Plants at a Glance: Indigobush

A back road trip through the Mojave Desert can be very time consuming, and I often remind myself that the purpose of the trip includes the journey as well as the destination. So many interesting things to catch my eye! Is that a tortoise sunning himself on the road, or just a rock? Are the cattle still hanging out near the tank around the bend? And during a good spring, what is that stunning plant?!

The flowers on the Fremont Indigobush are spectacular in the springtime. Photos courtesy: Alice Newton.
Fremont Indigobush (Psorothamnus fremontii) is a traffic-stopper whenever in bloom, and still remarkable even when it’s not. Previously known as Dalea fremontii, it is a low, pleasantly aromatic shrub occurring primarily in the Mojave Desert, but also extends into the Sonoran and slightly into the Great Basin Desert.

Although deciduous during the coldest winters, it has an extremely interesting branch structure that could be enhanced by intelligent pruning. The sharply-bent branches are a creamy tannish-white until they become twisted and gnarly gray as they converge near the base of the plant.

The flowers on the Fremont Indigobush are spectacular in the springtime. Photos courtesy: Alice Newton.
The leaves are small, linear, very aromatic, and are carried on the secondary and tertiary branches. But what grabs everyone’s attention are the electric-blue, pea-like flowers that emerge on the very tips of the branches in March, April and May. A clump of shrubs in full display can be seen for miles! Cool and wet winter and spring weather will allow them to bloom later and longer into the season.

Indigobush grows on coarse, sandy, gravelly soils usually with some gypsum content. It is a common companion to Las Vegas Bearpoppy (Arctomecon californica), but can be seen in Creosote/Bursage communities in washes along with Desert Senna (Senna armata), Globemallow (Sphaeralcea ambigua) and Rhatany (Krameria parvifolia).

Seed set occurs in late spring, taking several weeks (depending on temperatures) to mature in early summer. A single small seed is enclosed in each sticky capsule along the branch tip, with the same strong aroma as the leaves. The sticky resin is similar to that of pines and is hard to remove from your bare hands. The seeds are mature when the capsule just begins to open.

After collection, spread the capsules in a thin layer in a warm, dry area to after-ripen and release the seeds. Hand thresh only, as mechanical threshing usually damages more seed than not. Seed viability can vary widely, from as little as 5 percent, up to 95 percent.

Once mature, Indigobush seed, like many other desert legumes, demonstrates physical, or hard seed coat dormancy. The easiest way to break this dormancy is to place the threshed seed in a jar with water and coarse sand, and slowly tumble for no more than 24 hours. More vigorous mechanical or chemical scarification will generally damage them too much.

Sow in deep plug flats filled with a well-draining sand/peat or sand/coir mix, with daytime temperatures around 70-75 degrees F. Be patient, as full germination may not occur for more than two months. Try to keep the seedlings as dry as possible; they are very susceptible to damping-off, and fungicides may be phytotoxic. Transplant early into a larger container or outside into well-drained, sandy or gravelly soils.

Desert plants spend a great deal of energy creating extensive root systems, and may be root-bound before you know it. Be sure to protect tiny plants against herbivory with a wire-mesh cage or other barrier.

All desert plants benefit from a little extra water and fertilizer, especially if they’re being placed in a typical new-landscape situation with dead soil. However, avoid high nitrogen formulations. Mix a low dose of time-release formula with low nitrogen, high potassium and phosphorus, and chelated micronutrients into your planting area.

Water well at planting, but sporadically and deeply thereafter, gradually lengthening time between irrigations. Once Indigobush is established, it will thrive on a good watering about once a month in the summer, and every other month in the winter. Too long between irrigations will make it go dormant, but should recover quickly when watered. If your soil drains well, overwatering shouldn’t be a problem as long as the soil does not remain saturated.

After a few years the branch structure should be developed enough to begin shaping. Many small branches will begin to clutter up the interior of the shrub and should be carefully removed. Remember to selectively thin with an eye towards enhancing the twisted branch structure.

Never, ever shear this plant! You will ruin the structure you waited so long for, and lose all those wonderful, splendid flowers!

Showcase this plant by installing in front of a contrasting background or with creative night-lighting to enhance its interesting silhouette.

As the Vegetation Program Manager, Alice Newton is responsible for weed and rare plant management, arid land restoration and Song Dog Native Plant Nursery operations for the National Park Service at Lake Mead National Recreation Area.