Cocopeat and Watering
Cocopeat and Watering
In Hydroponics today, Cocopeat or Coconut Coiris now commonly being used by a large variety of gardeners. Over the last couple of years it has grown in popularity to the point that it rivals other systems in its use. As popular as it is, however, it presents unique problems when compared with using inert media such as expanded clay.
Watering represents the first major difference. At this point please note that all references watering or irrigation signify using nutrient solution. Even though inert media such as expanded clay hold a certain amount of water, they also have much larger air spaces than cocopeat. These air spaces allow the plant roots to easily obtain their necessary oxygen as well as water. In the case of cocopeat, these air spaces, or pores, are quite small. This results in these pores being easily filled with water displacing the air. Cocopeat thus holds more water than a medium like expanded clay. After drainage, the manufacturers suggest cocopeat holds around 250mls-300mls of easily available water per litre of cocopeat. How much of that water is actually available is also dependent on osmotic pressure of the medium or solution. For instance, at an E.C. of 3.5 (cF 35) there is only half the amount of water available to the plant compared with an E.C. of 0.5 (cF 5).
Frequency and duration of watering seems to create the most confusion when growing in cocopeat. As there are a large number of variables it is only possible to give general guidelines.
When estimating water requirements you need to take account of the cocopeat grade, plant water use and the climatic conditions. There three accepted methods of determining how much water to give your plants.
1/ Volume or Weight Measurement
As the easily available water holding capacity of cocopeat is, according to the manufacturers, generally about 250mls to 300mls per litre of cocopeat. You will be able to calculate the total amount of water available to each plant by multiplying the number of litres of peat by .25. E.g. you have 10 litres of cocopeat in a pot. The calculation is therefore 10 x 0.25 =2.5 litres of water available to the plant. This assumes that you are starting with cocopeat that has been flushed with water and drained. As you don’t want the available water to drop below 50% you will need to water again when the plant has used 1.25litres. So, how do you know when this has occurred? You can use scales under a pot and check the weight after watering. When the plant has used 1.25 litres of water then the pot will weigh 1.25kgs less. You would need to check the scales frequently as the plant will grow and put on weight daily. After one crop cycle you will have a pretty good idea of the amount of water (nutrient solution)required at each irrigation.
Instead of weighing the plants, you can also measure the volume of runoff. For example, a plant in 10Ltrs of cocopeatthat has used 50% of its 2.5ltrs of available water will require at least 1.25Ltrs of liquid. If in this case you were to add 2Ltrs you would only get (2Ltrs – 1.25Ltrs) = 750mls of runoff.
This is working with the same figures as above, i.e. assuming the cocopeat is holding 250mls-300mls of water/litre. You will often hear that cocopeat holds ten times its own weight in water. This is for absolutely dry cocopeat and should not be mistaken for thewater available to the plant. Total water and available water are not the same thing. Total water refers to the total volume of water retained by the cocopeat but as discussed this total volume may not all be available to the plant due to osmotic pressure.
2/ Evaporation Measurement
Using an evaporimeter will give you a very good idea of how much water has been lost to evaporation which can then be replaced through irrigation. An evaporimeter is essentially a pan of water with an open top and which is filled with water. The water in the pan evaporates over time and the amount of water needed to refill it after 24hrs is measured. This amount of water is then given to the plants over the next 24hr period. Generally, a plant factor is calculated and added to the water amount. This plant factor is a calculation used to determine how much water a plant would transpire compared with evaporation from the pan of water. Put simply, a plant will use a different amount of water when compared to evaporation according to its size and the type of plant it is. Thus a large plant will use more water than would evaporate from the pan and a small one less than the evaporation figure.Usually, this would end up being about 150% more water for large plants grown under lights.
Another way is by calculating the usage according to the climate of the area. Even though we are mainly growing indoors, the temperature outside affects the temperature inside unless full climatic control is installed. A large (1.5 metre high x 2m wide) plant on a hot day or in a hot grow room (+35oC) and with maximum ventilation in summer in a Mediterranean climate, (such as Perth or Adelaide in Australia and naturally, Mediterranean countries), can use as much as 12 litres of water a day. In a 10 litre pot, this plant will need to be watered 6 to 8 times with 2 to 3 litres of water per time. You do need to have some water running to waste normally 20% to 40% of the amount of the input water. Use correspondingly less with smaller plants, in cooler climates and in winter. If you are growing in a controlled environment (grow room), the temperatures will remain consistent may only need watering once every three to five days. Keep an eye on the drain water. If it starts to come through the bottom of the pot well before the irrigation cycle stops then the medium was probably already wet and you will need to water less.
3/ Leachate Measurement.
