Last January, during the wettest winter we’ve had in years, I was delighted to find a vernal pool at the Sepulveda Dam Wildlife Refuge. I had heard of them but never seen one.
A once-common feature of the California landscape, vernal pools are puddles in a valley’s alluvial plain. Though rare and precious now, the pools can still be found in protected areas after heavy rains. They linger into late spring, remaining sumpy “islands” of inch-deep water long after the surrounding land has dried out.
Vernals have occurred in all the big California valleys, including the San Fernando, but were most noticed in the richly-watered Great Central Valley. Before 1849, when Yankee interests began a total re-plumbing of the Central Valley’s hydrology, the flat lands would be pock-marked with vernal pools each February. (In especially rainy years they merged to form one vast Valley-wide vernal pool. In those flood years, as recorded in the 1850s and 1860s, one could launch a canoe at the foot of the Grapevine, paddle up the valley all the way to Stockton, slip through the Delta into the Bay, glide past San Francisco, ride through the Golden Gate on the tide, and have the whole Pacific as one’s oyster. I don’t think anyone ever tried it; who wants to canoe in the rain?)
Vernal pools evolve unique soils over time — the cycle of wet-dry attracts particular organisms. By mid-summer, they appear as island meadows of wildflowers. The most spectacular examples leave brilliantly colored concentric rings of flowers, each ring a marker of different growing conditions during the gradual shrinking of the puddle. The little pool in Encino cropped up with raggedy mustard (of course) but also a few encilia sunflowers, and an abundance of milkweed — indeed, “Mexican whorled milkweed,” Asclepias mexicana.
Late August — end of the season 
It took until August for me to realize these plants were milkweeds. When they finally started to puff out and shed their cotton-wool, it made my heart sing. Milkweeds are the most important plants for Monarch butterflies, which are facing catastrophic habitat loss. And just as the delight of this realization hit, I turned to find a glorious messenger fluttering around like she owned the place. I haven’t seen a big Monarch since several years ago. Glory be!
The Valley Village View 