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soap weed plant

U.S. Forest Service

United States Department of Agriculture

Plant of the Week

Yucca glauca range map. USDA PLANTS Database.

Close-up view of a soapweed yucca flower. Photo by Charlie McDonald.

Soapweed yucca flower stalks. Photo by Charlie McDonald.

Soapweed yucca plant growing in the foothills of the Sandia Mountains on the Cibola National Forest. Photo by Charlie McDonald.

Soapweed Yucca (Yucca glauca)

By Charlie McDonald

Soapweed yucca is one of about 40 yucca species, all of which are native to the New World. It grows in dry rocky soils throughout the Great Plains and is most abundant in short grass prairies and desert grasslands. These plants have a long history of beneficial use. As the name implies, the crushed roots of soapweed yucca produce a lather that makes a good soap or shampoo. The lathering substances called saponins are found in many plants, but are exceptionally concentrated in yucca roots. The dried leaves of soapweed yucca can be woven into baskets, mats, or sandals. The strong coarse leaf fibers can be extracted to make cordage.

Roots labeled “yucca root” are often sold in grocery stores. These roots are actually cassava or manioc (Manihot esculanta). This woody shrub has a starchy tuberous root that is a staple food in the tropical regions where it grows. The roots of true yuccas are generally too fibrous and too full of toxic saponins to be used as food.

Yuccas and yucca moths are the classic example of a plant and animal obligate symbiotic relationship where each organism requires the other to survive. Yucca moths are the only insects that can successfully pollinate yucca flowers and the developing yucca fruits are the only larval food source for yucca moths.

U.S. Forest Service United States Department of Agriculture Plant of the Week Yucca glauca range map. USDA PLANTS Database. Close-up view of a soapweed yucca flower. Photo by Charlie

Plant Profiles

Soapweed Yucca

With such delicate blue-green leaves, it’s a wonder this yucca is hardy to Canada. And “soapweed?” If you crush its roots in water, they release enough sort-of-soap that you can wash your clothes. This is a plant of surprising talents.

Soapweed yuccas are usually as alert as porcupines caught in tight corners: hemispherical and fierce. Mine is floppy because my soil is too rich, and my drainage too slow.

This Spring I’ll replant it in a raised bed filled with soil that’s gritty and lean. Then this yucca will become the fearless all-weather sculpture it was born to be.

Here’s how to grow this hardiest of yuccas:

Agavaceae, the Agave family.

What kind of plant is it?

Hardy evergreen perennial.

Zone 3 with fantastic drainage; normally Zones 4 – 8

Clumping and hemispherical, with a short trunk that prefers to snake slowly along the ground. In leaner settings than mine, the needle-tipped foliage is bayonet-stiff. Offsets gradually to form colonies.

Size in ten years

A main stem of growth, with foliage to about three feet tall and four feet across, accompanied by smaller offsets quite close to the mother ship.

Sculptural and grassy.

its narrow blue foliage: When the plant is growing in lean and sunny surroundings, the leaves are rigid. Each is tipped with a single needle-sharp spine. To avoid leaving blood samples, approach this plant only when you are operating fully in the moment.

its imperviousness to deer: They don’t want to leave blood samples on the leaves, either—especially when they would be given lingually. Ouch.

its hardiness: Yucca glauca is native to Minnesota. Enough said.

its symbiosis with its insect pollinators: Yucca moths lay their eggs in the flowers but also pollinate them. Only a few of the resultant seeds become the food for the developing larva. The yucca cannot set seed without the moths, and the moths feed on no other plant than the yucca.

its roots: These can be ground up and mashed, to release one of the precursors to soap.

Mid-Summer. The spikes of ivory flowers are not nearly as tall or showy as those of larger species such as Y. filamentosa. They are pleasant but are secondary to the year-round presence of the foliage.

The ivory flowers go with anything; it’s the pale blue foliage that needs some color coordinating. To my eye, this means near neighbors that celebrate blue, burgundy, white, blue, or pale yellow.

Both the color and the form of Yucca glauca inspire dramatic partners. The narrow and rigid foliage is the natural contrast to foliage that is rounder, or feathery, or large. Its pale blue color craves the complements of white; pale pinks, blues, or yellows; and burgundy. When possible, combine with plants that partner both in form and in color. They’ll also need to be sun-lovers as well as connoisseurs of sharp drainage.

