Hostas are primarily enjoyed for their attractive leaves, and those leaves require plenty of water to stay in good condition, especially when planted under trees that compete for water with the hostas. If your garden gets 1/2 to 1 inch of rain per week during the summer months, you may not have to additionally water your hostas. In drier conditions, your hostas will enjoy a drink from the garden hose to supplement the rainfall. Too much water can lead to fungal diseases, or to favorable conditions for garden slugs.