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"And the LORD God made ... trees that were pleasing to the eye ..." Gen. 2:9, New International Version.

"Bonsai isn't just something I do; it's part of what I am." Remark to my wife and daughter.

Saturday, August 4, 2012

Sun Screen, RPF (Root Protection Factor) 98.

     If you've been in bonsai for very long at all, you've heard and read warnings about allowing pots to get too hot. For any who don't know, a hot container means a hot root run: the heat is absorbed into the soil, and with the limited amount of soil in a bonsai pot, the temperature quickly climbs. Roots are not designed to resist heat like the above-ground parts of a tree; they're meant to live in the cooler environment of the soil. A dark-colored pot in direct sun, on a hot summer day, can easily get hot enough for root tips next to the pot's interior wall to be killed.

And as I've noted in other posts, we've had an unusually scorching summer, with a number of new records for heat in this part of the US. So about a month ago, I took a careful look at my containers to see which of them might be getting too hot.

Many granules that aren't charcoal gray are still dark in color.
My yamadori ponderosa is in a wooden training box, so I started to pass it by. But then I really noticed the dark color of its potting mix. The ponderosa's mix is 60% scoria, and much of that scoria is a dark charcoal gray in color. Some is almost black. I laid a hand on the surface of the mix, and was surprised at how hot it was! It wasn't too hot to touch, but its heat couldn't have been doing the roots any good. Even if the fine roots weren't being seriously injured, their efficiency had to be suffering.

Inspired by the sun reflectors that people leave inside their car windshields on hot days, I cut a sun screen from white perforated vinyl. The white color reflects the sun's rays away from the soil, and the holes, each about the diameter of a pencil lead, let air in and out. A side benefit is that the sun's radiation is reflected back up into the branches from below, giving them a little more lite. This is especially beneficial since the tree is a semi-cascade!

In spite of the stains, it does the job. A cut (teal arrow) lets me fit a cutout (blue arrow) around the trunk.

A sun shield like this could be made from any waterproof material that: first, reflects the sun's rays; and second, lets air thru. Even a small white-painted wooden lattice would do the job.

I still have a fairly large roll of the perforated vinyl. (I got it some years ago, from the projection-screen company for which I worked at the time.) As I use scoria more and more in my potting mixes, I expect to make more of these sun screens.

A small piece of the same material reflects extra light from below to a young Fukien tea that I want to encourage to grow as a semi-cascade.

:-)  :-)  :-)


  1. Very good idea. I take it that you have not found that the reflected heat from the vinyl onto the underside of the foliage is not an added danger. We haven't that problem here in Northern Ireland this year. We need something to deflect the water from rain drowning the trees.

    1. Thanks, Mike. With a ponderosa, I'm not concerned about reflected heat. The tree's native range gets hotter summers as a regular rule; this baby got higher temperatures than any it's seen here, during its first 80 or so years growing out west.
      Now with a Japanese maple, it might be a different matter.