Tiburon mariposa lily opens in summer light

The summer air carries the scent of dry grass and salt from the bay. Here on the slopes above Tiburon, the longest days of the year have arrived, and with them, something rare unfolds in the coastal scrub. Step outside if you can, or find a window that opens to summer light. The heat builds slowly in the morning, and by noon, the hillsides shimmer.

In this narrow band of serpentine soil, the Tiburon mariposa lily opens its three-petaled flowers to the June sun. Each bloom spans three inches across, white petals marked with purple chevrons at the base, surrounding a cluster of yellow anthers. The lily grows from a bulb buried six inches deep in the rocky soil, sending up a single stem that can reach two feet tall. This threatened wildflower exists nowhere else on earth except these serpentine slopes in Marin County. The plant spends eleven months of the year as an underground bulb, emerging only when the soil warms and the rains have stopped.

The mariposa lily shares this harsh ground with other rarities. Long-rayed brodiaea, also threatened, clusters its purple flowers on nearby slopes. Marin dwarf-flax, another endemic, opens tiny white flowers among the rocks. These native plants have adapted to serpentine soil, which contains high levels of magnesium and heavy metals that most plants cannot tolerate. The invasive pale flax and common cat's-ear struggle here, giving the natives an advantage in their own territory. The rocky, nutrient-poor soil that challenges most plants becomes a refuge for species that evolved with it. Each lily plant may live for decades, blooming only when conditions align perfectly. The bulbs can remain dormant through drought years, waiting for the right combination of winter rain and summer heat.

Bees and butterflies find the mariposa lilies during their brief flowering window. The flowers produce nectar deep in their cups, accessible to long-tongued pollinators. Each successful pollination produces a three-chambered seed capsule that splits open in late summer, releasing seeds that may take seven years to produce their first flower. Most seeds never germinate. Those that do must navigate the narrow window between too much moisture and too little, between the spring rains and the summer drought. The population here numbers in the hundreds, each plant a survivor of this precise timing.

Close your eyes and feel the summer heat building on your skin. Somewhere on these slopes, white petals catch the light, opening wider as the temperature climbs. The air shimmers above the serpentine, and the scent of warming earth rises from the ground.