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Argyroxiphium sandwicense

Silversword

Asteraceae

Hawaiian Islands, alpine zones of Haleakala and Mauna Kea

Attracts Pollinators Drought Tolerant

SunFull Sun

WaterLow · drought-tolerant

HardinessZones 9–10

Overview

Argyroxiphium sandwicense is a monocarpic rosette perennial of Hawaiian alpine cinder desert, forming a dense hemispherical mound of sword-shaped leaves 1-2 feet (30-60 cm) across. Leaves are 4-10 inches (10-25 cm) long, 0.4-0.8 inch (1-2 cm) wide, and clothed in dense silvery hairs that reflect high-elevation ultraviolet radiation and trap moisture. The rosette persists 15-50 years without flowering, then bolts a single erect stalk 3-6 feet (1-2 m) tall carrying 50-600 nodding heads with maroon to purple ray florets and yellow disc florets. The plant dies once seed ripens, so reproduction rests entirely on that one flowering event. Two subspecies occupy separate mountains: subsp. macrocephalum spp. on Haleakala, Maui, between 6,900 and 9,800 feet (2,100-3,000 m), and subsp. sandwicense spp. on Mauna Kea, Hawaii island, between 8,500 and 12,300 feet (2,600-3,750 m). Both are listed under the United States Endangered Species Act. Feral goats and sheep removed most wild plants before fencing was installed, and disruption of native pollinators by introduced ants limits seed set in the remaining populations. Cultivation outside the tropical alpine zone seldom succeeds, since the plant requires cool nights, intense sun, and sharply drained cinder.

Native Range

Argyroxiphium sandwicense is endemic to Hawaii and confined to two volcanic summits. Subspecies macrocephalum spp. grows in Haleakala crater and on the outer slopes of that mountain on Maui, and subspecies sandwicense spp. on the upper slopes of Mauna Kea on Hawaii island. Both occupy open cinder and lava substrate above the tree line, where annual rainfall is 15-50 inches (380-1,270 mm) and night temperatures drop below freezing through much of the year.

Suggested Uses

Grown almost entirely in botanical collections, conservation nurseries, and outplanting programs within the Hawaiian alpine zone. Planted in fenced restoration plots on Haleakala and Mauna Kea at spacings of 3-6 feet (0.9-1.8 m) on open cinder. Outside that habitat it is held under glass in cool alpine houses on cinder substrate at low humidity.

How to Identify

The rosette is hemispherical and densely packed, with narrow sword-shaped leaves radiating from a single unbranched stem base. Leaf surfaces carry a thick coat of appressed silver hairs, so the plant reads metallic gray in full sun. Leaves are succulent and gel-filled when cut, unlike the dry-leaved bunchgrasses sharing the same habitat. A flowering plant is recognized by the single erect purple-tinged stalk 3-6 feet (1-2 m) tall crowded with nodding composite heads 1-2 inches (2.5-5 cm) across.

Appearance

Size & Dimensions

Height1' - 6'

Width/Spread1' - 2'

Reaches mature size in approximately 15 years

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~6 weeks

Summer Fall

Flowering occurs once in a plant lifetime, between June and November with a July to September peak. The stalk elongates over 4-6 weeks, then heads open in sequence from the base upward across a further 4-8 weeks. Seed ripens 6-10 weeks after flowering and the plant dies, leaving a dry standing stalk that releases achenes through the following winter. Flowering age ranges from 3 to 90 years, with most plants bolting between 15 and 50.

Detailed Descriptions

Flower Description

Maroon to purple ray florets with yellow disc florets

Foliage Description

Silver-gray

Growing Conditions

Sun Requirements

SunFull Sun

Requires 8-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range6.0 - 7.5(Neutral)

357912

Soil Types

rocky sand

Drainagewell drained

Water & Climate

Water Needs

Low

Frost Tolerance

hardy

Time to Maturity

15-50 years to flowering

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Argyroxiphium sandwicense requires full sun for 8 hours or more, cinder or coarse volcanic gravel with rapid drainage, and pH 6.0-7.5. It draws most of its water from fog and dew rather than rainfall, and the leaf hair layer must stay dry at the crown, since overhead irrigation in warm lowland conditions rots the rosette within weeks. Night temperatures near or below freezing appear necessary for a normal growth cycle, and plants held at sea level in the tropics decline and die within 1-3 years. Root disturbance is fatal because the plant carries a single shallow root system spread through loose cinder, and foot traffic near the rosette compacts that substrate and kills plants outright. No fertilizer is applied. Seed is short-lived and germinates on cool, moist cinder without pretreatment.

Pruning

No pruning is carried out on the rosette, and removing leaves exposes the growing point and kills the plant. Dead outer leaves are retained where they lie against the base, since they shade the root crown and slow evaporation. After flowering the dry stalk stands until it collapses on its own and is left in place so seed disperses. Plants damaged by trampling or grazing do not resprout from the base.