I can't say this too often.  Watering is the key to growing beautiful hostas.  Most garden soils have all the nutrients necessary to grow nice plants, but I suspect that very few receive enough natural rainfall to provide the water they need to stay looking good from spring to fall.

Hostas are native to Korea, Japan, and coastal China, areas of abundant rainfall. The average yearly rainfall in Tokyo is 60 inches, in Seoul it's 54.   Most states in the Midwest get about 30-40 inches of rain, while most in the South and Eastern Seaboard get 40-45. In St. Louis it's 37 inches, in Philadelphia it's 41. Rainfall above 50 inches per year is mostly limited to the Deep South.  So for most of us, natural rainfall provides only half to two-thirds the water hostas receive in much of their native habitat. And, in much of the U.S., we have hotter summers.  The average high temperature in June in Tokyo is 80 degrees and in Seoul it's 77. The average high in June in St. Louis is 85.

On top of that, compared to many perennials, they are not very efficient at using water, especially when it's hot.  Hostas, even the small ones generally have a great deal of leaf surface, and while these large leaves are evaporating water on hot summer days with wild abandon, their roots can't keep up. Since hostas evolved in a climate that is wetter and cooler than ours, their roots and leaves are not really designed to cope with the hot, dry summers that we get in much of this country.

The answer is simple enough.  Water them.  I can't tell you how many people have asked me how often they should water their hostas. Some people just have to make things complicated.  Again we must rely on that essential garden tool, common sense.  If it's dry, water.  If it's wet, don't.  That's all there is to it.  Don't let your plants dry out, especially when it's hot.  If they dry out, they get those ugly burn marks along the leaf edges, and in extreme cases the whole plant may fry to a crisp like these.

It's all pretty simple, and it's probably the most important thing you can do for your hostas.  There's just one complication.  Hostas love water when they are actively growing, but not when they are dormant.  In fact, staying wet in the winter and early spring is the main cause of crown rot and root rot.  So the answer to keeping your hostas watered is not to plant them in heavy, wet soil.  Your hostas need to be planted in well drained soil full of organic material, so they stay moist during the growing season, but are not too wet when they are dormant. Soils with lots of organic matter hold water well, but also have plenty of open spaces for good drainage and air circulation.