Garden guides, both online and printed, specify plant height, width, exposure, and water needs. Some even list soil conditions or vary water needs by specific geography. What is missing, though, is water per square foot, the single-most revealing sustainable gardening metric.

While water use of an individual plant matters to overall consumption, my analysis these past two years found that the number of plants and its corollary, water per square foot, are even more important. A small plant may be low irrigation, but filling a space takes many, each of which is irrigated. A plant that covers a lot of ground, on the other hand, means fewer plants and fewer emitters.

Rather than gauge a plant’s suitability by garden guides’ accounts of its water use, gauge it instead by water per square foot. Using known water use and size, you can calculate it as:

Water/Square Foot = Plants/Square Foot * Water Use/Plant

To stretch precious water, the best plant choices turn out to be wide low-water-use ones. A few plants can cover a big area with little irrigation. Prime water-efficient varieties with their mature widths include:

• Agave ovatifolia ‘Whale’s Tongue Agave’ (3-4’ wide)
• Aloe rubroviolacea ‘Arabian Aloe’ (4-6’ wide)
• Myoporum parvifolium ‘Putah Creek’ – Creeping Myoporum (spreads to 15’)
• Prunus ilicifolia ssp lyonii ‘Catalina Cherry’ (10-15’ wide)

By contrast, low-water-use plants such as Aloe variegata ‘Partridge Breast Aloe’ or Senecio crassissimus ‘Vertical Leaf Senecio’ are not truly water efficient. When it takes 7 plants to fill the same space that a single wide one would, it also takes 7 times as much water.

Although it is tempting to view regular-irrigation plants as relative water hogs, an area populated with wide ones has less aggregate consumption than it would with narrow, low-water-use varieties. The total plant count is so much less, which which more than makes up for each plant taking a bit more water. Examples that pass a sustainable garden’s water efficiency test include:

• Jasminum angulare ‘South African Jasmine’ (spreads to 8’)
• Hydrangea quercifolia ‘Snow Queen’ – Oak-leaf Hydrangea (6-8’wide)
• Prostanthera ovalifolia ‘Variegata’ – Variegated Mint Bush (3-5’ wide)
• Plectranthus ciliatus ‘Drege’ – Spur Flower (2-3’ wide)

Not surprisingly, consumption can vary by climate zones and even microclimates. The 2014 WUCOLS (Water Use Classification of Landscape Species), funded by California’s Department of Water Resources and directed by University of California Cooperative Extension, categorized species’ water needs in six regions as determined by local experts. Organized by city, their online database of over 3,500 plants’ water use is searchable by botanical name, common name, plant type, or water use.

Sun/shade exposure and soil conditions also affect a plant’s consumption. Some considered drought-tolerant may be quite thirsty in hot sun or sandy soil. Others purported to need regular moisture may thrive with little water in partial shade or clay soil. Comprehensive web sites such as SanMarcosGrowers.com are easily navigable for this type of cultural information. The encyclopedic Sunset Western Garden Book (2012) details every plant’s exposure, water needs, and in which of 29 climate zones it grows best.

Perhaps future guides, databases, and even plant tags will incorporate the pivotal water/square foot metric. They should. In the meantime, just calculate it yourself and let it be your guide to select and place plants for a more sustainable garden.