Brian Kemble is curator at the Ruth Bancroft Garden in Walnut Creek. His monthly column focuses on drought-tolerant plants and dry gardens.
While some succulents thrive in full sun, many need a little shade.
In most cases, they want partial shade rather than deep shade. This may be provided by placing them on the east side of a shrub, rock or wall, where they will get direct sunshine in the morning while being spared the full brunt of the hot afternoon sun. Another option is to plant them in the dappled shade of a shrub or tree that has a canopy that’s not so dense.
The Palo Verde tree, Parkinsonia aculeata, is an excellent example of a drought-adapted tree providing this kind of light shade. In its native region of the southwestern United States and Mexico, it is often found with some of the smaller kinds of cactuses growing beneath it, benefiting from the shade it provides as well as the accumulation of organic matter.
It should be noted that Parkinsonia aculeata is not the only tree called Palo Verde, which translates as “green stick” and refers to the green trunk and branches. The name is also applied to other species of Parkinsonia as well as the closely related genus Cercidium.
All of these modest-sized trees are members of the Pea Family, and they possess specialized roots that are able to work a kind of magic called “nitrogen-fixing.” This involves taking nitrogen in a form that can’t be used by plants and converting it to a usable form. Since nitrogen is a major component of fertilizer, this is a significant benefit to all plant life in their vicinity.
Parkinsonia aculeata is a modest-sized tree, attaining a height of about 15 to 25 feet. Like many desert trees, it has some thorns on its branches, but these are small compared to those of the mesquite or some of the desert acacias.
Its leaves are composed of many tiny leaflets, giving the tree a fine texture. The leaves are shed in the winter months, but the green trunk and branches are appealing features all year long.
The Palo Verde has an attractive yellow flower with an orange eye, produced abundantly in the late spring and early summer, and continuing intermittently through fall. They are about three-fourths of an inch across and pleasantly fragrant. The pods that follow are several inches long and look like long narrow pea pods, as befits a plant belonging to the Pea Family.
An interesting variant on the Palo Verde is the hybrid named Parkinsonia “Desert Museum.” It is thornless and very fast-growing, and is reported to produce even more flowers than Parkinsonia aculeata.
In either case, these trees are tolerant of alkaline soils, perform very well with little watering and do not require much pruning or care. However, they are not adapted to a purely winter-rainfall climate, and must have a little water from time to time over the summer months. They should be planted in a sunny position.
Email questions on drought-resistant plants to info@ruthbancroftgarden.org. Learn more about the Ruth Bancroft Garden at www.ruthbancroftgarden.org.