I had plans to talk more about demonstration gardens and why they are so important, especially for our youth. However, that discussion will have to wait until next month because this week, on our way to Costco, we opted for the scenic route (which promised wildflower viewing). If you haven't driven down Dillon and Thousand Palms Canyon Road, you're truly missing out. Now, back to our story. As we were driving down Thousand Palms, we spotted a group of caterpillars crossing the road. Mark, being the indulgent husband he is, pulled over so we could take a closer look. They turned out to be White-Lined Sphinx Moths—or rather, their caterpillars. They were on the move. So, why am I so excited?
If you've ever purchased evening primrose from the nursery, you might already know the answer. Evening primrose serves as a host plant for these particular moths. The relationship between the plant and caterpillar is symbiotic. The caterpillar feeds on the plant, while the plant benefits from the pruning effect of the caterpillar, stimulating new growth. While we don't necessarily need caterpillars every year, an absence of them can lead to overgrown plants. This not only shortens the plant's lifespan but also detracts from its aesthetic appeal. While we plant for various reasons such as environmental conservation, supporting wildlife, and water conservation, we also just want pretty plants.
Now, let's discuss what might happen to your plants this year. If caterpillars find their way to your yard—congratulations! They will likely devour your evening primrose, possibly down to the ground. But fear not! This is nature's pruning. Your plant will rebound, likely with multiple branches sprouting from where there was once just one. It's similar to how pruning a shrub encourages new growth. Eventually, these caterpillars will transform into beautiful moths, serving as pollinators.
But what if several years pass without caterpillars, and your plant becomes overgrown? You'll know when it happens—look for brown leaves at the base, leaving a long stem exposed (see photo above). At this point, you must take on the role of the caterpillar. Trim the plant back, leaving about an inch of stem. Don't be afraid! You'll likely find seedlings under the original plant—nature's way of hedging bets for survival.
You might wonder if other plants operate similarly. Yes, there are many! Take our local milkweed, Asclepias subulata, for example. While it doesn't require pruning for aesthetic reasons, it relies on predation from monarch and queen caterpillars to stimulate new growth. In late April or May, check for new, soft growth. If absent, selectively trim some stems, perhaps every third to fifth one. The plant doesn't need to be cut to the ground or shaped with shears (eek!). However, if caterpillars do consume it entirely, rest assured—the plant will recover and become bushier. I would suggest planting additional milkweed so the caterpillars have more plants to feed on.
When contemplating pruning, consider why you're doing it. Avoid pruning simply because you feel you should or because your neighbor does. Instead, think about how the plant would naturally be pruned in the wild—perhaps by frost, fire, or animal predation. This will guide you on how and when to prune to maintain the plant's health and beauty.
