Do bonsai trees need fertilizer?
Yes, most if not all bonsai absolutely do. Your plants may live for a while, even a long time, without fertilizer. But bonsai trees need fertilizer for optimal growth, fruiting, flowering, and overall health.
Why is Fertilizing Important?
Bonsai are trees grown in little pots. What we’re doing is by definition taking a plant out of the environment under which it would normally live, putting it in something else, and trying to make it grow the way we want. In the ground, a plant would get nutrients from the soil. It would reach out its roots out to collect nutrients from decomposing leaves or mast (nuts seeds etc.) from the forest floor. Even plants in gardens need fertilizer as previous year’s plants absorb nutrients in the soil that need replenishment.
In bonsai, to keep water moving we plant trees in well aerated, well draining, growing mixes, which may not have any soil in them at all. Growing a tree in lava rock or fired clay pellets changes the nutrients the tree receives from its soil mix. These nutrients from the soil are vitally important. Trees cannot properly thrive without fertilizer. It is important to fertilize appropriately though because too much fertilizer, or fertilizer applied at the wrong time can also be harmful.
What Should I Fertilize With?
Fertilizers are meant to bring plants the macronutrients and micronutrients they need to thrive. Most fertilizers you purchase focus on the three nutrients that plants need the largest volume of: nitrogen (N), phosphorus (P), and potassium (K). The other macronutrients are calcium, magnesium, and sulfur. There are also micronutrients such as boron, chlorine, copper, iron, manganese, molybdenum, and zinc which plants need in small amounts.
Nitrogen (N) is needed to help plants make proteins, divide cells, and grow new tissue.
Phosphorus (P) helps plants grow roots, flowers, and fruits.
Potassium (K) helps the plant’s overall health including protecting it from disease.
Fertilizer labels list the levels of the top three macronutrients in a series of three numbers with dashes between them. This code represents the percentage of these key macronutrients in the package. Thus a 0-10-10 fertilizer has zero nitrogen in it. A 20-20-20 fertilizer has 20% of each of the major three macronutrients and is sometimes referred to as a “balanced” fertilizer.
Lets say (for clean math) that you bought a 100-pound bag of fertilizer. If you had a 100-pound package of fertilizer that was 16-4-4 you’d have 16 pounds of nitrogen, 4 pounds of phosphorus and 4 pounds of potassium. But those numbers are a percentage, not poundage. So if you had a 50-pound package of that same 16-4-4 fertilizer you’d have 8 pounds of nitrogen, 2 pounds of phosphorus and 2 pounds of potassium. The rest is usually filler or inert substance.
It’s easy to come to the false conclusion that the higher the numbers the better, but it’s not true. Only nutrient ratios can be reported that are immediately available. That means organic fertilizers naturally have lower numbers, but are not inherently better than inorganic fertilizers.
Fertilizers come in liquid or solid form and are organic or inorganic. Organic fertilizers come from plants and animals. Inorganic are synthetic or from man-made materials. Many liquid fertilizers allow for immediate intake, but you can get synthetic liquid fertilizers in slow or controlled release form as well.
Different type of fertilizers can be used to different effect. For example, bonsai hobbyists in the north might kick-start their plants in the spring with synthetic fertilizer as the microbial life in their bonsai slowly wakes from dormancy. Without such a fertilizer the plant wakes slowly, including its roots. That means the plant would be slow to uptake these nutrients. Using a quickly absorbed synthetic fertilizer encourages earlier growth.
For overall, year round use, you might consider a slow or controlled release fertilizer, which breaks down slowly and enables you to fertilize less. Some all-purpose fertilizers incorporate the excellent qualities of organic slow release fertilizer but also include a bit of synthetic fertilizer that can be accessed immediately. These combo fertilizers can be a good solution for some hobbyists, a sort of best of both worlds.
The necessary trace micronutrients mentioned above are contained in many all purpose fertilizers and many organic fertilizers. A quick label check will show if they’re included or not.
Generally speaking for the beginner I’d recommend a well-balanced organic slow release fertilizer. If you’re moving on to tailor your fertilizing needs you can supplement with synthetic fertilizers to accompany your basic standard fertilizer.
