Okay, now you can clean up the garden.
Wait for warmer weather
According to Savvy Gardening’s Spring garden clean up done RIGHT, we should wait until the daytime temperature warms to a consistent 10°C, so insects that have been overwintering in your garden have had a chance to wake up and leave their winter sheltering spot. The Xerces Society provides additional guidance for figuring out when its safe to start in their article Don’t Spring into Garden Cleanup Too Soon!
I was surprised to discover that only some butterflies migrate. The North American Butterfly Association’s Questions and Answers page describes the types of butterflies that do migrate and which ones migrate in large groups, like Monarchs. However, other butterflies and moths overwinter here in one of their life stages, such as in a chrysalis camouflaged on a branch, or as an adult under leaves, bark or rocks. Mourning Cloaks, Question Marks, and a few others emerge early in spring, and will drink sap if there are no flowers blooming yet. Then, if the weather cools again, they seek cover to wait some more.
The Wild Pollinator Partners blog post Another good reason to hold back “cleaning up” the garden, describes recent research showing that after queen bumblebees emerge from underground in spring, they hang out in leaf litter as they slowly wake up. If we clean up leaves too soon, we risk raking and bagging queen bumblebees.
Leave the leaves
Continue to leave the leaves on the ground; they will decompose to become a free mulch substitute. For a tidier look, remove leaves near garden edges and start a leaf mulch pile elsewhere in your yard. You could also focus on leaving leaves just in shady areas where you’re trying to mimic humus-rich, woodland conditions.
Leave stems 15-inches above ground
Typically, perennials are cut back to a few inches above the ground. Instead, cut plant stems to 15 inches, so they can still be used as nests. Add stems to the top of your leaf pile or start a brush pile. New growth will soon cover the awkward stems.
If you find this too untidy-looking, leave only pity or hollow stems standing, and cut the others back. Stem-nesting bees will use stems of Joe Pye weed, cup plant, sunflowers, goldenrod, coneflowers, swamp milkweed, bee balm, raspberries, elderberries, roses, and sumac.
I have seen birds eating seeds from uncut stems left from the previous year.