Does anyone know what this is?

I first noticed this white growth around the back edge of the pot at Christmas time. The plant [Peace Lily
Spathiphyllum] seems healthy, some minor tip burn and a few leaf holes; these I attributed to being in too much sun [ southern window ] and the white fly or aphid problem I am having with my oxalis. I moved it to a northern facing window in the fall. The white substance is easy wiped off with dish soap and water. This plant was outside this summer and thrived; so much so that i repotted it before bringing it in. So I am fairly sure the pot and soil was clean.

Clay pots will do that. It can be caused by minerals in hard water or chemicals in fertilizer.

I kind of guessed that's what it was, but haven't seen such a heavy layer before. Is it a problem for the plants, and what can be done about it? I try to let my water sit overnight to let some chemicals evaporate. For sure I am in a hard water area, would boiling than cooling water help?

I don't think anything simple will remove minerals from tap water. You could try saving some rainwater, or melting snow and let it warm up to room temperature, or even buy some deionized water at the grocery store. (They have it with all of the other bottled waters. Some people like to use it in their irons.)

I wouldn't worry about it - plastic pots do the same thing except it is on the inside of the pots. Take a scouring pad to it with bleach water or some other cleaner if it bothers you. If it is mildew then you might have a little more concern but then it is easier to clean away than mineral deposits.

I wondering if i shouldn't be fertilizing this plant right now? that would cut down on minerals.

I don't think fertilizing would help decrease mineral deposits, in fact you'd probably have more in the form of salts. If the plant is actively growing or looks pale then a dose of fertilizer would be good for the plant but not the pot.

http://www.ehow.com/how_4895050_clean-mineral-deposits-flower-pots.html

Many minerals are in fact necessary for plant growth and flowering. Minerals are even in rain water. IF your municipal water is called "hard", it has a lot of minerals. "Soft" is the opposite. The reason this white "ring" shows up so often with unglazed pots is because the water + minerals are absorbed by the porous clay, leaches to the outside surface, and the water evaporates, leaving the minerals behind. If your water has a heavy iron content but otherwise has few minerals, that ring would be black.

In other words, other than esthetic, this white ring is unimportant. You should water your potted plants with plain water between feeding your plants with water-soluble fertilizer so that minerals don't build up in your potting soil. This will also lessen that white-ring. There will usually be a lot more concentration of minerals in the fertilizer than in the tap or rain water. If you REALLY want to be cautious, use distilled water between feeding since distilled water should have no minerals in it.

Ken

Thanks so much for all the helpful answers. Here is a photo of this plant last year indoors when blooming. It also bloomed when it was outside last summer.