Irrigation in Conventional Landscapes
The irrigation schedule is determined primarily by the landscape type (conventional landscape vs restoration). The schedule is further modified by the types of plants used (perennials, woody plants or grasses and sedges) and the site conditions (cool and foggy along the coast, warm and dry inland, etc.).
The first step is to determine whether your goal is conventional landscaping or restoration. A drought tolerant native landscape is a type of conventional landscape.
Conventional landscapes are kept reasonably moist throughout the year, so that the plants do not experience damaging moisture stress. The concept of "reasonably moist" changes with the design and the plant selection. For example, an English perennial garden will be kept very moist, so that the plants grow and remain lush from spring through early fall. Plants are watered multiple times per week, often using an overhead irrigation system. A native plant garden uses less water than an English garden, and some plants may even go into summer dormancy from the restricted water supply, but the irrigation concept is the same. All plants are ensured adequate water to keep them alive and to maintain an appropriate level of growth and vigor.
To maintain vigorous growth, most gardeners water the plants at least once per week. It is common to irrigate two or three times per week. They are looking for a sweet spot where:
A well designed conventional landscape will use a minimum of inputs and may beautifully mimic a natural system, but at its heart, the landscape is reliant on outside inputs to survive. The regulated soil moisture from irrigation means that the roots will grow and adapt to a modified water regime. Although many plants will find adequate natural water sources in the soil, most will not. If irrigation is ended, many plants will die and others will become severely stressed. The landscape will decline, and only a few remnant plants will survive.