Assorted potting mixes
Potting mixes for a Japanese maple can use a wide range of materials but they must have one essential attribute – good drainage. Peat based potting mixes, of the type that’s commomly available in garden centres and diy outlets don’t, on their own, meet the requirement above. The problem is further compounded by their use in large containers where the plant might be in the same pot for five years or more. A multi-purpose potting mix is just that; a grade that is the best match for a very wide range of plants grown for the short term in small pots.
Whilst peat has two desirable attributes – it’s cheap and it retains moisture, the drainage potential is lacking in the sort of scenario we might to want to use it in. The main stumbling block is compaction as the weight of water it can hold in the pot will gradually compress it and the action of regularly watering the surface helps to accelerate that compaction.
Wood/bark chip grades
The solution is to add a skeleton, for want of a better word, that resists compaction but allows the peat to fill in the spaces and provide the water holding capacity we need. There are a number of additives we can use and their suitability is governed to a large extent by the size of the pot. Bark or wood chip, perlite and vermiculite are the most suitable extras for improving peat and have the advantage of being a similar weight and density to peat, allowing for easy mixing and no separating out once in the pot. For that reason, gravel, stone chippings or similar should not be used as they will gradually migrate over time towards the bottom of the pot due to the disparity in density.
Vermiculite (left), Perlite (right)
Vermiculite is best suited to small pots up to 8” dia. using either a medium or coarse grade, the medium being the most readily available. Perlite can also be used for small pots but because of it’s more rounded structure and better draining potential, can be used for larger pots up to 12”, particularly if the coarse grade rather than medium is used. Chipped bark, wood chips or a mixture of both, depending on the size used, are suitable for the smallest pot right up to containers of several feet in diameter. For pots up to 12” my recommendation would be to use chips graded approximately ½″ and for larger pots ½″ to 2″. For small pots anything much below ¼″ is going to have little or no effect as we’re approaching the same size as the peat particles.
Peat/bark mix in use
As to the types of chipped bark or wood to use, anything is suitable; from composted chipped pine bark such as that used for orchid composts (and the most expensive option) to home produced wood chips. Sterilizing is not necessary as the fungi that will colonise the chips are concerned only with eating decomposing wood and will not affect your Japanese maple. Shredded bark or wood is not really suitable as it will not flow freely when mixing with peat and tends to form a more plate-like structure within the pot.
We now come to the question of concentration. The often seen instruction to add 10% perlite or whatever to improve drainage is a non starter. Going back to the ‘skeleton’ analogy, for it to work the additives need to be in contact with each other, or at least the majority. For that reason, anything less than 40% is not going to work effectively but for the purpose of simplicity a 50/50 mix is ideal. The percentage additive can be increased up to 70% but it’s water holding capacity is going to reduce. That’s not a problem as watering frequency can be increased – it’s far easier to put water in than to take it out! This method will provide the ideal growing medium for your Japanese maple – perfect drainage but still with the ability to retain moisture, and overcomes the problem of creeping compaction and the side effects that causes with long term potting of a Japanese maple.
In a subsequent article I will cover soil based potting mixes.
