Who might be overwintering in my garden?

Photo credit: Living Prairie Museum

By Cameron Ruml – Living Prairie Museum

Cameron Ruml is the interim Curator at the Living Prairie Museum, with over 10 years of experience in ecosystem management, tallgrass prairie restoration, and environmental education at the City of Winnipeg’s Naturalist Services Branch. Cameron holds a Bachelor of Environmental Studies, is licensed as an arborist, and is dedicated to preserving and promoting the natural world. When he’s not at work, you can find him exploring the great outdoors whether it’s on the trail or on the water – he’s always up for an outdoor adventure!

On a late-October day at the Living Prairie Museum, one of our museum staff members was working in our museum gardens right before the first snow of the season. He sunk in a spade to loosen and re-locate a perennial narrow-leaf sunflower, and to his surprise, out popped a little wood frog (Lithobates sylvaticus)! It was preparing to overwinter just below the soil surface, under the leafy plant material on the ground.

Wood frogs have the miraculous ability to overwinter while frozen solid. They are alive, but in a state of suspended animation for up to eight months of the year. No heartbeat, no breathing, no muscle movement. Frogs are cold-blooded, so their body temperature is about the same as the surrounding environment. However, unlike wood frogs, most frog species in Manitoba overwinter deep underwater and never freeze.

Northern leopard frogs, for example, cannot tolerate freezing, and spend the winter lying on the bottom of ice-covered water bodies. They need to stay in direct contact with liquid water to continue respiration, by absorbing oxygen and diffusing carbon dioxide through their skin. Most frog species in Manitoba need larger, deep bodies of water that never fully freeze to survive the winter. For this reason, leopard frogs are unlikely to be found overwintering in urban gardens, unless you’re lucky enough to have a very deep pond!

Wood frogs have a different strategy. When ice starts to form on their skin, it triggers the frog’s liver to produce glucose which the heart flushes into every cell in its body. This prevents ice crystals from forming inside of their cells where it would crystallize, expand, and cause damage. The ice instead forms in the body cavities outside of their cells.

When warmer temperatures arrive in the spring, their heart starts beating again, the brain activates, and their muscles begin to move. This ability allows them to get an early start to breed and lay eggs in spring melt-water pools that dry up by early summer, while most other frogs need permanent water bodies to breed.

As for the frog we accidentally dug up: we gently returned it to its original resting place in the garden. The temperature that day was about 3 Celsius, and our surprised gardener observed it attempting to “shimmy” back down into the soil. He covered it with about the same amount of leaf litter as he found it beneath, since the frog should know best where it needs to be.

If you have wood frogs or any species of frog in your garden, consider yourself lucky! Frogs eat invertebrates: insects, arachnids, worms, slugs and snails. They are part of a healthy ecosystem and help to keep garden pests in check.

So, this got us thinking, what other sorts of creatures might you find overwintering in your urban garden?

A common message we share at the Living Prairie Museum is to “leave the leaves” while undertaking your fall yard chores. Many insects spend the winter in the larval stage in the leaf litter and standing dead perennial top-growth, so it’s best not to mow it down and to leave it undisturbed throughout the winter.

To illustrate how this works in more depth, check out this great resource by the Xerces Society for Invertebrate Conservation, on creating nesting and overwintering habitat for pollinators and other beneficial insects: https://xerces.org/sites/default/files/publications/18-014.pdf.

Manitoba’s butterflies overwinter in all different life stages: as eggs, larvae, chrysalis or adults, depending on the species. Black swallowtails are one of the most likely species to find in your garden (fig.2). Their caterpillars munch on carrot family (Apiaceae) plants like parsley, dill, fennel, carrot tops, or on native golden and heart-leaf alexanders. They overwinter as a chrysalis, attached to the base of plant stems, twigs or hard surfaces near the ground.

Photo credit:  Black Swallowtail Chrysalis, Sdetwiler, wikimedia commons, CC-BY-SA-4.0  https://creativecommons.org/licenses/by-sa/4.0/deed.en
Black Swallowtail – Papilio polyxenes. Fig. 2:  Black swallowtail adults emerge the following spring after overwintering as a chrysalis. Photo Credit: Living Prairie Museum

Many insects on the other hand, overwinter as adults. Lady-beetles are an example that you are sure to have seen searching into the nooks and crevices around your house and yard to find an overwintering spot, often finding their way all the way into your home!

On the subject of insects who find their way indoors: another likely place to find overwintering insects is in your firewood pile! Many spiders, small beetles, sowbugs, ants, small flies and wasps overwinter in wood piles, only to wake up when the logs are brought inside. (fig.3)

Fig.3 Wasp overwintering in a wood pile. Photo credit: Rob Currie

In short, it’s best to leave the standing dead stems and vegetation undisturbed in the fall, leave the leaves on the ground, and you have a good chance to avoid disturbing any creatures hunkering down for winter in your garden.

Published: January 2023