Rock rose growing out of a crack in the asphalt. This picture was taken in California in April, 2008.
I remember noticing the white rock rose for its beauty. I would see two or three flowers on a plant that appeared to be weed like. It would be along cracked sidewalks or in rocky and dry grassy places. I never imagined rock roses could form pretty garden shrubs as seen in the following picture taken in California, near the San Francisco area.
When it came to learning the binomial name of the rock rose that I only recognize by sight, I had to look into the family Cistaceae and various genera in the family.
The Cistaceae (rock-rose or rock rose family) are a small family of plants. This family consists of about 170-200 species in eight genera, distributed primarily in the temperate areas of Europe and the Mediterranean basin, but also found in North America; a limited number of species are found in South America. Most Cistaceae are subshrubs and low shrubs, and some are herbaceous. They prefer dry and sunny habitats. (http://en.wikipedia.org/wiki/Cistaceae)
Keeping in mind the rock rose I know, we continue with a selective description: The plants often have showy yellow, pink or white flowers, which are generally short-lived. The flowers are bisexual and they usually have five petals. The petals are free, usually crumpled in the bud. They have five sepals, the inner three of which are distinctly wider, and the outer two are narrow. Sepals form the calyx of a flower which is a sort of whorl enclosing the developing bud, and opens up with the blooming flower. The sepal arrangement is a characteristic property of the family.
The stamens (the pollen producing reproductive organs) are numerous, of variable length, and sit on a disc; filaments are free. The ovary is superior (above point of insertion of the petals, sepals and stamens). The fruit is a capsule, usually with five or ten valves. The seeds are small, with a hard, water-impermeable coating, weighing around 1 mg.
The fruit of rock rose is a pentagonal capsule, 5–7 mm long.
Two important ecological properties, mycorrhizal ability and fast renewal after wildfires, gives Cictaceae the chance to thrive in Mediterranean habitats.
A mycorrhiza is a symbiotic association between a fungus and the roots of a higher plant that has specialized tissues for conducting water, minerals, and photosynthetic products through the plant. Most Cistaceae have the ability to create symbiotic relationship with root fungi of the genus Tuber. In this relationship, the fungus complements the root system in its task of absorbing water and minerals from the soil, and thus allows the host plant to dwell on particularly poor soils.
Cistaceae have also adapted to the wildfires that are common place in their native habitats. The plant seeds have a hard coating that is impermeable to water, and thus the seeds remain dormant for a long period of time. This together with their small size allows it to establish a large seed bank rather deep in the soil.
Within Cistaceae, eight genera are recognized, including five in the Mediterranean (Cistus, Fumana, Halimium, Helianthemum, Tuberaria) and three in the temperate regions of North America (Crocanthemum, Hudsonia, Lechea). It was difficult to know if the genus I needed to look into was Halimium, Helianthemum or Cistus. Halimium is a genus of 12 species in the family Cistaceae native to Europe, NorthAfrica and Asia Minor, with the center of diversity in the western Mediterranean region. Helianthemum is a genus of about 110 species distributed throughout the Northern Hemisphere, especially in the Mediterranean.
I concentrated on Cistus, a genus containing about 20 species They are perennial shrubs found on dry or rocky soils throughout the Mediterranean region, from Morocco and Portugal through to the Middle East, and also on the Canary Islands. They have showy 5-petaled flowers ranging from white to purple and dark pink.
Cistus salviifolius, common name Sage-leaved Rock Rose in the genus Cistus is the rock rose I know and it is favored by gardeners. The name Cistus from the Greek word kisthos (κίσϑος) means basket, while the species name salviifolius refers to the wrinkled leaves similar to those of the sage.
Cistus salviifolius is a typical maquis plant of the Mediterranean. It is a rock rose that has been introduced into California. The stems are covered by clumpy hairs. This bushy shrub reaches on average 30–60 cm in height. The five white petals have a yellow spot at the base, forming a corolla 4–6 cm in diameter. The stamens are also yellow and the anthers, the part of the stamen that produces the pollen, shed abundant yellow pollen. This plant is pollinated by insects and especially bees. The flowering period extends from April through May. The leaves are evergreen. No water or fertilizer is necessary from April until mid-October.
They are a joy to come by in the most unexpected places.