Water needs are determined for plants that have become “established” in the landscape (Fig. 6). “Established” means that substantial root development has occurred in the soil adjacent to the root ball; that is, the landscape soil becomes the principal reservoir of water rather than the root ball soil. The time for establishment varies among species and with soil conditions, but it generally occurs by the second or third year after planting. After establishment, roots of trees, shrubs, groundcovers, etc., become intertwined in the soil, creating a common root zone.
Reference Evapotranspiration (ETo) Conditions
ETo is defined as water loss from a large field of cool-season grass 4 to 7 inches tall that is not water stressed. Although ETo can be measured directly, it is usually calculated from weather data. Daily ETo information for many regions of the state is available through the California Irrigation Management Information System (CIMIS). Evaluations are made for site conditions equivalent to those used for ETo measurements, such as full sun, no extraordinary winds, no shading from nearby structures or plants, and no heat inputs from nearby sources such as buildings, pavements, or reflective surfaces.
As an exception, shade-requiring species, such as Japanese aucuba (Aucuba japonica), are evaluated for shade conditions. Shade species are considered to be plants that show visible injury when exposed to full sun for some part of the day. Since species vary in their shade requirements, any species requiring some shade to avoid injury in the region is evaluated for shade.
Groundwater Not Available
Although some species of plants develop root systems deep enough to extract groundwater, such as valley oak (Quercus lobata), groundwater is not available in all planting sites. A species capable of extracting groundwater may not be able to do so simply because water is not available. Therefore, evaluations were made for conditions where the only sources of water are rainfall and irrigation. In areas where groundwater is available and a species is known to have the capacity to use groundwater, adjustments in irrigation scheduling should be made for that species or group of species.
Plants Must Be "Irrigatable"
In some cases, the soil surface around plants may be covered with impermeable pavements or other materials. This is particularly the case for trees in parking lots or along streets. Such pavements act as barriers to the infiltration of water into the root zone, making the plants very difficult to irrigate. In other cases, the soil volume capable of holding water may be so small and may dry so rapidly that it becomes very difficult to maintain available water in the root zone. In both cases, the amount of water identified as being needed to maintain good quality may not be sufficient simply because the plants are not irrigatable. Our evaluations assume as a standard condition that the species can be irrigated, that is, that applied water can enter and be held in the root zone long enough for plant uptake.