Hostas are noted for having a fairly high need for water. This is probably true because their native lands receive in the neighborhood of 60 inches of rain per year. That level of precipitation is only matched in the Pacific Northwest in the United States and Scotland in the U.K. Parts of New Zealand also receive rain in those amounts.

On the other hand, the Midwest and Northeastern U.S., England and most of Europe receive somewhere between 20 to 40 inches of rain annually. This points out the need for supplemental irrigation in these areas especially during the hot, dry weather of late summer.

Although we have a tendency to look at each requirement for plant growth as an individual unit, the fact is that they all work together. So, when we talk of hostas as shade plants, light intensity is not the only factor. Sure, light is needed for photosynthesis but it also warms the leaf surface. Light at higher intensities encourages more rapid evaporation i.e. transpiration, resulting in higher water demand.

Since hostas originate in a part of the world where they get a high level of rainfall and relatively cool temperatures, they may not be structured so that they can pull enough water from their roots in order to replace what is lost from excessive evaporation. Growing hostas in the shade keeps their leaf surfaces a little cooler, thus reducing the rate of transpiration.