Foothill palo verde.

Identification

Foothill palo verde (Parkinsonia microphylla) is the species of palo verde native to upland Sonoran Desert positions; its sister species, the blue palo verde (P. florida), is the riparian sibling found along washes. The two are easy to tell apart in spring: blue palo verde flowers are uniformly bright yellow and the trunk is steel-blue-green, while foothill flowers have a single white petal among the four yellow ones, and the trunk is a softer yellow-green. Foothill is also distinguished by the shorter, finer leaflets that give the species its name (microphylla, "small leaves"). Mature size in cultivation runs 15–25 feet tall and 18–25 feet across.

The tree's defining feature is its bark. Like most desert legumes, palo verde drops its compound leaves during drought to conserve water; unlike most, it then carries on photosynthesizing through chloroplasts in the green bark of its trunk and branches. A leafless palo verde is a tree still working.

Ecology

Foothill palo verde is the famous "nurse tree" for young saguaros: 80–95% of mature saguaros in Saguaro National Park grew up under the shade of a foothill palo verde, sheltered from frost and direct summer sun until they could put out roots of their own (Drezner, 2014, Journal of Arid Environments). We see this on residential lots: a foothill palo verde planted in 2018 will already have a small saguaro under it by 2026 if the homeowner doesn't pull it out as a "weed."

The tree flowers in mid-April through mid-May, becoming a column of pale-yellow blossom that supports cactus bees, halictid bees, and a few specialists like Centris pallida. The flowers are followed in June by light brown beans that the seed-cleaning rodents in the lot will plant for you, often in inconvenient places.

Water budget

Foothill palo verde is the lowest-water tree in our atlas after the shrubs. Established trees need 3–6 gal/wk in summer, and that should arrive in deep, infrequent doses rather than daily light watering — a single 50-gallon pulse every two weeks during the hottest stretch is closer to ideal than 6 gallons every Saturday. The tree resents standing water and will drop branches in waterlogged soil. We never plant it on a greywater outlet for this reason.

The species is also the easiest to overwater into killing. A new palo verde that gets daily 5 gallons during summer will look great for two years, then collapse from root rot during the third winter; we have replaced four trees in our first eight years of work for exactly this reason. The basin we install for a palo verde is sized to hold a single 1-inch storm event and to dry out completely between events. Underwater is the goal.

Planting notes

We plant 5-gallon nursery stock from Desert Survivors or Spadefoot, never larger; the species establishes terribly from larger sizes. Hole twice the rootball width, no soil amendment, no fertilizer, no stake. We do install a 4-foot trunk guard for the first two summers because young palo verde bark sunburns easily on the southwest exposure.

The species also self-seeds reliably from any mature tree on the lot — meaning that after planting your first one, you'll likely have one or two volunteer seedlings each summer in adjacent basins. We let the homeowner decide which to keep and yank the rest in November.

Subsystem pairing

Foothill palo verde pairs with EARTH-1 only. We never run greywater (GWY-1) to it — the higher constant water and the sodium load are both problems. We very rarely run cistern-fed drip (CIS-1) to it, because the tree wants long dry intervals between deep waterings, and most controllers schedule the opposite. We sometimes run AC condensate (COND-1) to a young palo verde during establishment, but we cap the flow at the equivalent of 4 gal/wk and shut it off entirely from October through April.

This is the receiving plant we install most often in the right-of-way under a curb-cut basin. We pair it with desert milkweed on the rim of the basin and call the planting done.