Bonsai trees need slow-release organic fertilizer pellets (3-3-3 NPK) applied to soil. For fast-draining soils, place pellets in tea bags to protect drainage. Junipers feed spring through late summer; pines need different timing: fertilize early spring for vigorous growth, skip spring and resume mid-summer for refined, shorter needles.
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Bonsai need water, air, and sunlight in order to photosynthesize and grow. Combination of these 3 elements is enough for a tree to manufacture the sugars and starches it requires from its leaves. As the keepers of these trees, we can govern how a tree grows to an extent with the use of fertilizers. It is easy to get bogged down with complex feeding regimes and variations of fertilizer components. The backbone of a fertilizing regime are slow-release organic fertilizers that are available as pellets and are laid onto the surface of the soil. It is not necessary to use many pellets as they will swell to 2 to 3 times their size once watered. The pellets can be spread evenly throughout the soil as shown here. As the tree is watered, the pellets break up and are absorbed into the soil itself where they slowly release their nutrients on a continual basis. Although slow-release organic fertilizers are relatively weak, with an NPK of around 3-3-3, they are composed of natural ingredients that are broken down by bacterial and fungal activity in the soil. Liquid chemical feeds that can be as strong as 20-20-20 at the peak can then be used to promote maximum extension and vigor in trees that require coarser growth. Some tree species, such as this Sabina juniper, require a very large-grained, open, fast-draining soil that cannot be routinely changed. This applies to almost all coniferous species, particularly junipers, spruce, and pines. For trees such as these, the fertilizer pellets can be inserted into empty tea bags available from health food shops, and these are then placed onto the soil surface. When the tea bags of fertilizer are watered, the water takes nutrients from the bags into the soil without impeding the drainage. Ensure that you pin the tea bags to the surface with bonsai wire or bamboo pegs so they can stay in place. Alternatively, use fertilizer baskets. Junipers can be fed with organic slow-release fertilizer from early spring, indicated by this— the soil of the junipers beginning to dry out rapidly over a 2-3 week period, which tends to be in March in the UK. And this fertilizing can carry on until late summer when growth stops. Pine species are atypical in terms of fertilizing. Fertilizing a pine as the buds elongate into candles in spring will encourage longer candles followed by longer needles. This is useful if strong and vigorous growth is required at the expense of having the longer needles, for instance when developing new trunks. If short needles and refined growth are required, withdraw the fertilizer in spring as the needles start to extend and until the needle size has set around midsummer. Then you can start feeding hard while the new buds for the following year begin to set.
