Part 28 of a series by Eugene Howell
The bonsai pot is not just for holding the soil in which the bonsai grows. The pot’s appearance is very important to the overall impression your bonsai gives to the viewer who’s admiring it. We covered pot selection in part 13
Today however, I want to talk about cleaning a bonsai pot. Granted, while your bonsai sits in your own backyard, it’s likely that you’ll be the only one looking at it and therefore dirty pots may not be something of much concern. However, when it comes time to display your bonsai trees, whether in a show, exhibit, or even a club meeting, it’s important to clean up the pot so that it adds to the impressiveness of the bonsai.
While sitting in the sun, rain, weather, and frequent watering, the outside of bonsai pots become dirty and unattractive from mildew, algae, moss and mineral deposits. While these are not harmful, they are unsightly and detract from the beauty of the tree planted in the pot; so it is worthwhile to give each one a thorough cleaning periodically, especially before being displayed to the public.
There are three factors that determine the frequency your pots will need cleaning.
First is the way the pot was made. If it is glazed on both the inside and outside, mineral deposits, algae and moss are slower to build up. The pores of the clay have been sealed by the glaze so little material sticks to the surfaces. An unglazed pot, on the other hand, has pores and will absorb liquids. This will cause mineral salts to build up within the pot walls and these will eventually travel completely through the walls and begin to show on the outside surface of the pot. There are those that associate mineral salts build-up on a pot with poor quality or shoddy workmanship in making the pot, but this is not the case. All unglazed pots, regardless of cost, will absorb moisture and minerals into its pores and eventually show them on the exterior surface. Unglazed pots are generally considered “masculine” in bonsai, so they fit well with strong looking trees. It’s therefore not practical to assume that only glazed pots should be used.
Second, the type of water used will greatly affect how fast mineral deposits accumulate. City water has had much of the mineral content removed and therefore takes a longer time to cause deposits on the pot. Well water has a high content of iron and calcium and will quickly begin to show on bonsai pots.
Third, the amount of fertilizer given to the tree will affect the speed of mineral salts build up. Since all fertilizers are soluble in water, if the water is not taken up by the tree or washed from the pot during the next watering, the mineral salts will eventually be deposited on the pot. If the pot is unglazed then these mineral salts are deposited in the pores of the pot and eventually show on the outside surface of it.
How to clean a bonsai pot
There are two chemical methods of making the job of bonsai pot cleaning a little easier. Both methods require the tree to be removed from the pot. If it has been in the pot for more than a year-or-so the root ball should be dense enough that this can be done without much trauma to the tree. However, while it is out of the pot the root ball must be kept constantly moist.
Make a mixture of 1 part vinegar to 4 parts water and completely submerge the empty pot in this for an hour or so. When removed from the solution, gently wire-brush the pot to remove the stubborn mineral deposits. The other chemical method is to use 1 part bleach in 6 parts water and soak the pot for 24 hours. Following this, the pot should be soaked in water for an hour to make sure the bleach has been completely removed. This last method has the secondary benefit of sterilizing the pot. After soaking for 24 hours in bleach, few (if any) disease organisms will still be alive. In the gardening world, this is a standard method of eliminating the spread of disease from plant to plant. Not only are the pots soaked, but the tools are also. However, bleach has a tendency to rust tools if not properly cared–for after the soak, so it is not recommended for expensive bonsai tools.
If the objective is to clean only the outside of the pot to temporarily get it ready for a show, then simply wiping the outside with a sponge soaked with vinegar will greatly improve the pot’s appearance for about a day or two. And for an especially easy temporary fix, wiping the outside of the pot with WD-40 just before the show will also hide dirt and grim for about a day.
Since trees can usually remain in the same pot for a couple of years before the mineral deposits begin to show, having to clean them once during that period should be about all that’s needed.
Photo credit: flickr Creative Commons, Temple de la littérature (Hanoi) by Jean-Pierre Dalbera