coir growers plenty of time to identify the problem before the substrates dries to a level where the plants become water stressed. In contrast, substrates such as perlite and expanded clay dry out far quicker, leaving far less time to identify an issue with a pump or pump timer failure. Other than this, coir, more so than other hydroponic substrates, such as rockwool, perlite and expanded clay, tends to protect the plant roots from high ambient air temperatures. This is related to two factors. 1) Water conducts heat. However, when we irrigate coir, the water tends to make its way to the lower regions of the coir, leaving the top layer relatively drier. 2) Coir has low thermal conductance, meaning coir makes a great insulator against heat. Therefore, the dry top layer of the coir acts as an extremely effective buffer-zone between heat and the lower damp regions of the coir where the bulk of the root mass is found. This is particularly important when the plants are young and haven’t formed a canopy over the growing area which acts to shade the substrate/pots from direct light and heat. As substrate temperature and oxygen levels are interrelated (the warmer the substrate the lower its oxygen holding capacity) this insulation plays an important role in promoting a more ideal root zone environment which aids in preventing root disease and encourages high levels of water and nutrient uptake.
- One significant benefit of coir growing is that the substrate provides an ideal environment for beneficial bacteria and fungi which suppress pathogens such Pythium, Fusarium and Phytophthora. Coir promotes the growth of beneficial microflora (bennies) because of compounds called lignins. Not only will bennies thrive where lignins are present, helping to minimize pathogens, but if you add bennies to your hydroponic system via applying them to the nutrient tank/reservoir and/or substrate their numbers will explode. This is particularly true for beneficial fungi such as Trichoderma which are naturally occurring in raw untreated coir. Numerous studies have shown that coir acts as an ideal substrate for the mass production of ,, Therefore, when bennies are used in coir growing, the substrate provides an ideal environment in which bennies such as Trichoderma harzianum can thrive. This results in high numbers of bennies in the ‘rhizosphere’ (the region of the substrate in contact with the roots of a plant where nutrients are readily available for uptake. It contains many microorganisms and its composition is affected by root exudation), which acts to prevent root disease, promote adventurous root growth and potentially offer benefits to plant growth beyond disease prevention.
Trichoderma: Benefits to Plant Health and Growth
I’ll touch on this one briefly because I go into great detail about beneficial bacteria and fungi on pages….
In short, beneficial microflora such as Trichoderma spp. act as very effective root disease preventatives. Trichoderma spp. have been used as biological control agents against a wide range of pathogenic fungi e.g. Rhizoctonia spp., Pythium spp., Botrytis cinerea, and Fusarium spp. Phytophthora palmivora, P. parasitica and different species can be used (e.g. T. harzianum, T. viride, T. virens) to control the various pathogens. Among them, Trichoderma harzianum is reported to be most widely used as an effective bioinoculant.
Additionally, plant growth promoting benefits are also exhibited by some species of Trichoderma. For example, in research conducted in a controlled hydroponics system, Chet et al (2006) note an increase, at protein level, in the activity of chitinases, b-1,3-glucanases, cellulases and peroxidases in cucumber roots previously inoculated with T. harzianum strain T-203. The capability of T. harzianum to promote increased growth response was verified in the hydroponic system. A 30% increase in seedling emergence was observed and these plants exhibited a 95% increase in root area.
Similarly, research with T.harzianum strain T-203 conducted with cucumbers grown in an axenic (free from pathogenic microorganisms) hydroponic system demonstrated increased growth response as early as 5 days post-inoculation resulting in an increase of 25 and 40% in the dry weight of roots and shoots. A “significant” increase in the concentration of copper, phosphorous, iron, zinc, manganese and sodium was observed in inoculated roots. In the shoots of these plants, the concentration of zinc, phosphorous and manganese increased by 25, 30 and 70% respectively.