The round foliage of purple smoke bush glows when the sun passes through it; this shrub is never less than large, though, and would need to be planted only to the North or East of the Yucca so as not to block the all-important hot sun from the South and West. For plants with round purple foliage that can be planted even on the sunny side of the Yucca, consider purple-leaved sedums such as ‘Matrona’ or, much lower and almost a groundcover, ‘Vera Jamison’.

There are plenty of hardy cacti to explore; prickly pear is also native, amazingly, to Canada. While they require fantastic drainage whereas yucca merely prefers it, Yucca glauca will go toe-to-toe with any of them when you’re gardening outside the cacti’s prefered haunts of out-and-out deserts: The grittier the soil, the better.

A feathery partner every bit the equal of Yucca glauca in hardiness as well as craving for sun and good drainage is leadplant, Amorpha canescens. It also has the same native habitat. Junipers can provide feathery growth at any scale, and therefore at any side of the Yucca. In front, the resolutely prostrate habit of many Juniperus horizontalis cultivars is usually accompanied by grey-blue foliage that isn’t much of a color contrast in Summer—but it often flirts openly with burgundy in the Winter, whereas Yucca glauca holds to its blue year-round. Burgundy and blue: What garden doesn’t need more plant combinations and color pairings that are at their most vivid when it’s cold?

Yellow-needled junipers are the way to bring year-round contrast in color and texture. Juniperus conferta ‘All Gold’ is prostrate and low enough to be in front of the Yucca. J. chinensis ‘Gold Coast Improved’ is just a bit higher, so would be at the side. Choose just one or the other.

Yucca glauca will also grow in regular soil as long as the drainage is still good, in which case your additional choices are wide indeed. Can the Yucca be backed with dark green broadleaf foliage? Osmanthus or holly foliage is usually darker than that of box or rhododendron.

Because the Yucca is reliably evergreen, you can add deciduous and even herbaceous partners. I was once commanded to do a garden that featured pink, and wound up combining Yucca glauca with Spirea ‘Anthony Waterer’. Soapweed’s dangerous and blue foliage saved the bed from wallowing in soft textures and pastel colors.

About the only plants to avoid would be actual grasses, which would look repetitive at best, floppy at worst.

Where to use it in your garden

Yucca glauca is only happy towards the front of a bed, where its access to sun is minimally impeded. Its sculptural habit would be enhanced by planting into large pockets in ledge, or out of the way of traffic in a large sweep of gravel.

Full sun and soil that’s well-drained. The lax growth of my Yucca glauca shows how overly-moist my soil is—and, probably, that it’s also too rich. Yucca glauca looks better when the soil is lean and mean.

How to handle it: The Basics

Plant in Spring; water once and you’re done for the season. The plant can grow untended for years at a time, although you’ll want to clip off the flower stalks later in the Fall, by which time the yucca moth larvae have long since dropped to the ground, where they spend the Winter burrowed into shelter.

How to handle it: Another option—or two?

Yucca seems like a plant-and-forget-it plant, until you realize that there are a lot of brown leaves at the base that need to be clipped off. The foliage of Yucca glauca is so painful that this periodic maintenance needs some strategizing. The best tactic is to do it in late Winter, when it’s still so cold that you’d be wearing a reasonably heavy coat and gloves anyway.

Quirks or special cases

Yucca is long-lived, but any given rosette of foliage still has a life-span. After your clump is five or so years old, it’s not unusual for a rosette to take a pass on another year of rigid beauty. You’ll know by late Winter, so you can remove the deceased while still protectively garbed. If needed, free it from the colony by using your longest-handled loppers.

None. As is typical of Yucca species and cultivars, as long as the climate isn’t colder than each can tolerate—which would only happen with Yucca glauca if you were trying to garden near the Arctic circle—and the ground isn’t too poorly-draining, the plant is self-reliant and almost indomitable.

Yucca glauca is one of the less adventuresome of the many Yucca species, most of which seem to cross-pollinate as well as mutate casually, even opportunistically. Soapweed yucca has few cohorts, but I’d grow these if I could ever find them: ‘Stricta’ is more vigorous, with larger and branchier spikes of flowers; the flowers of ‘Rosea’ are tinted pink on the outside.

On-line and, occasionally, at retailers.

By seed, by division of the clumps, or by cutting the fleshy roots into sections in early Spring and placing them horizontally on the soil. They soon sprout leaves as well as roots.

Yucca glauca is broadly native to central North America, from Alberta and Saskatchewan to Texas, New Mexico, and Arizona.

A Gardening Journal ]]>