Lava woes

Several students have asked why I prefer pumice to lava. They are interchangeable for the most part. In all, lava is a pretty nice particle. Drains well, holds a good amount of moisture, and roots grow well in it. If you’ve a lot of it on hand, or it’s cheap where you live, sure, go ahead and use it. But I have a beef with this particle on several counts, and mostly I’m comparing it unfavorably to pumice.

1. Lava is heavy. If you have larger trees, those pots and boxes get ponderous very quickly.

2. The gas exchange appears higher in pumice than in lava. Pumice gets much lighter when it’s dry. From my experience, this promotes better root growth.

3. The most egregious problem with lava is how hard it is. If you repot many trees in the spring, and are doing detail work on rootballs, you will likely discover that lava destroys your tools. Tweezers wear down quickly. Chopsticks become nubs. Root scythes lose their teeth fast. Your formerly sharp root scissors now has dings in its edges. Scissors get dull fast with lava in the mix; pumice, on the other hand, is soft and the scissors will cut right through it. All volcanic materials are abrasive, but lava is the worst.

For drainage we often choose whatever is most easily found where we live. I know many in the Midwest who cannot easily find pumice. But when I have the option, I always choose pumice over lava for the interior of the soil mass. If the looks of your soil are an issue, use some small lava and akadama as a top dressing in the last 3/4″. That is best use of lava.

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  1. Jeremiah Lee says:

    I found a supply of pumice at a local landscaping supply yard. I sift it out to different grades, but it is still very dusty. I usually rinse it out and let dry before mixing with Akadama. Do you do anything besides rinsing before using the soil?

    • crataegus says:

      Just sift it for the size you’re using, then add to the akadama. No rinsing necessary, as when you first water the repotted bonsai you should water for about 5 to 10 minutes, according to size and depth of pot, to flush out all the fines and dust from the pumice and akadama that is created from chopsticking in the soil to marry with the root system. So, prior rinsing is not needed. We create a lot of dust in the pot when potting.

  2. Brian says:

    Hey Michael,
    So about that Akadama…
    Is there a close, readily available domestic equivalent?
    Calcined clay, oil dry, etc.???
    Environmentally, I have issue with continuing to rely on a finitely finite export that needs to be transported across the Pacific. Thanks, Brian

    • crataegus says:

      No there is no equivalent that I have seen, that is, what kind of roots are created. From my observation, there is nothing that makes a root ball like it. Sadly, I agree, there is a problem with importing it. We do have akadama beds in Oregon, Washington— it’s a volcanic ‘clay’—but the demand is not high enough yet for anyone to dig it. Most of the the best materials for bonsai are volcanic. Pumice is as well. We have plenty in the Northwest but my clients in the Midwest can’t find much. We do have a few problems with everyone having access to superior soil components. We need someone willing to dig and bag these domestically!!

  3. Scott Barboza says:

    I use mix of equal parts akadama, pumice, and haydite or seramis sieved to 1/2″-3/8″, 3/8″-1/4″, 1/4″-1/8″. I discard the fraction above 1/2 and below 1/8. I use the coarse cut for the drainage layer and the fine cut for the plants I’d like more water retention. Do you have a size range you recommend?

    • crataegus says:

      Haydite is unnecessary if you’ve got pumice, and pumice is by far the better particle. I see strange root systems in haydite mixes. My soil mix can range from the fine soil of 1/8″ for some shohin/accent plant applications to 1/3″ for trees I’m seeking a big root and a big shoot on. Most of my mixes are in the 1/4″ range because I’m still developing many of my trees, growing out branches and the like.

  1. […] and lava. Michael Hagedorn recommends a similar mix of akadama and pumice. He also discusses the differences between lava and pumice in the […]

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