Why do bonsai trees need fertilizer?
Bonsai trees are grown in small containers, not in the ground like a regular tree. And even though bonsai could become big trees if they were planted in the ground, while they're confined to a pot, they should be treated carefully.
In the ground, tree roots seek out water and nutrients. Think of sun, water, and air as the food that a tree consumes, like carbs, protein, and fat for people. Beyond those three major elements, trees need other nutrients that they normally can get from soil in the ground. Those necessary elements are not always present in pots. That's why fertilizer is necessary for bonsai trees.
Just as you take regular vitamin supplements in addition to eating food, bonsai trees require fertilizer to optimize their health. Your regular diet might give you most of the nutrition you need, but vitamins ensure you get everything you may have missed. Fertilizer is normally a combination of mineral or plant/animal material that contains what bonsai need regularly.
How often should I fertilizer my bonsai tree?
Bonsai tree will do fine for a while without fertilizer, but eventually, growth will slow down, or the tree may start to have major health problems. Not all problems are caused by or solved by fertilizer, but it's a good place to start if you're trouble shooting. If you're using store-bought fertilizer, your should apply the fertilizer to your bonsai tree every two weeks at the application rate and dilution specified on the label.
What kind of fertilizer should I use for bonsai trees?
There are a bewildering array of fertilizers available; it's not surprising that people get confused! We recommend a liquid mineral-based fertilizer for indoor trees and we use mostly the same for trees living outdoors. You can use organic fertilizers, but since they're derived from leftover organic material (e.g. chicken bones, feather, and cottonseed), keep in mind they smell [bad] and can attract flies and other insects. So you may not want to use them on indoor trees. Manufacturers dig up different ores and process them to be available in the right proportions for mineral-based fertilizers.
We carry Bio-Gold, which is an organic fertilizer from Japan that can be used both indoors and outside.
If you're interested in digging deeper, below we provide more details about specific mineral and organic fertilizers.
What is NPK? 5-5-5?
Those numbers on fertilizer labels can be confusing - the three numbers represent the percentage by weight of the product that is Nitrogen, Phosphorous, and Potassium; three critical elements for tree growth. The fine print on most labels also includes percentages for Calcium, Magnesium, Manganese, Iron, and a host of other elements you may recall from high school chemistry class.
Each of the elements in fertilizer are needed and typically available in slightly different amounts depending on the origin of the fertilizer. The numbers themselves mostly tell you whether the fertilizer is balanced (5-5-5) or specialized (0-10-10), and can be deceiving. Generally, the higher the numbers the more concentrated the minerals in the fertilizer. Higher concentration is neither better or worse, it just requires different methods of application.
Examples of Mineral Fertilizers
Miracle-Gro - the best known fertilizer in the US - is derived from a variety of minerals and animal waste products and refined into a water-soluble crystal form that is readily available in many stores. Proponents of the fertilizer believe it delivers high doses of minerals efficiently and with minimal waste. Opponents argue that the use of "Urea" or synthetically created Ammonia causes harm to beneficial soil microbes that ultimately are needed to keep plants happy and healthy.
Dyna-Gro - originally developed for cannabis cultivation, this is a precise formulation that avoids some of the problems caused by cheaply derived nitrogen sources. It comes in a liquid form that is easily diluted and can be used at every watering or every few weeks.
Osmocote is the best-known of the time-release pellet type of fertilizer. It contains the same type of minerals as Miracle-Gro, coated with an osmotic coating that releases a small amount of fertilizer each time it gets wet. The advantage of this type of fertilizer is that it can provide nutrients even when you forget to fertilize. Use caution when applying and following the directions. Time release can be problematic in climates where temperatures exceed 100 degrees regularly as the osmotic coating is heat sensitive.
Examples of Organic Fertilizers
As mentioned above, Bio-Gold is considered a premium bonsai-specific fertilizer. It is imported from Japan and is formulated to avoid attention from animals and to not smell while decomposing. The nutrient delivery is reliable and uniform when applied, however the price is high compared to most other fertilizers. (11 lb bag is $80 as of this writing.)
Fish Emulsion is made from seafood waste products (think of the last time you got a skinless boneless fish filet) that are treated and sometimes augmented with minerals. Fish emulsion is typically a safe fertilizer and unlikely to burn plants even in high concentrations. The organic components are natural, and sustain soil bacteria. Fish emulsion like other organic fertilizer can clog up soil pores and lead to poor water penetration over time. The most common complaint about it though is that it smells like the ocean!
Kelp Extract or hydrolized kelp, or many other kelp preparations contain low levels of nutrients. They are a fertilizer but also deliver other related organic goodness that's not measured in the N-P-K rating including hormones and humates. (warning - rabbit-hole topic!)
Processed and/or Composted Chicken and Rabbit waste is a common component of inexpensive organic fertilizers. Its use in bonsai is perfectly fine, but it will smell while decomposing and releases nutrients slowly. It's beneficial from the perspective that animals like raccoons and mice are not likely to scavenge it.
Powdered and Pelleted Fertilizer blends like "Dr. Earth" or "EB Stone" are simply cocktails of many different organic components, formulated with specific plants or conditions in mind. Some contain beneficial microbes inoculations as well. The exact named purpose is not super important as many of them are nearly identical. All are typically suitable, although they have the same drawbacks as other organic fertilizers. Generally pellets are less problematic for soil than powder. This type of fertilizer can be problematic in that it sometimes attracts the attention of nocturnal scavengers.