Deep, regular watering is a must for maintaining healthy, lush, perennial plantings. As many plants die of overwatering as die without enough water. Light frequent sprinklings are harmful to the perennial garden because small amounts of water are delivered only to the top inch or two of soil. The deep, anchoring roots of the plants will not receive enough moisture to support the whole plant.
How do you know when to water or how much to water? There's a cheap and easy way to tell. Stick your finger into the soil next to one of the plants, to a depth of 3-4 inches. If that soil is dry, it is time to give the planting area a deep, thorough soaking. It takes approximately 1 gallon of water to soak one square foot of soil to a depth of 4 to 6 inches, the primary root zone for most perennials. Plants will thrive with one to two inches of water per week; especially if they are well established, planted in good soil, and mulched properly with 2 or 3 inches of organic mulch. Good garden soil, when watered deeply, will retain that moisture for several days.
Xeric plants need regular, deep watering the first year they are in the ground, after that, they can survive with much less water than many other perennials. In fact, most native and xeric plants need to be watered only once every 10 days to two weeks, depending on the individual plant. Drip irrigation and soaker hoses are much more efficient at delivering water than sprinkler systems. Sprinklers, especially overhead types, are only 40-50% efficient when it comes to getting the water into the soil. Drip irrigation and soaker hoses, have a 90% efficiency rating. If you have a soaker or drip system, turn it on and check a spot at the very end of the hose system. When the soil at that end is damp at a depth of 4 to 6 inches, you have delivered enough water to the garden.
Watering is best done on an "as needed" basis. Hotter weather, rainfall, cloud cover, amount of daylight – will affect your garden's water needs.