Anza Design & Nursery

When it comes to Manzanitas (Arctostaphylos spp.), there is a lot going on in San Luis Obispo County. And this becomes apparent in the case of two Manzanitas growing in areas around the Morro Bay Estuary. 

The Morro Bay Estuary is situated along California's central coastline and lies roughly 70 miles north of Point Conception. To the east of the estuary are the Los Osos and Chorro Valleys...And they are home to two rare and endemic species of Manzanita. 

The Chorro and Los Osos valleys are home to Arctostaphylos osoensis (Oso Manzanita) and Arctostaphylos morroensis (Morro Manzanita). These plants grow nowhere else in the world, and they are restricted to areas surrounding the Morro Bay Estuary.

Morro Manzanita is generally more prevalent than Oso Manzanita. Morro Manzanita primarily inhabits the stabilized sand dunes at the south end of the Morro Bay Estuary, with much of the population occurring within Montana De Oro State Park. Many of the upland dune complexes in the area date back to the Pleistocene.

Then there is Oso Manzanita. Oso Manzanita is by far more restricted than Morro Manzanita. While Morro Manzanita grows throughout the upland dune complexes surrounding the towns of Los Osos and Morro Bay, Oso Manzanita basically hugs the slopes and rock outcroppings surrounding Cabrillo Peak. Oso Manzanita occurs in only a handful of occurrences. Yes, like 6 or 7 stands. 

Oso Manzanita seems to hold a close bond with two peaks that divide the Chorro and Los Osos valleys. Its 6 or 7 stands are in areas around Cabrillo Peak and, to a lesser extent, Hollister Peak.  Primarily, Oso Manzanita seems to prefer the south and west facing rock outcroppings of Cabrillo Peak.

Cabrillo Peak is one of the former volcanic peaks that divides the Chorro and Los Osos valleys. The peak, like the others in its chain, is basically a big chunk of volcanic rock that formed long ago (~22 – 26 mya). This chunk of volcanic rock is thought to be the result of when a dike ran through the underlying geologic plates and allowed molten igneous rock to well upward, where it then cooled near the earth’s surface. Over time, erosion stripped away the earth’s outer surfaces and exposed the mass of igneous rock that we today call Cabrillo Peak.