Bonsai need feeding. The concept that trees need to be starved to keep them small is one that is totally wrong and will lead to failure every time. A plant, any plant, whatever size and wherever it is growing needs nutrients just as humans need food.
For those of you who have read our article on Bonsai potting mediums you will realise that the use of inorganic potting mediums such as Akadama, Kyodama and baked clay granules makes for a perfect physical environment in which to grow a tree but there are no nutrients present in any of the raw ingredients. This means that you can add exactly what you want and can monitor the effect of your feeding regime for future reference to produce the desired results. For those of you using organic soil ingredients, there are nutrients dormant in the soil but you have no way of knowing how much and can not predict at what rate they will be released into your root ball.
Lets get the technical stuff out of the way now before anyone nods off, just so we have some reference if we need it later.
The main components of any fertiliser are Nitrogen, Phosphorous and Potassium, universally identified by the N-P-K ratio shown on the packaging. Loosely speaking, Nitrogen(N) is used by plants for leaf growth, Phosphorous(P) for root-growth and Potassium(K) for flower and fruit production. There are then a number of trace elements that are essential in small quantities and are not really worth worrying about because they are included in just about all proprietary feeds.
The N-P-K ratio will guide you in the proportions of each of the main nutrients. A perfectly balanced fertiliser will logically have the numbers all the same eg 5-5-5 or 20-20-20. The numbers are an indication of the strength of the feed so 5-5-5 would be considered fairly mild while 20-20-20 is pretty strong. Adding more 5-5-5 will not increase the strength and using less 20-20-20 will not weaken it.
You have a choice. Soluble powder or liquid fertilisers that are diluted with water or slow release fertiliser that comes in granules or pellets and breaks down slowly over time. The liquid feed is ideal if you are totally dedicated to your trees and can be there at the right times to feed them. However since most of us have other distractions in life, pellets can be useful in providing a slow trickle of feed which will maintain good health in your absence. I recommend a combination of both.
We've touched on what each component does but let us now expand on the use of each.
Nitrogen (N) is the high sugar, high carbohydrate component of the diet. It encourages strong leaf and shoot growth. It is useful in certain evergreen species as a kick start in the spring where we want a certain amount of lush growth to form fresh pads but in deciduous species it should be left out until after the first set of leaves have hardened and only then used lightly as it will lead to overlong internodes.
Phosphorus(P) encourages the growth of roots and flowers. It should always be given as part of a balanced regime because a plant will only take up as much phosphorus as it needs.
Potassium (K) Helps the wood to harden, as well as increasing the roots ability to absorb both nutrients and the water they're dissolved in. It is an important part of the autumn diet as it prepares the tree for winter.
So the basic growing season will consist of a balanced diet on a weekly basis during the spring with a tendency towards extra nitrogen to encourage strong but controlled growth. During the height of summer growth will grind to a halt. Feeding should be reduced or suspended at this time. A second burst of growth in early august can be accompanied by a balanced feed before switching to low nitrogen feed for the run up to winter which will harden existing growth, form healthy new buds and do little to encourage more fresh growth that will not have time to harden before winter.
There are of course exceptions to this general approach. There are too many to cover in detail but here are a few examples where thought is required.
Indoor trees (for those convinced that they exist!)will be active most of the year but will slow down during winter. They should be fed all year round but less should be given during winter. It is tempting to encourage lush growth in the winter months but without the light levels to support the new growth it will tend to be leggy and will ultimately need to be removed.
Lime-hating plants such as Azaleas may require regular supplements to maintain their acidic soil ph and chelated iron levels. However, my Azaleas get fed in the same way as other species and act as a good indicator of correct feeding. If they require an acid supplement it suggests to me that something is lacking in my general feeding regime or my acid soil mix is exhausted. Ericaceous fertilisers in liquid and pelleted form are widely available if needed.
Many Pine species and Juniper species also benefit from twice yearly 'acid' feeds.
As mentioned deciduous trees in general, especially those at the fine ramification stage of development will benefit from a reduced amount of Nitrogen during spring and summer.
