Vernal Pools
Plant Communities of Mount Diablo State Park

Vernal pools have only recently gained the publicity -- some would say notoriety -- they deserve. Long called hog wallows as well as other degrading names, vernal pools are specialized habitats within our grasslands. Wherever clay soils form small depressions underlain by cementlike hardpans, vernal pools appear. As their name indicates, these are spring pools, filling with water during winter rains, slowly dying as days lengthen and soils warm. Such miniature wetlands are whole ecosystems unto themselves, with special circumstances: seeds, perennial roots, and bulbs must start growth when covered with water, yet they must wait until water levels recede to put on their full growth, blossom, seed, and die. Growing while covered with water is particularly difficult since little oxygen mixes with water, yet plants require oxygen for healthy growth. How vernal pool annuals manage germination under these circumstances is still poorly understood.

The floral displays of vernal pools are impressive indeed. As water evaporates from each level a ring of flowers appears. Most flower species occupy a specific level in their pool, and rings of flowers constantly change. It is not uncommon to see complex swirls and whorls of color like a fine Persian carpet. Whites may blend with yellows, or yellows segue into purples and blues and, lower, mix with more whites.

Vernal pool annuals include downingias, lobelia relatives with perky blue flowers marked and splotched white, yellow or dark purple; glue-seed (Blennosperma mana), with pale yellow daisy flowers and white pollen; vernal pool mint (Pagogyne spp.), with minuscule, white forget-me-not-like flowers; button parsley (Eryngium spp.), with spine-edged parsley-scented leaves and tight buttons of spiny-bracted green or bluish flowers; fragrant clover (Trifolium variegatum), with small heads of honey-scented white and purple flowers; and annual forms of golden monkeyflower (mimulus guttatus), with perky, golden-yellow two-lipped flowers.

Vernal pools are easily destroyed through habitat degradation, as for example when fields are leveled for agriculture or suburbanization. Since they most often occur in desirable, low, rolling foothill country or on valley bottoms, vernal pool habitats are the first to be developed in any given area. Sadly, the East Bay has lost most of its vernal pool habitats. Many were destroyed before the importance of these special wetlands was appreciated.

Excerpted from MDIA's book Plants of the East Bay Parks by Glenn Keator, Ph. D.

© 2019 by Mount Diablo Interpretive Association