Additionally, I myself have found that when using bennies the incidence of fungal pathogens such as Botrytis and Aspergillus is greatly reduced. An immediate explanation for this may be that some species of Trichoderma colonise the plant tissue and induce plant defence mechanisms that prevent Botrytis and other fungal pathogens such as ‘Aspergillus’ taking hold in a crop.
Benefical fungi such as Trichoderma spp. are ideal for use in indoor ‘hydro’ growing because they thrive in slightly acidic environments while beneficial bacteria such as Bacillus spp. thrive better in neutral to alkaline environments. Given that optimal pH for indoor crops is 5.5 – 5.8 (slightly acidic) this makes Trichoderma the ideal bennie for hydroponics.
Run-to-Waste Growing (RTW/DTW) – Overview
RTW/DTW growing is the preferred growing system for the commercial production of crops such as tomato, cucumber and peppers because when compared to recycling growing, RTW/DTW provides more control over the nutrient that is being fed to the plants.
In recycling systems the feed solution (water + nutrient) is fed to the plants and reused. During the course of each feed the plants preferentially remove nutrients at differing levels as it passes through their root systems. The nutrient that is not taken up by the plants and/or absorbed by the substrate is then returned to the nutrient tank/reservoir. Therefore, in a recycling system, because some nutrients such as N, P, K are preferentially uptaken by plants at high levels, while others (e.g. Ca, Mg and S) are taken at much lower levels, the nutrient that returns to the tank/reservoir is altered from the feed solution that was initially fed to them. Put simply, preferred nutrients get depleted, while less needed nutrients accumulate in solution. The feed and recycling process is then repeated over and over again and on each occasion (each feed) the nutrient values are further changed as plants preferentially remove nutrients at differing levels and ratios. As a result, after several days of recycling, the solution that was initially placed fresh in the nutrient tank/reservoir can be greatly altered. This can quickly lead to deficiencies in some nutrients and excesses of others. Additionally, the unused nutrient salts can accumulate in the substrate increasing its EC and impacting on growth. As Savvass et al. (2009) found in studies with recycling hydroponic systems, an imbalance in the nutrient solution is generated by excesses of the ions least consumed by the plant (normally SO4–, Ca2+ and Mg2+), which disrupts the balance of the nutrients and often increases the EC to levels that affect growth and yield. 
While these imbalances can be overcome by monitoring and correcting the nutrient solution, this requires implementing practices (e.g. lab analysis of the nutrient solution or individual nutrient species ion monitoring through the use of scientific testing equipment) that are typically not employed by novice hydroponic growers.
In RTW/DTW growing systems nutrient from the reservoir/tank is fed to the plants and the nutrient and water that isn’t taken up by the plants or absorbed by the substrate is not returned to the nutrient tank/reservoir, but instead runs off into a catchment tank/reservoir as waste. This waste is then disposed of and not run through the system again. What this means is that the nutrient being fed to the plants is not altered by plant uptake and then returned to the tank/reservoir. Therefore, nutrient deficiencies and/or excesses are unlikely to occur and any such nutrient excesses or deficiencies in a RTW/DTW system would be a result of adding an imbalanced nutrient or too little or too much nutrient to the tank/reservoir in the first instance. This is just one reason why I promote RTW/DTW growing to novices in Integral Hydroponics. Quite simply, RTW/DTW growing, when handled correctly, promotes a better nutrient status in the root zone than does recycling.
Another issue associated with recycling systems is the potential spread of plant root pathogens, where the presence of just one infected plant will put the entire crop at risk. Oomycetous pathogens in particular, such as Pythium and Phytophthora, can easily spread and propagate explosively under favorable conditions, causing serious damage.
Growing RTW/DTW significantly reduces the risk of root zone pathogen cross contamination between plants. This is because the plants aren’t being fed a recycled nutrient that can become contaminated by a single diseased plant and as a result of then feeding the contaminated nutrient to the rest of the plants infect the entire crop.
