A combination of factors including weather, plant type, shade, slope, soil, and previous research is used to determine watering requirements.

Water plays an important part in a plant's life processes, from nourishing photosynthesis to maintaining the simplest cell wall. Native and adapted non-native plant species that have been preserved or planted and established properly seldom require supplemental water. Many times they flourish solely on natural rainfall.

When these plants do require supplemental water, or for plants that have not been fortunate enough to receive proper planting and maintenance, we have established guidelines or watering best management practices. These practices are based on plant physiology and soil characteristics.

Last month, we discussed how we derive the grass irrigation recommendations. We can use the same baseline estimate and adjust for trees, shrubs, and perennials by applying different species factors. We can also compare current monthly deficits with historical precipitation and adjust accordingly during the latter part of the month.

In either case, we do not rely solely on or recommend watering events based on a calendar date. We do use a combination of factors including weather, plant type, shade, slope, soil, and previous research to determine watering requirements.

What we have found is that for most sites in San Antonio, turf may be watered once a week from April through October and once every three to four weeks during the winter. That includes the acceptance of occasional dormancy in the peak of the summer by Bermuda, buffalo and zoysia grasses.

Trees, shrubs, and perennials can be watered once a month from March through October. Shrubs and perennials may have to be watered once every two to three weeks during the summer. However, properly established drought tolerant plants can forgo irrigation in late summer. That is the beauty of native and certain non-native species – sometimes no water is needed.

Mark Peterson is a conservation project coordinator for San Antonio Water System.