When to Begin Fertilizing in Spring?

It’s likely not a good impulse to want to lasso students to slow down their spring fertilizing, but I admit to the urge. And yet I get it: the trees are growing, so we reach for the fertilizer by instinct. 

The problem with this is that by the end of summer everything, even old geriatric bonsai, look like young goats in a pasture, jumping around everywhere. We need a bit more selectivity. 

Illustration by Sergio Cuan for one of the fertilizing chapters in my recent book Bonsai Heresy: 56 Myths Exposed Using Science and Tradition

Some plants really do need fertilizer right out of the gates. If you plan on decandling black pines, just before they begin visibly growing is the prime start time. Chojubai could use some early in the growing season as well. But those are rare; for the most part, for developed bonsai, let them grow a beat of time before fertilizing. Maybe wait a month or two.

We have, though, many undeveloped, younger bonsai. Many of those can benefit from some fertilizer in spring as they begin growing. This translates into greater caliper, more buds, faster build. Very old collected trees don’t need lots of fertilizer; for these a mild push is all they need, later in the spring.

I’d encourage the use of a slow release of some sort. Organic fertilizer balls or cakes are excellent (or perhaps small amounts of osmocote or apex, synthetic slow release fertilizers). Liquids work, but are labor intensive to get the right dose to the right plant. Another advantage to solids: if you get frequent rains in the spring and fall, with wet soil for weeks, the bonsai still get fertilized. If the pot is sopping wet you’re less likely to want to water in fertilizer.





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  1. Pauline Muth says:

    Mike, what about us with very short growing seasons (zone 4)? Waiting a month or two seems too late.

    • crataegus says:

      Hi Pauline— it is a concern. If talking about highly developed bonsai, generally the guideline is to do the majority of yearly fertilizing after growth has hardened off. This way we avoid long internodes, needles, big leaves, thick twigs, etc. But where that is on the calendar will be certainly related to where you live. In Japan, Nagano Prefecture, a high country area (also a fairly short growing season), this was mid-May. In Oregon I’m at early May. Vermont might be June. And then fertilize more strongly as the season advances well into fall. As you’re pointing out, it will be site-specific. Some find benefit from a very mild dose as things are coming out, and that’s a possibility, though I think it’s highly related to how much fertilizer the tree received in the fall.

  2. vlad says:

    Re chojubai: It takes probably more than 3 weeks before the organic ferts decompose/are transformed into nutrients. In spring due to the weather it may take even longer. How do you prevent “the shortage” of nutrients? Anorganic? Fall fertilizing?

    • crataegus says:

      Many resort to a few doses of chemical fertilizer in the early spring to solve that issue. I’m unconvinced of forcing plants to take up nutrients when they wouldn’t be doing it naturally anyhow, so I think the organic release timing is likely better than we think. Maybe. I’d like to see more data on that. But maybe put organic on earlier—before the plant needs it—so it can start breaking down. Good question-

  3. Boris says:

    Isn’t the idea of ion-exchange in e.g. Akadama to serve as buffer for fertilizer? Also, about osmocote, I haven’t seen much discussion on this user friendly fertilizer. Any drawbacks? Here at 2234m / 7329feet altitude we can get spring starting late May too.

    • crataegus says:

      Interesting point about buffer to fertilizer. As I understand it, akadama doesn’t hold onto too much fertilizer, which can be a good thing if fertilizer is overdosed. One of the biggest problems with Osmocote or other chemical pellet type fertilizer is that when it’s spent the husk remains there. So often you can’t tell what is still active and what isn’t—you can squish it to find out if any remains in the inside, but that’s fiddly—and so careful calendaring will prevent over or under-dosing. It’s not ideal, really, as it also washes away on mounded surfaces. I list it as a possibility for those who have animal problems and can’t use organics, or possibly for young stock development, or maybe in your case cold springs. It has its uses. And then it’s easy to abuse, too… Thanks for the comment-

      • Boris says:

        I haven’t read your book yet, (I’m adding stuff in Stone-lantern to get free shipping) but in one of the podcasts you mentioned the difference between new, mid- and mature bonsai, and their different soil regimes. In a video from Bjorn B., https://youtu.be/vGw-CeuSdNA , he explains the regime he uses for different stages of bonsai’s life, using osmocote+ for early age. Including late fall fertilization to be ready for Spring.
        As with any chemical reaction, this would be thermally activated so I guess that if temperatures are low, the fertilizer doesn’t move, so it doesn’t hurt to add it and have left overs in the spring.