This technique requires you to measure the conductivity of the water draining from the plants after irrigation. For tap water or bore water. First, you measure the conductivity of the fresh nutrient solution. After irrigating, measure the conductivity of the leachate. This is the water draining from the bottom of the pot. This should measure about 3CF; 0.3E.C. or about 240ppm higher than the original nutrient solution. If the drainage water conductivity is higher than these guidelines, then you are not irrigating enough and you should increase frequency and volume of irrigation. With all run to waste systems, you would usually need to have 20% to 40% running out of the medium depending on the quality of the water supply and the stage of growth of your plants. A safe minimum figure is about 30% run to waste but it may be less when the plants are small and much more with larger plants and water that is a bit saline. Generally, in the first couple of weeks of growth you will get a lot of the water running through the pot as, at this stage, uptake is low but a reasonable amount of water is required to keep the medium moist and there is no change in the conductivity between what goes in and what comes out. That is, if you water with one litre of water, you will get most of that one litre of water draining out. Try to water young plants in such a way that you get just a little bit of drainage.
With larger plants the idea behind giving an amount of water that will produce the 20% to 40% runoff is to wash out the unused nutrient salts and the accumulated sodium chloride (salt) that is introduced into the medium along with the bore or tap water. If you are in doubt then 40% run to waste would be a happy medium. It is a good idea to give two or three waterings with plain water once week to help flush out salts. On no account should you do this for more than one day at a time.
For de-ionised water the same rules don’t quite apply. As there is no accumulation of sodium chloride, the conductivity of the leachate is unlikely to rise above that of the input solution because the plants are removing a lot of the nutrient salts. If the leachate is kept 0.3E.C. below the input solution it will be about right. If the leachate is reading more than 0.3 EC below the input, the reduce the amount given at each irrigation.This is virtually the opposite of the above scenario. The amount of water run to waste will usually be closer to 10%-20%. Incidentally, as in all growing, the best crops will always be obtained with de-ionised or rain water or water with a very low background conductivity.
So why aren’t the plants watered continuously as in NFT, for instance? Coupled with the need for water is the need for air. Cocopeat, along with soils and potting mixes of all sorts consist of solid particles interspersed with air spaces. When the medium is saturated just after a watering, the air is driven out of the medium to be replaced by water. As the medium begins to get drier through plant uptake, drainage and evaporation, air begins to re-enter. This process continues until there is no further water to be lost.
A question often asked is “is it possible to have a recirculating system using cocopeat?” The answer is yes. Our trials have shown that there is no problem running a recirculating system provided you use a nutrient designed for cocopeat and you renew the cocopeat after each growing cycle. This recirculating system is not continuous as in NFT but is intermittent. You don’t change your irrigation practices but just re-use the drainage water. It pays to dump the solution weekly as the nutrient will become too unbalanced if left too long. Also, it pays to use Regen-A-Root, Bio Balancer and Wiltguard in the solution to prevent pathogen buildup. The guidelines regarding conductivity of solution and drainage still apply to recirculating solutions. In this case, just maintain the solution within the desired range with the addition of either water or nutrient but in general, you will find that the nutrient conductivity will be constantly rising. The frequency and duration of watering guidelines also apply.
We would suggest that you water a minimum of four times a day for large plants and adjust the duration according to the leachate conductivity. Even though this seems a bit complicated, in practice it is quite easy. One of these irrigations can be at night. In practice, it is better to irrigate smaller amounts more frequently and the exact amount can be worked out using the weighing and/or leachate conductivity methods. Can you water too often? Technically yes, but for practical purposes you could water 15 or 20 times a day and not overwater provided you followed the guidelines already given. Many commercial growers water this often with just a little water at a time and judge the amounts by measuring the leachate.
Additions to Cocopeat
We have sometimes seen growers putting some other material, such as expanded clay or perlite, in the bottom of the pot to hopefully improve drainage. Does this work? The short answer is no. If they want to improve drainage it would be better to mix the clay or perlite evenly through the cocopeat. Even though the speed of the water travelling through the pot may increase, other problems such a larger saturated area (perched water table) at the bottom of the pot, may arise. All media have a certain amount of saturated area (called the perched water table) at the bottom when water has stopped draining from the pot. This area is always the same depth in the same medium regardless of the depth of the pot. For example, if the particular cocopeat you are using has a perched water table of 4mm in a 200mm deep pot then the perched water table will be still 4mm even in a shallow dish. This means that there is always a saturated area at the bottom of the pot straight after watering. Obviously, how long it remains saturated will depend on how often you water but it is easily possible for it to be saturated most of the time. The plant roots in this saturated area are deprived of oxygen. In a deep pot, the ratio of saturated medium to total medium is quite small. In a shallow pot this ratio increases markedly. If you put drainage material in the bottom of the pot then you are just making the medium shallower and increasing the ratio of saturated medium and the problem with this is that the ratio of air to water reduces and the plant roots are deprived of oxygen in this more saturated medium. If you are trying to lighten the mix then you are better to mix perlite thoroughly with the cocopeat in a ratio of about 60% cocopeat to 40% perlite
The most practical method to use is the leachate measurement method. Irrigate a minimum of 4 times a day for plants over about 3 weeks old and keep the conductivity within the correct range. If your plants need more water then increase the frequency of watering rather than the amount. This means that it is better to give four (or more) smaller irrigations than one large one.
If in doubt check with your retailer or e-mail our Agronomist at email@example.com