The exception to this general recommendation is if you have a lot of a particular kind of bonsai, such as the hobbyist that specializes in only taking on junipers or azaleas. In that case I’d recommend looking up if that particular species has fertilizer preferences and adjusting your purchasing accordingly. Some notable examples of bonsai with special nutritional needs are: palms need high potassium and low phosphorus, citrus bonsai needs high nitrogen, and azaleas need slightly higher nitrogen levels.
How Do I Fertilize My Bonsai?
Though it’s possible to fertilize through the leaves, fertilizer is generally applied in or on the soil mix and is absorbed through the roots as nutrients are carried across them with water.
Some bonsai hobbyists will incorporate fertilizer when they repot a tree. You may or may not choose to do so, but eventually that fertilizer too will be used and new fertilizer will be applied. If applying liquid fertilizer, thoroughly wet plants first. If applying time-released fertilizers in solid form, you may want to cover the fertilizer. Or you might disturb the top of the soil mix to allow solid fertilizers to penetrate into the mix.
Feed your bonsai according to the instructions on the label. Not more or less. Never feed your bonsai more (in volume or concentration) fertilizer mix than is listed on the label.
When / How Often Should I Fertilize?
Just as you should fertilize according to label directions in fertilizer volume and concentration, you should also fertilize in frequency according to label instructions. Like any other measurement, the amount of fertilizer or concentration may depend on the frequency of application.
Typically fertilizer should be applied on all plants when it’s needed. Bonsai though by nature need nutrients that aren’t available in their pots or from their growing mixes. That is, we know that bonsai will need nutrients from fertilizers because they don’t naturally have access to them in their small pots.
If the plant has a dormancy period, don’t fertilize during that time. That could be as much as between late-September to mid-April but that depends on the climate where you live. Bonsai should be fertilized during the rest of the year, especially during periods of active growth.
Even after picking a fertilizer and following instructions, the schedule on which you fertilize should consider what growing mix you’ve planted your tree in, how large a container you’ve chosen, what season of the year it is or where the plant is in its growing cycle, the age of the tree, the health of the tree, and the nutritional needs of the plant in question. That’s pretty complicated. Some of those details involve amateur assessment and a degree of subjectivity, as well as possibly things like soil tests.
I’ve heard recommendations for when to fertilize your bonsai from weekly to once or twice a year. Such simplified instructions are given because they’re rules of thumb different bonsai hobbyists and professionals have developed that met their needs and worked well for them. I won’t disagree with any of them. Instead I suggest that these rules of thumb should be considered, as well as the factors above to decide on your fertilization schedule. I suggest new hobbyists start with buying their fertilizer and following the instructions on the package for quantity, strength, AND frequency. After establishing that schedule consider modifying it according to what works for you and your bonsai.
Should I Fertilize Pre-Bonsai and Bonsai in Training Differently Than “Finished” Bonsai?
Yes, for a few reasons. Generally older, established trees have stores of nutrients and need less fertilizer than young plants in their growth phase. Some pre-bonsai are in large nursery pots whereas finished bonsai may be in very small pots. Last, the soil mix used may be different for the ten-gallon nursery pot pre-bonsai and the finished bonsai in the small pot. All these factors should be considered when deciding on a fertilization plan for each bonsai.
Normally, necessary macro and micronutrients are delivered to or accessed by bonsai when water moves through the soil mix and brings these nutrients to the roots. Nutrients are then taken into the plant and moved around inside the plant via osmosis. Soil pH can impede that work from occurring if the soil is too acidic or alkaline.
The neutral pH for soil is 6.5, and most plants do well in the 6.0 to 7.0 range. However, there are several common bonsai plants that prefer acidic or alkaline soils. Environmental influences can alter the pH of the soil over time, such as elements in the water you use, or exposure to minerals leeching away from the concrete foundation of a house or other building materials.
Before you undergo any treatment to adjust pH it’s important to first test the soil and be sure of what the levels are. There are many store bought products that can alter pH, as can some home-remedy items. Lime or wood ash can raise pH, sulfur or aluminum sulfate can lower pH. pH levels should also be adjusted slowly over time.
Basic pH tests for the major macronutrients can easily be bought such as HERE. Those looking for more detailed analysis can send soil samples to their local county extension office for laboratory testing, or may find other groups willing to conduct soil tests.