|One of the most important parts of bonsai cultivation, and maintenance, is in your feeding regime. This is because your bonsai soil mix is very free draining, and therefore the nutrients quickly leach out of the soil. This is why the most popular method of feeding your trees with fertiliser, is to use a slow release fertiliser, which basically releases nutrients as the fertiliser breaks down after watering. Now these feeds are basically organic manure based pellets, or cakes that can be a little bit smelly in the summer, so they’re not that popular for indoor bonsai, which I can quite understand. Generally with most indoor plants there are soluble feeds on the market, but as I explained above you will have to apply very frequently, most manufacturers recommend about once every twelve days during the growing period.
It is important that you only fertilise your healthy trees while they’re in active growth, because you can burn the fibrous root tips of a dormant or sick tree.
N.B: Active growth is when the bonsai tree is putting on new growth tips, and the trees sap has risen. For most species this is from spring until autumn, even with indoor bonsai that are kept at a warmer temperature they slow down over the winter period due to shortened daylight hours.
The tree takes in the most food when it puts on the most active growth, so feed accordingly
You will find on all fertilisers a break down of composition on the side of the packaging, which will relate to its N.P.K ratings this is basically what each symbols stands for, and primarily what it does to the tree:
N: is for Nitrogen that enhances the foliage growth, so for bonsai you do not want this to be too strong for refinement, because the tree will get very long internodes (spaces between the leaves).
P: is for Phosphorus that enhances meristems, and buds.
K: is for Potassium that enhances flowering and fruiting in a tree, which is why tomato feeds are very high in this, so it is good for bonsai that you wish to flower.
There should also be trace elements present in your fertiliser, such as Zinc, Iron, Manganese, Boron, and Copper that are all beneficial in stopping Chlorosis (leaf yellowing).
As You can see in this picture,
it is possible to feed your Bonsai heavily if the tree is healthy.
When feeding your bonsai it is important to know what stage you are at with the tree, because your feeding regime should be dependent on whether you are refining the branch structure, or growing on to achieve a thick trunk and primary branch structure.
Therefore to achieve refinement on your outdoor bonsai you should be using a slow release rapeseed based fertiliser that consists of rapeseed, manure usually poultry, fish blood, and bone, plus trace elements. This is a good tick over feed for your bonsai, which should last you about four to six weeks before you need to re-apply. Most of the ready-made Japanese rapeseed cakes have an N.P.K of about 8.6.4, so it is a nice soft feed. If for example you have an acid loving tree such as an Azalea, you can supplement the rapeseed based feed with a soluble buffer feed such as Miracid to help improve its Acidity levels. Obviously if you are living in a hard water area you might want to collect rain water, to water your Acid loving plants, alternatively you can use acidifying feeds more frequently. Be very careful with chemical water softeners, because they can be harmful to plants.
If you decide to mix your own fertiliser be careful not to use too much poultry manure (Rooster), or trace elements, because it is the strongest part of the fertiliser composition, so it can be easy to burn the roots if over applied. Some growers in Japan mix a soft systemic insecticide in the feed to prevent fly eggs being laid with in it, when applied onto the bonsai pots.
For speed growing your bonsai material providing the tree is healthy, you can use chemical fertiliser such as Growmore, Vitax, Sierra, etc, which will produce much faster growth especially when field growing. This can also be helpful in the spring to use a gentle chemical feed to green up species like juniper, after the frost has dulled the colour. I use nothing stronger than an N.P.K of 10.10.10 with Trace elements for this job.
Indoor bonsai can be fed with very low N.P.K slow release chemical fertilisers during full growth but generally soluble manure based fertilisers are used
When applying the slow release feeds you should always place them away from the main trunk, because the immediate upper roots can get burnt, also to improve the trees root spread (Nebari) you should make the roots fight to the edge of the pot, rather than back on itself hence causing crossing roots.
Watering is a problem when the feed pellets are on top of the soil, because they can easily be washed off, so you can either place in Japanese fertiliser baskets that hold the feed on the soil surface, or you can push into the soil around the edge of the pot. Do not try to crumble the feed down or mix it with water, because the feed will not break down and release gradually, and will be quickly flushed through the soil medium with regular watering.
Some bonsai keepers mistake Vitamin B1 root promoters such as Superthrive as feeds, but they are simple root stimulants to induce rooting, they are NOT fertilisers, so you will not get balanced growth from these products.
WHEN NOT TO FEED!
It is important not to feed when the tree is not growing, because the roots can be burnt. This includes directly after repotting because the roots have been damaged through root pruning, so you should wait for the tree to come into growth before applying feed.
The fertiliser on a tree that has been defoliated should be taken off, again waiting until it starts growing.
Sick trees should NEVER be fed, because the suffering tree would not be able to cope with the intake of nutrients.
Trees that have been top pruned heavily and restyled during the growing season should be only lightly fed with a foliar feed; manure based soluble fertiliser (Maxicrop) to begin with, until the tree picks up again in growth.