Xylococcus bicolor

The name Xylococcus derives from the Greek word for "wood berry," which refers to the rock-hard stone the seeds are encased within. The specific epithet, bicolor, refers to the leaves, which are a dark green on the top surface and a wooly-white on the bottom.[8]

There is growing concern that this species is unable to produce young individuals in the wild. Despite producing thousands of drupes annually, many intensive searches had failed to find seedlings, and it was believed that the species could no longer reproduce by seeds. Complicating this predicament, the seeds of Xylococcus are still perfectly viable, with 61% of stones having at least one seed filled with live endosperm. One hypothesis put forward was that the seeds needed to pass through the digestive system of the now-extinct California grizzly bear to germinate, or that the stones containing the seeds need to be broken by the bear's strong jaws.[4][8][14]

Some experts, like Paul Zedler at the University of Wisconsin-Madison, contend that Xylococcus reproduction might simply encompass a larger timeframe than human observers might expect, similar to the long reproductive cycles in species like the giant sequoia, Sequoiadendron gigantea.[14] The giant sequoia annually produces 300,000 to 400,000 seeds, with many seedlings emerging from the duff on the forest floor, but only very few of these seedlings thrive unless conditions are correct. This means that despite the massive seed production, the giant sequoia seldom has young plants that can maintain the density of old groves.[15] Some specimens of Xylococcus can live up to a century or more, and like the giant sequoia, produce thousands of their seeds annually through their drupes, meaning that perhaps their reproduction occurs on a longer ecological time scale.[14]

In 2008, a bioblitz in San Diego led to the discovery of 53 seedlings and possible saplings growing in Florida Canyon, Balboa Park. It was discovered that the following February, none of the 2008 seedlings had survived, although new ones had emerged, meaning that none of the seedlings could survive a full year. Many other seedlings have been found successfully growing, but they are difficult to monitor, and often in the path of development. Very few seedlings have been documented to survive more than year, although one 30-year old seedling has been found.[4][8]

Xylococcus bicolor is cultivated as an ornamental plant by specialty plant nurseries, for planting in California native plant and wildlife gardens. It forms an interesting shrub with flowers that hummingbirds like. It prefers to have its leaves in full sun, but likes its roots in the shade. This species is tolerant of most soils, but requires excellent drainage. It needs fast draining soil of pH 6–7, in USDA zones 7–10.[18]

This plant is best suited on north-facing, dry rocky slopes. It grows at a moderate pace in a rounded shape, usually about 8 feet (2.4 m) high by 6 ft (1.8 m) wide. It prefers regular light watering over the first summer after planting. After establishment, this drought-tolerant shrub should survive the dry months with no supplemental water, though it can usually handle summer watering infrequently. It provides white and pink blooms over winter and spring, and is hardy down to 20 °F during these months.[19]

This species is notoriously difficult to propagate, and wild seedlings are seldom to survive past a year. Wild seeds are viable, and can be grown successfully provided proper protocols are followed. The fleshy outer layer of the fruit first must be removed, with some putting the fruits with water in a blender to remove the outer layer. Stones are then removed from the blender, and broken open with wire cutters. As the stones are brittle, around half of the seeds can be released undamaged. Seeds that are damaged in this process can be soaked in 0.3% hydrogen peroxide and copper soap to curb pathogens.[4][20]

Seeds may also be soaked in water to improve germination. However, they germinate best when stratified at 55 °F, often accomplished by placing them in a refrigerator. The treated seeds are placed in a food container on top of fine, moist potting soil, and within two weeks, around half of the seeds will have radicles emerging. Once these radicles emerge, it is recommended to place the seeds into pots, in a soil mix with extra perlite to improve drainage. The delicate plantlets need shade and periodic misting in this period. Eventually, cotyledons will emerge within fibrous sheaths. When the sheaths fall off and release the cotyledons, misting may be stopped, but shade is still required for root development.[4][20]