Manzanitas: Early Bloomers

The timing of these earliest bloomers in California native flora is keyed to the seasonal migration of hummingbirds and the emergence of bumblebees and native bee species.

An Early Spring in the Garden

Bart O’Brien
RSABG Director of Special Projects

After this past summer’s heat, and the welcome arrival of significant early fall rains, many of the manzanitas (Arctostaphylos species) in the living collection at Rancho Santa Ana Botanic Garden have burst into bloom seemingly overnight. How does this happen so quickly in manzanitas and their close relatives (Xylococcus bicolor—mission manzanita and Ornithostaphylos oppositifolia—palo blanco)? It all comes down to a successful strategy of adaptation to our mediterranean climate – nearly all of these plants (yes, there are one or two exceptions) prepare to take full advantage of late fall and winter rains and pollinators by producing fully formed yet dormant inflorescences at the tips of their branches at the end of their spring growth cycle (you can tell that their growing season is over when their bark peels in late May or early June; this tells you that they have entered summer dormancy). These claw-like structures are called “nascent inflorescences” and that is indeed what they are—preformed, ready-to-go flower clusters just waiting for the cooler temperatures and rainfalls of late fall and early winter that triggers their rapid expansion into full bloom. The timing of these earliest bloomers in California native flora is keyed to the seasonal migration of hummingbirds and the emergence of bumblebees and native bee species.

Typically, here at RSABG, the first of the manzanitas to flower is the rare and endangered Refugio manzanita (A. refugioensis) from Santa Barbara County. Most years, these plants begin flowering in late October or November and are followed by a progression of different species and cultivars with the greatest number of plants usually blooming in January and February. The last manzanitas finish blooming sometime in March or early April. Another similarly diverse group of plants that has nearly the same blooming season are the currants and gooseberries (Ribes species). You will almost always see some chaparral currants (Ribes malvaceum) beginning to bloom in late October or November, too.

The production of nascent inflorescences in these three closely related plant genera is unique in California native flora—and this writer cannot think of any other example of this phenomena worldwide. All other plants produce stems, leaves, flowers, and fruits in comparatively slow to moderate progression. Manzanitas and their kin are great examples of plants rapidly responding to environmental cues.

How did this come about? I can speculate that the most widespread species, kinnikinnick (A. uva-ursi) is a plant that is found predominantly in cold arctic and alpine environments, and in fact its distribution is circumboreal. In such harsh conditions it is likely advantageous to the plant to be among the first to bloom in order to take full advantage of favorable pollinator and climatic conditions. Carrying that early-blooming characteristic to California’s more extreme Mediterranean climate (California has the longest dry season and the coldest winter temperatures of the world’s Mediterranean climate areas) would likely have the same advantages.

Particularly of note to those of us here at the Garden is the taxonomic importance of manzanita nascent inflorescences. From the time that California’s manzanitas were first discovered up until the late 1930s, botanists had a very hard time discerning the differences between species of manzanitas. They could see that there were many different kinds, but had great difficulty in characterizing this diversity. Alice Eastwood, Willis Jepson, Katherine and Townshend Brandegee, Marcus Jones and many others seem to have missed out on this, the key diagnostic feature of manzanitas. However, once Albert E. Wieslander described the importance of this characteristic in his 1939 paper (and coined the phrase “nascent inflorescence”), all students of the manzanitas quickly realized that this indeed was the main key to differentiate the incredible diversity of the group. (There are more taxa of manzanitas in California than there are of any other woody plant group). There’s more to this story – and how Jepson “stole” Wieslander’s concept (he called these structures “embryonic panicles”) and a number of his species—but that will have to wait.

To be continued...