      • crataegus says:

        Hi Boris, yes I agree with with Bjorn! The osmocote is useful for the younger plants. And if you put it on in fall, often you can squish it in the fingers and find there’s still fertilizer left by springtime. It’s a temperature released fertilizer, so every ten degrees lower adds a month of longevity (roughly).
        Tricky thing with this stuff is it’s hard to tell what’s the new stuff and the old stuff, if you apply more. Best to calendar applications, and do the squish test.

  4. Pauline Muth says:

    Be careful with some like osmocote…these types of man made fertilizers do promote fast growth..too fast for some species. The quick growth can make for too thin cell walls. This reduces the natural protection against fungi. I switched to all organic several years ago and my fungus issues especially on pines disappeared.

    • crataegus says:

      Very true, thanks for the comment, they are to be used with discretion. Those with animal problems have fewer options, but otherwise staying with organics has a wide range of benefits.

  5. shawndalton says:

    Hi Mike, I just bought your book up in Harrisburg PA from Superfly bonsai. Chuck, the nursery manager highly recommended it. I’m new to bonsai but have been growing anything that I can get my hands on since I was a teen. I just subscribed to your newsletter and blog and look forward to reading it and your book.
    Thank You
    Shawn D.

  6. tom c says:

    Great post – and I thought your book was great! What are you thoughts about fertilizing mature ezo spruce? From my understanding, they push quite a bit later into the season than other species. Do you still recommend waiting for new growth to harden off or fertilize a little prior to the push of new growth?

    • crataegus says:

      Thanks Tom, glad you enjoyed Bonsai Heresy! I don’t find ezo to push later than other spruce, they harden off in mid-spring, and around then would be a good time to begin fertilizing. They like a fair bit of fertilizer, too. Hungry plants.

  7. Victor Taboada says:

    Hi Michael,

    In Bonsai Heresy you make a compelling case against using 0-10-10, preferring single-digit organic fertilizers. Recently I heard from a respected bonsai expert that nitrogen in very early growing season tends to burn the new roots, so, in his opinion, it was a good idea to start the spring with 0-10-10, which encourages roots, and switch to a fertilizer with nitrogen sometime in May…what are your thoughts?

    Thank you so much for sharing your knowledge with all of us!

  8. Victor Taboada says:

    Hi Michael,

    In “Bonsai Heresy” you make a good case against using 0-10-10 in favor of single-digit, more balanced fertilizers. Recently I heard from another bonsai specialist that using fertilizers with nitrogen early in the growing season tended to burn the new root growth, and that it was better to start the season with 0-10-10 to promote roots and switch to a regular fertilizer (with nitrogen) perhaps in May or so. What are your thoughts?

    Also, I have found that liquid chemical fertilizers, when used per the manufacturers instructions, lower the pH of the resulting solution to very extreme levels, in the case of 0-10-10, again, when used per the instructions, the resulting pH could be in the order of 2.5, which gives me pause before feeding that to the plants. I guess my question is, when using liquid chemical fertilizers, like Dyna-Gro, do you follow the recommended dosage, or do you instead target a pH?

    Thank you so much for sharing your expertise with us!

    • crataegus says:

      It’s an interesting claim, I’ve not seen that but it doesn’t mean it doesn’t happen. I’ve seen more issues with old collected trees and setback after fertilizing too much too soon.
      As for pH, if you’re fertilizing with liquids frequently then you may wish to add some baking soda to it to buffer, though if used only now and then and in the 4-5 pH range…likely not any issues with that.
      Here again I think the organics may well edge out the chemicals. Milder, less likely to create fertilizer burn or a pH issue.

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