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Potting with coir

Nick Johnson
27 May 2011
Blog team: 

Join Nick Johnson as he explains the use of peat-free mixes in the Tropical Nursery.

Each year in the UK, around 2.5 million cubic metres of peat are sold to amateur and commercial gardeners. In Great Britain, over 94% of the 69,700ha of peat bogs have been damaged or destroyed.

Since 1997, Kew has been committed to finding alternatives to growing plants in peat based composts. There are many in the trade nowadays but for large scale horticulture, one of the best mediums is coir. This is made from shredded coconut fibre and is a versatile product, as well as coming from a sustainable source. The only major drawback is the miles it has to cover to get here! We use coir as the basis for our potting mix in the nursery, named ‘Kew Mix 3’. It was developed in the late nineties and consists of coir (45%), Silvafibre (commercially available well rotted leaf-mould) (45%), loam (equal parts sand, silt and clay)(10%), Kieserite (Trace) and a slow release fertiliser (15.9.11).
 

A comparison between raw coir fibre and KewÕs ÔMix3Õ

A comparison between raw coir fibre and Kew’s ‘Mix3’

Many people in the industry have said they have difficulties switching from peat to coir and that they prefer to use peat. Believe me, when I first started using it I had the same problem! But, to use coir successfully, you need to change the way you pot plants and the way you water. Once you adjust your way of doing things, coir is actually a remarkably good substitute and in some cases it even outperforms peat.

When potting with coir, we use the soft-potting method. Coir tends to lose its air field porosity when firmed. This means that it will hold too much water and not enough air. Eventually this will cause the roots of the plants to rot and die. The pot is tapped gently as we go, and at the end it is tapped onto a hard surface to settle it in. No firming-in is needed. We tend to pot the plant quite high in the container. Once it is watered in, this should cause the compost to sink enough to give us a good watering gap.
 

Jude Goddard teases the roots in preparation for potting up.

Horticultural apprentice Jude Goddard teases the roots in preparation for potting up

When watering, you should always remember that coir drains differently to peat. Peat drains evenly, so if it looks dry on the top, chances are it will be dry throughout and require watering. Coir doesn’t do this. Even if it looks dry on the top, it could be very moist a third of the way to the bottom. As with other mediums, overwatering can cause the compost to ‘sour’ and the roots to die. So we pick up the pots and feel the weight of the compost. After a while, this becomes like second nature, you soon get the hang of how heavy a moist pot is in comparison to a dry one. I think after nearly ten years of using coir I could water a nursery blindfolded, as long as I can pick up the pot and feel the weight. I might trip on the hose though, so I probably won’t be trying this anytime soon!

- Nick -

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Comments

29 May 2011
Comment: 
Thanks VERY much for your tips on using coir. I am going to try out your loose potting and weigh-before-you-water techniques at home, because I think one of my problems (and probably a lot of other gardeners too)is that the bottom of the pot gets sodden but you don't realise how wet it is. So the plants don't thrive and the peat-free compost gets the blame!