By Susan Lewitt, for Let’s Talk Plants! February 2025.
Are Vernal Pools All Wet?
Biodiversity is something very important to preserve especially in San Diego, which is a biodiversity hot spot. One reason for our great variety of life is the many microclimates that occur here giving us a wide diversity of habitats. One habitat that is often overlooked is vernal pools. Before development through housing, agriculture, as well as urbanization, vernal pools existed in many parts of San Diego. Due to all this development, approximately only 3% of this now rare ecosystem still exists.
As gardeners, is there something we can do about this to hold on to these fine, but rare species?
They only occur after winter rains soak the ground and fill these pools. This pool of moisture will linger for many weeks. The results are some very fascinating, but short-lived beauties.
Here is the response that I got when asking this question from Justin Daniel, the past president of the California Native Plant Society, San Diego Chapter:
“By and large vernal pool species are annuals that have limited to no use in a regular garden setting. During the dormant season, they are hidden entirely even in the ideal settings.”
He continued, “However, paired with bulbs like Brodiaea and golden stars, they can be great seasonal pot features where a large shallow clay lined pot, for example, becomes a colorful bowl in the spring and early summer. To encourage year-round color, I'd pair with a meadow palette with blue eyed grass, … plant in some bulbs, but largely keep the area as low grass with annuals popping in. They are tough to maintain as these areas quickly get overrun with weeds that disrupt the soil surface. Ideally, vernal pools are fringed with cryptogamic crust, allowing moss and liverworts to thrive. Weeds should be dealt with in the early stages consistently, slicing out roots or using garden shears to destroy the root at the base. Pull small grasses at the first leaf, before the roots expand, but careful not to pull out native seedlings as well!
If choosing perennial plant species found in nurseries, I'd suggest sticking with small leaved slow growers like chamise, Adenostoma fasciculatum, black sage, Salvia mellifera, Cleveland sage, Salvia clevelandii, deerweed, Acmispon glaber, monkeyflower, Diplacus puniceus, and California buckwheat, Eriogonum fasciculatum. Bahiopsis laciniata and yerba santa, Eriodictyon crassifolium, are also good choices around the pool edges in a more (of a) sage scrub component. Cacti (Opuntia, Ferocactus, Cylindropuntia, and mammillaria) and succulents (Crassula or dudleya) can also be good outside of the pooling area.”
It may be difficult to have a real vernal pool garden in your landscaping, but some of the vernal pool species may work. Brodiaea is one such genus. There are some Brodiaea species that occur in San Diego County and are usually available at the California Native Society’s Native Plant Sale in October. One is Threadleaf Brodiaea, Brodiaea filifolia. It naturally occurs between San Diego, Orange and Riverside Counties in any remaining vernal pools and grassland habitats. Another one is Dwarf Brodiaea, Brodiaea jolonensis. It is found on the coast and coastal mountain ranges from Southern California to Northern Baja California. Both of these species have sweet purple, lavender, or blue flowers that appear in the spring and they reach a grand height of roughly 6 to 12 inches tall.
The best strategy to help preserve our native biodiversity is to avoid non-native invasive plants in your gardens, especially if you live within a mile of any natural habitats. Birds will take seeds, and transport them intentionally, or unintentionally to places where we may not want those seedlings. Other ways of preserving these sensitive habitats include keeping off-road vehicles out of protected areas, removing seeds from your hiking shoes before you enter a different area, and not taking any plants out of the wild without a permit. If you know of an area that is considered a vernal pool, please help it survive by showing respect and not trampling the area with excessive activity, especially motorized and non-motorized vehicles.
Planting native species in our gardens, saving natural areas from harm, and adding more natural areas to be protected, goes a long way in protecting our biodiversity and human health as part of 30x30, a project to protect the natural world from extinction.
References and resources on Vernal Pools:
Susan Lewitt is a member of the California Native Plant Society (CNPS), participating in their Native Gardening Committee, and their Conservation Committee.
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