When you’re growing tens of thousands of plants, the choice of compost mix is considered from all angles: economic, environmental and what’s best for each plant, and lately I’ve been experimenting with the nursery’s standard mix.
Let’s start with a bit of background: Peat has been the traditional choice for growers. Peat is an excellent growing medium especially favoured for its ability to manage water and hold nutrients that would otherwise leach from the soil. However, peat is extracted from unique lowland marsh habitats that are scarce across Europe, and can take centuries to develop. The extraction of peat destroys the habitat completely as it has to be dug up. Finding a comparable growing medium to replace it is a tough task, but a necessary one from an ecological standpoint.
This is where coir comes in. Coir, a substrate of coconut fibres, is fast becoming a very common and popular growing media, especially as a peat substitute. Coir was once considered a largely unusable waste product, too fine for rope and clothing making.* Companies dumped the waste coir in large holding yards and it simply sat, as it can take over twenty years to decompose the thick fibrous walls of cellulose and lignin. It was then experimentally used as a medium for run-to-waste hydroponic systems, and is now found in lots of standard potting compost mixes.
The big advantage of coir is its water retention and distribution throughout the medium. Coir is hydrophobic and when a container of coir comes into contact with water it distributes it more evenly around the container. Coir also holds the water well just like peat but actually has a higher available water capacity than peat (a large percentage of water peat holds can’t actually be accessed by the plant).
Another big plus of coir is it’s naturally disease free (assuming it’s been stored correctly). It also creates an undesirable habitat for insects, vastly reducing the chances of infestations or attacks on your plants. But coir isn’t sterile: it contains high levels of Trichoderma, a genus of fungi that are beneficial to root development and plant growth.
Coir has a relatively high nutrient holding capacity, and nutrients are easily accessible to the plant. The medium also releases potassium incredibly slowly. But, nutrient supply is also where coir’s Achilles heel can be found. Coir has a big problem with calcium drawdown and, due to it’s pH of around 6.5, prolonged lime treatments aren’t an option. Most people apply gypsum which increases the calcium content without altering the pH, though other pH-neutral liquid nutrient solutions are available too.
The other major issue with coir is a lack of air porosity. A lower porosity is bad for root development. The solution is simple though. By adding 1 part perlite or vermiculite (3mm+ granules) for every 2 parts coir, the air porosity can be increased enough to promote strong root development.
When propagating some rare plants (around twenty species in total), I used one tray of coir plus osmocote and another tray of 2:1 coir perlite plus osmocote. Plants were positioned together and irrigated evenly. The results were even greater than I expected. Below is a photo of two Myagrum perfoliatum: the left plant is in coir, the right is in the coir perlite mix.
The addition of a material like perlite or vermiculite will also decrease the overall calcium drawdown of the mix and give a higher nutrient and water holding capacity. 2:1 coir perlite is the mix that we will now use in the nursery, previously an almost pure coir mix was used but losses have been unsustainably high.
Another mix I would recommend is 1:1:1 coir, perlite, vermiculite for seed sowing. I also like to add some osmocote. Though many people will dispute the benefits of osmocote at seedling stage, recent studies have shown it to be effective in strong seedling development. It makes sense, seedlings in the wild don’t magically begin in soil with no nutrients so why should they when grown in the nursery.
In conclusion, using coir requires a lot of trial and error. Its uses very much depend on the type of work you are doing. As with most things horticultural, it’s not one size fits all. Though most compost mixes are like that and most professionals elect to mix their own rather than buy readymade. So if you are mixing your own peat based medium or wish to try a new growing media mix coir may just be for you.
* Coir is a term which actually encompasses all forms of coconut fibre scraped off during commercial processing but for the sake of this article will only refer to the horticulture substrate