One other benefit of growing RTW/DTW is that when compared to recycling systems it reduces the daily nutrient tank maintenance time. For instance, what I do is set up a 200L nutrient tank/reservoir and feed it onto a 100L tank through a float valve system (300Ls in total when a fresh batch of nutrient is made up). This means I can stock up on a lot of nutrient working solution. Because the solution isn’t recycled, EC checks, water top ups, and nutrient dumps aren’t required (although I do check the solution pH every day).
Doesn’t RTW/DTW growing mean using more water and nutrients than in a recycling system?
In short, yes it does. Aside from the advantages RTW/DTW systems provide they have one primary disadvantage when compared to recycling hydroponic systems. They use more water and nutrients. In fact, some countries (e.g. the Netherlands) in order to minimise eco harmful nutrient (e.g. nitrate and phosphate) run-off have introduced regulations that limit the growing of commercial crops in RTW/DTW systems. Therefore, one consideration re RTW/DTW growing is that the waste has to be disposed of carefully. However, keep in mind that regulations limiting RTW/DTW growing were introduced because of the huge-scale of hydroponic production in the Netherlands (100% of many greenhouse crops being produced hydroponically) and the potential cumulative impact that this could have on the environment. Conversely, where small home grows are concerned and where waste is disposed of correctly (e.g. dilute it and use it to lightly fertilize your garden, avoiding run-off into the water table and water ways) environmental concerns don’t present as much of an issue.
One study that compared recycling to RTW/DTW showed the average water and nutrient saving when growing tomato in a recycling system is 24 % water and 34 % nutrient. Inline to this, I find that the RTW/DTW growing methodology I practice tends to use about 30 – 35% more nutrient than ‘DWC’ (Deep Water Culture; e.g. Bubble Buckets), my preferred method of recycling growing. However, when considering the extra nutrient and additive use, this equates to perhaps $30.00 extra worth of consumables per cycle. What you then need to consider is whether this extra $30.00 will be compensated for due to benefits to yields; i.e. will 5-10% extra yield gained through RTW/DTW growing compensate for the $30.00 more spent on nutrients and additives?
 Lokesha, R., D.M. Mahishi, and G. Shivashankar. 1988. Studies on use of coconut coir dust as a rooting media. Current Research, University of Agricultural Sciences, Bangalore 17(12): 157-158.
 Evans, M. R. and Stamps, R. H (1996). Growth of Bedding Plants in Sphagnum Peat and Coir Dust-Based Substrates
 Khedari, J. (2005) Development of fibre-based soil–cement block with low thermal conductivity
 Saju, K. A., Anandaraj, M. and Sarma, Y. R. 2002. On-farm production of Trichoderma harzianum using organic matter. Indian Phytopathology, 55: 277–281.
 Rini, C. R. and Sulochan, K. K. (2007) Substrate evaluation for multiplication of Trichoderma spp.
 Usharani, S., Kumar, R. U. and Christopher, D. J. (2008) Effect of organic substrates and inorganic nutrients on rhizosphere competence of Trichoderma viride. Annals of Plant Protection Sciences, 16: 526–528.
 Iris Yedidia, Alok K Srivasta, Yoram Kapulnik and Ilan Chet (2001) Effect of Trichoderma harzianum on microelement concentrations and increased growth of cucumber plants
 Haupt, M. R.(2007) An investigation into the use of biological control agents as a sustainable alternative to synthetic fungicide in treating powdery mildew in tunnel cucumbers. University of South Africa
 Savvas, D., Sigrimis, N., Chatzieustratiou, E., & Paschalidis, C. (2009). Impact of a progressive Na and Cl accumulation in the root zone on pepper grown in a closed-cycle hydroponic system. Acta Horticulturae, 807, 451-456.
 Tuzel, I.H., Tuzel, Y., Gul, A. Meric, M.K. Yavuz, O. Eltez, R.Z. (2000) Comparison of Open and Closed Systems on Yield, Water and Nutrient Consumption and Their Environmental Impact