Willows flowering, bees arriving to drink

The air carries a faint sweetness along the South Platte, where narrowleaf willows line the water's edge in dense stands. Their slender branches hold clusters of pale yellow catkins, each one heavy with pollen that dusts your fingers if you brush against them. The morning light catches these flower spikes like tiny lanterns, and the sound of running water mixes with a deeper hum.

Narrowleaf willow (Salix exigua) releases its pollen now, in late spring, when the days have warmed but the nights still hold coolness. These willows are dioecious, meaning individual trees produce either male or female flowers, never both. The male trees bear the showy catkins that release clouds of yellow pollen into the air. Female trees hold smaller, greener catkins that will develop into the cotton-like seeds that drift on summer breezes. This separation means the willows depend entirely on others to move pollen between trees. They have evolved not for wind pollination, as many assume, but for insects.

The hum grows louder as you approach the flowering branches. Golden Northern Bumble Bees (Bombus fervidus) work the catkins methodically, their bodies coated in yellow dust. These threatened native bees emerge from winter dormancy precisely when the willows bloom, a synchrony refined over thousands of springs. The queens, heavy with eggs, need protein-rich pollen to develop their ovaries and establish new colonies. Worker bees gather both pollen and nectar, packing the pollen into specialized baskets on their hind legs. Nevada Bumble Bees (Bombus nevadensis) join them, smaller but equally determined, their black bodies striped with yellow bands that distinguish them from their golden cousins. Both species have declined dramatically across Colorado, making their presence here along the South Platte increasingly precious.

The willows offer abundant rewards. A single male catkin can produce millions of pollen grains, and each tree bears hundreds of catkins. The nectar flows freely in the cool morning hours, before the heat of midday slows production. The bees visit flower after flower, inadvertently carrying pollen between male and female trees. This relationship benefits both partners: the bees gain the season's first major nectar and pollen source, while the willows achieve the cross-pollination necessary for genetic diversity in their offspring. The timing is critical. Too early, and late frosts can damage the flowers. Too late, and the bees have moved on to other blooming plants. But when the synchrony holds, as it does this morning, the partnership flourishes. The invasive Western Honey Bees (Apis mellifera) also work these flowers, but they lack the specialized relationship that native bees have developed with native willows over evolutionary time.

Close your eyes and listen to the layers of sound: water moving over stones, leaves rustling in the breeze that carries the scent of willow pollen, and underneath it all, the steady drone of bees at work. When you open them, watch how the morning light illuminates each grain of pollen as it transfers from flower to bee to flower, a exchange as old as this riparian corridor itself.