On Friday, I led a workshop at Wilson Elementary School in Petaluma. The workshop was jointly hosted by the garden teacher at the site and the School Garden Network of Sonoma County . The content was Next Generation Science in the Garden, developed by Life Lab–leaders in garden-based education.

One of the activities in the workshop invited participants to observe milkweed plants. Using the prompts I notice…I wonder…It reminds me of…., learners asked questions about the yellow “pests” (oleander aphids) that were sucking the juices from the milkweed specimen I brought from my yard. In my effort to select plants that were not flowering (because the ladybugs and butterflies would be seeking the nectar), I inadvertently brought plants that had fingernail-sized monarch caterpillars on the leaves!

This time of year, Western monarch butterflies are preparing to overwinter along the coast of California. When they emerge from their chrysalides in late summer or early fall, they are biologically adapted to respond to the changing day length. The butterflies in this final generation of the season must store enough energy to make the journey to their roosting colonies, where they congregate until the temperatures signal it is safe to mate and migrate north again. The caterpillars in my yard were right on time.

According to monarchwatch.org, the peak time of year to observe this phenomenon in Napa (latitude 38 degrees) is mid-to-late September. Although I had noticed a single monarch butterfly visiting my milkweed last weekend, I did not think to go look for eggs or larvae. To create the most memorable experiences in the school garden, constant vigilance is key. Although teachers cannot plan the precise moment when a monarch will visit, knowing the approximate timing of this natural occurrence certainly helps. Indeed, when I looked at previous year’s garden photos, I found that monarch caterpillars can be found in my yard within a two-week window.

But first, the garden needs to have milkweed. It’s the only food source for monarch caterpillars. The monarch larvae in my yard feast on narrow-leaf milkweed (Asclepias fascicularis) propagated and gifted to me by a UC Master Gardener of Napa County. It differs from the increasingly popular tropical variety (Asclepias curassavica) found in most nurseries.  The variety of milkweed matters. I prefer the native species because it is adapted to the seasonal shifts in my area. The California native fascicularis stops blooming right around the same time as the last generation of monarchs should be making their way south. In contrast, the curassavica continues to bloom well into late fall/early winter, thus potentially “tricking” the butterflies into sticking around too long.

I’ve experienced monarch migration multiple times, but it never ceases to amaze. To create monarch memories in your school garden, be sure to plant the right milkweed. For more information, visit the Xerxes Society, Monarch Watch, or the USDA Monarch Education site.