When it comes to watering, each terrarium will differ slightly depending on several factors. Jar size, amount of soil, climate, lighting, etc. All of these factor into making a watering schedule. Terrariums with tightly fitting lids will lock moisture in more easily than those with any type of gap or those without any lid at all, resulting in a longer period of time between watering. These terrariums can be lightly sprayed once every few months, possibly longer. However, if you own a terrarium that is not tightly sealed you will want to water more frequently because some is lost through evaporation. For these types of containers you may need to add water every week or two. One way to gauge whether or not more water is needed is to look for condensation on the glass. If its present you can hold off watering for awhile. Another method is by looking at the leaves of your plants and moss. Are they wilting? Dry and crispy? Then it's time to water!
Also essential to the health of your terrarium is the use of the correct type of water. But isn't all water the same? Not when it comes to watering your terrarium plants. The wrong kind can eventually harm plants, not to mention leave hard water stains on the glass which are next to impossible to remove. Moss is one such example of a plant that is sensitive to the type of water used. It's hardy plant; withstanding freezing temperatures, minimal to no light (for a limited amount of time), and complete drought. But when it comes to the chlorine, chemicals, and heavy metals present in tap water moss will quickly suffer. In fact, using tap water even once or twice can potentially cause moss to begin to yellow and eventually die. But why is that? For starters there is nowhere for these harmful substances to go in an enclosed glass case with no drainage and they accumulate very quickly. While a plant is able to filter out some of these chemicals and salts, they cannot do so effectively on a regular basis and stay healthy.
So what kind of water should be used? Pure spring water, rain water, or distilled and reverse osmosis water is best. Water labeled as "purified" isn't suitable because it will have minerals and other substances added for flavor. While it takes a little bit of extra effort, the payoff will be a happy terrarium that lasts for many years to come!