Rock-Rose

Rock-rose (or rush-rose; Helianthemum scoparium) is a small, early-blooming perennial found in open areas in the chaparral. In The Reserve, it seems to prefer the tops of sandstone bluffs.

The small, thirsty leaves are shed at the onset of the dry season, and the stems, which also contain chlorophyll, become the primary photosynthetic structure. The bare green twigs give the plant a unique tufted appearance that gives rise to the Latin  species name, scoparium, meaning broom.

Rock-rose is a “fire-follower”, one of many plants of the sage scrub and chaparral that have seeds that can remain dormant for many years and are stimulated to sprout by a wildfire. Thus “fire-followers” are extremely abundant for a few years following a fire, decreasing in abundance thereafter until another fire starts the cycle anew.

Description ^2,3,4,59

Rock-rose is a small subshrub, usually less than 1.5 feet (0.5 m) high. Many slender green (photosynthetic) stems arise from the base, extending upward and outward. Leaves are linear, usually less than one inch (2.5 cm) in length, with smooth margins. Leaves are sparse and fall after the main growth period, leaving a spray of bare stems that contain chlorophyll and continue to photosynthesize through the dry summer months. At this stage, a plant with its tuft of bare green stems resembles a small deerweed (Acmispon glaber) or spiny rush (Juncus acutus).

The yellow flowers occur in loose clusters of few to many. They are shallow-dish-shaped, radially symmetrical and bisexual, less than one inch (2.5 cm) in diameter. There are five sepals; three inner sepals are broad and triangular but the outer two are much smaller, linear in shape and inconspicuous, so the calyx appears to have only three sepals, which are green tinged with red.  The petals are obovate to fan-shaped. There are 10 or more greenish-yellow stamens in multiples of five, Stamens are of different lengths and somewhat flaccid in appearance, each with a rounded yellow anther. The pistil is yellow with an ovoid, superior ovary; unlike some,⁴ ours appears unlobed. A short, stout style is topped with a pale stigma with tiny hairy or finger-like projections making it resemble a pom-pom. Flowers often appear after the winter rains begin, but the main bloom time is Mar-July.¹

The larger inner sepals persist and enclose the dry, ovoid seedpod which splits open when mature, releasing several small, hard, dark seeds that have rounded or angled sides. Seed coats are very hard and water-impervious.

Classification ^2,11,59

Rock-rose is a dicot angiosperm in the rock-rose family, Cistaceae. Plants in this family have five-parted flowers and stamens in multiples of five. The overlapping sepals are of unequal sizes and the superior ovary has a short style. Seeds have a hard, impervious coat and may remain dormant for many years.⁴¹

Cistaceae is a small family and contains some useful, drought-tolerant garden plants, such as the ornamental rock-rose (Cistus spp). Rock-rose is the only representative of this family native to the Reserve, although the non-native garden rock-rose occasionally occurs.⁴⁸

Ecology ^14,226

Like many annuals and some subshrubs of the chaparral and sage scrub, rock-rose is a “fire-follower”, a plant that becomes extremely abundant after a wildfire, persisting for a few years before disappearing again²²⁶ – “a ‘Big Bang’ kind of reproductive strategy.”²²⁷ In a study after the 1995 Vision Fire in Point Reyes National Seashore “the most spectacular examples of short-lived, fire-adapted plants … were several legumes, … mixed with broom-rose Helianthemum scoparium, Cistaceae.”²²⁷

Most “fire followers”, including rock-rose (broom-rose), produce seeds that have a very hard, water-impermeable seed coat. Without the uptake of water, germination cannot occur, and seeds are slowly buried in the soil where they accumulate in large numbers. A wildfire breaks down the seed coat and thousands of seeds may germinate during subsequent rains. Different species respond to different stimuli under different circumstances.²²⁸ In the case of rock-rose, about 25% of the seeds germinate without fire, but germination is doubled by the heat of a fire.²²⁸ Before the Point Reyes fire, rock-rose had been listed as of doubtful occurrence in the area. After the burn, thousands of rock-roses formed “extensive turf-like patches” and persisted for at least five years before becoming rare again.²²⁷ 

Human Uses ¹⁶

The Kumeyaay boiled the flowers for a tea that helped mothers during a difficult birth.

Interesting Facts ²¹

“Helios” is the Greek word for sun. “Anthemon” is the Greek word for flower. Thus Helianthemum refes to the fact that rock rose opens in the sun.