On Root Rot

Roots are our main business. Naturally, we tend to be preoccupied with the green areas that we organize with scissors or wire…but really, top notch leaf, needle and shoot health is only possible with lots of healthy roots.

Root rot is often the result of low soil media oxygen levels, as water saturated media can asphyxiate the roots. Other root rots are diseases, like the feared sudden oak death caused by Phytophthora (which reproduces in soggy soils).

In a pot, root rot often starts with a mistake in watering. A tree gets too dry…roots die…then it’s overwatered…and then it rots. Large areas of root rot are often indicated by wilting or branch death.

While root rot can happen with any potted plant, several genera seem prone to it. Some shrubs with fine hair roots are high on that list, such as azalea and holly. In Japan, hollies are often repotted at a faster cycle because of susceptibility to root rot.

What does root rot look like? Rotted roots are darker or duller than healthy roots, usually black, dark grey or dark brown, and squishy. They also smell, so bring your nose to the party. Healthy roots of most plants are blonde to reddish brown, and smell pleasantly earthy.

A small patch of root rot on a holly. The grey patch is decomposing roots, the surrounding roots are healthy.

Another patch on the same holly—the dark grey roots are rotted. In severe root rot situations half the root mass may be rotted, possible with nearly any mismanaged potted plant.

Things that might tip you off for root rot are wilting tops, leaves losing their luster, or twig death. It’s best to cut away any black areas until you reach healthy tissue.

Chronic root rot problems may be due to fine, silty soil media, overwatering or poor drainage. If the problem seems persistent, perhaps pot up with a greater percentage of pumice or perlite for greater aeration, increase soil media size, drill more holes in the pot bottom, or adjust the watering cycle. While spacing out watering would be the natural instinct, root rot often starts with desiccation—so ‘moderating’ the watering cycle may be a more useful concept.

March 2021 Bulletin Board:

  • The Spring Seasonal-lite dates are up: May 15-16, or May 29-30; this online course covers many details of spring growth, including correcting tight root balls, ‘It’s NOT my fault!’ mystery spring shoot loss, watering single flush pines, fertilizing, decandling, and, like all of our online courses, includes a design segment
  • Also! Our Accent Plant course Wild For Accents will be July 10 this year (last year we had 30 students including some internationals); this one is just plain fun, and—surprisingly—summer is a good time to compose many species into accents

6 Comments

  1. Gordon Pybus says:

    Michael, Thank you very much for this post. And thank you very, very much for the Repotting Roundtable episode in Bonsai Wire, extremely helpful. Being fairly new to Bonsai, I have been focusing on the wrong things: pot size and shape, wiring, tools, pruning style, etc, and ignoring completely the roots. I was getting closer by trying to pay more attention to my watering (overwatering) but both the episode and your post brought home to me the fundamental importance of soil composition, as it relates to root health. Unfortunately, I’ve lost several of my favourite trees, maybe because I gave them more attention (ie., overwatering in insufficiently aerated soil). As a novice, I initially thought water was “food.” I’ve now zeroed in on some coarse Perlite to improve soil composition, and am getting ready to save some of my (least favourite) trees. Oh well, I certainly won’t forget this lesson. Thanks again. And be sure to pass along to Jonas and Andrew, etc.. Very timely and helpful.

    >

    • crataegus says:

      Hi Gordon, glad it was useful! And the Roundtables are really fun for the four of us. We’ll have to do more of them.
      In truth, you’re right about water being food for plants—it is part of what makes up the carbohydrate they make with it. But, roots also need oxygen, because they respire…so, they need to have a happy exchange in the wet/dry cycle. What that should look like is what a lot of our arguing over soils and such is about…

  2. Paolo says:

    I wish the seasonal lite videos were also available without been interactive (limited to 10).

    • crataegus says:

      Thanks the comment Paolo, I’ve been trying to make that work but with the private sessions I’ve had to limit the numbers. I may also launch an altered course someday without interactive. Thank you-

  3. tpacker25 says:

    I also found the Repotting Roundtable episode very helpful, particularly as to what type of soil mediums to use. The Bonsai Wire podcast has turned into a welcome and informative respite from the rest of the world! Keep up the good work.

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