Pine development is a subject all its own with back budding being a constant battle. In the past emphasis has been placed on starvation techniques and unconventional feeding regimes. Nowadays the consensus of opinion favours a normal feeding regime to maintain optimum health with training results being achieved through correct pruning.
The next major decision that you need to make is whether to use organic or chemical fertilisers - or a mixture of the two. Some of what follows may sound somewhat untechnical because my knowledge is based on experience in my local environment with my unique trees planted in my particular soil mix rather than on science. Take from it what you will and apply it with common sense.
In my experience, organic feed is a safer option as a basis for a feeding regime. Organic feed will breakdown in a more sympathetic way than chemicals. The conditions that release the nutrients are largely triggered by climatic factors such as warmth and moisture. This tends to occur at the same time as the tree is making demands of its own. I also find that you cannot overfeed with organic as the tree will only take up what it needs and the surplus will be flushed away.
On the other hand chemical fertilisers will release their dose regardless of sudden frost or heavy rain so the timing of application has to be considered with more care. Get this wrong and unexpected bursts of nutrients may occur at the wrong time.
So what does all that mean in practical terms. This what I use.
Pelleted chicken manure in large, cheap buckets is the foundation for my feeding regime. They usually sell it off cheap in the Autumn at garden centres.
It is applied to all my evergreens at the beginning of March and replenished monthly in April and May. Deciduous trees and azaleas receive the same but a month later.
In June when everything is growing well I use Chempak 3. This is a chemical fertiliser with an NPK 20-20-20. I have always used it at full strength. Some will advise using it at half strength. Either way the important thing with any chemical fertiliser is to make sure the soil is watered before feeding to prevent burning the roots.
If you have a lot of trees growing on or the spring has been particularly wet or dull, Nitrogen may still be needed in abundance and you can swap to Chempak2 which has an NPK of 25:15:15. Conversely if spring has been very sunny you may decide that enough Nitrogen has been applied from the chicken manure and switch to Chempak 8 which has NPK of 12.5:25:25.
I apply Chempak weekly throughout June on all trees.
In July I do not feed.
In August and September I use Chempak 8 weekly.
That is the core of my feeding regime but there are other supplements that I use.
In February, March and April I use liquid seaweed in a sprayer and apply this foliar feed to the evergeens. I continue to use seaweed as a stimulant throughout the growing season as a drench every couple of weeks.
On top of that I use rape seed pellets. If you apply these to the soil surface they look unsightly and smell even worse so I fill a plastic bottle with the pellets and then add water. The smell is just as bad in the bottle but mine has a pull up lid and I add a squirt to the watering can most days which is much appreciated by the trees.
I don't claim to have particularly green fingers so trial and error within safe limits has brought me to this regime.
There are various feeds available that claim to be specifically designed for Bonsai. They are perfectly acceptable but ask yourself what they are made of, are they organic or chemical and what makes them worth their inflated price tag. There is a danger that an expensive feed will be used more sparingly. Good for your pocket but not ideal for your trees. I prefer to use affordable feed that will deliver the required results.
A word of caution!
Now you are armed with all these ideas on how best to feed your trees it is easy to get carried away especially if using chemicals. If in doubt about a new product try a reduced dose first and see if it does what you want it to do. If you miss a feed do not be tempted to strengthen the dose or feed at more frequent intervals to catch up and if you suspect that you may have overfed a tree, water it liberally several times until water runs freely through the root ball to flush out the excess. Trees should not be fed when sick or dormant as they will be unable to use the nutrients provided leading to excessive levels of toxins in the soil.
For many years the traditional approach to feeding after repotting has been to wait six weeks. I agree that chemical fertiliser would probably have a detrimental effect but have found that organic feed added after repotting does no harm and provides a background of nutrients that will be there to support the roots as they re-establish. I go one step further and add a small amount of growth stimulator, bonemeal and rape powder to my potting mix.
We have only begun to touch on the subject of feeding in this article. Specialised approaches abound but hopefully you will have gleaned the knowledge of a good basis from which to explore. As with finding the right potting medium, only cautious and educated experimentation in your own growing environment will show what is right for you.