"Jim Lewis" wrote:

I have a couple of Bonsai trees (Zelkova Schneidria)
which are starting to show a white powder-like substance
around the lower part of the trunk. I am not sure if this is
an indication that the tree is unhealthy , or merely a harmless
lichen. It is possible to scrape the substance off, but
generally it re-appears. Any advice? The tree is indoors, in a mostly
unheated room of the house where the termperature is around
2-8 C.

It probably is a calcium deposit from minerals in your water
supply. Unsightly but not dangerous.
Usually this occurs when you water your trees by submerging them,
pots and all in water. That's not the preferred way to water;
you should water from the top -- like rain.

Yes, I did indeed water by submerging, but not for a while.
I will try the method you suggest. How much water per day
is advisable?

Jim Humphreys