Wait until spring daytime temperatures are above 50 degrees for five consecutive days before cutting back dead flower stalks and removing leaf litter to give overwintering bees and caterpillars time to emerge. Cut plant stocks leaving 12-18 inches tall. Many bees will nest in this material.
Any gardener will tell you it’s not a good idea to plant your tomatoes outdoors until evening temperatures are reliably in the 50s. The tender tomato will shut down and suspend growth and fruiting if subjected to temperatures below 50 degrees or above 90 degrees. If it’s time to plant tomatoes in your area, chances are conditions are neither too hot nor too cold for pollinators to be out and about.
Cool-season lawns begin growing when soil temperatures reach 50 degrees. In all but the warmest climates, if grass is growing enough to justify regular mowing, it’s probably a safe bet that most pollinators have emerged.
To help pollinators the most, wait a little longer to mow.
In northern states mid-late April should be the earliest you consider cutting back perennials and clearing garden debris. Keep in mind that some bees don’t emerge until late May, so the longer you can tolerate your “messy” garden the better.
Apricot, peach, plum, and cherry trees are the earliest to bloom, coinciding with the emergence of many ground-nesting bees. Apple and pear trees bloom between mid-April and mid-May. If you can manage to wait until apple trees are no longer in bloom, then you should be safely in the clear from disturbing those pollinators that emerge a little later than the rest.
Leave stems 15-inches above ground
Typically, when we clean up our gardens, perennials are cut back to ground level. Instead, cut pithy or hollow plant stems to 15 inches, so they can be used as cavity nests by solitary bees, like Mason Bees and Small Carpenter Bees. Some stems that are used for nests include Joe Pye Weed, Cup Plant, sunflowers, goldenrods, coneflowers, Swamp Milkweed, bee balms, raspberries, elderberries, roses, and sumac. In my garden, I have seen Small Carpenter bees use stems of Anise Hyssop, Ironweed, and Purple Coneflowers.
New plant growth will soon cover the awkward stems that remain.