To mulch or not to mulch--impact on native bees?

Is mulching in fall a good idea? I don't have an always-yes or always-no answer for this. Mulching has its pros and cons. Most of the people reading this post are probably already familiar with the benefits of mulch. That it keeps in moisture, reduces weeds, and some people like the way a freshly mulched bed looks. But in some situations the negatives can be pretty strong, and should be taken into consideration...read on.

The strongest arguments on the positive side are 1) a 2-inch layer of shredded hardwood mulch in the fall can reduce the weeding you need to do in early spring, in areas where weeds would be noticeable or unwelcome. And 2) shredded hardwood mulch can protect your soil from moisture loss and erosion during the winter months. I'm talking strictly about landscape situations here, not vegetable or crop production.

I'm also assuming we're using what is generally considered optimal organic mulch (according to a 1990 study that compared organic mulches), namely locally produced wood chips (or if you don't have that, commercially available natural hardwood shredded mulch, which is the closest next best thing).

I often like to mulch a new bed that I've just installed, just once, because it helps get the new plants going. And if you are using the newspaper-and-mulch method for killing lawn ahead of planting, you've already got the mulch there.

However, I've been doing research on pollinators for a new video course, and finding that the case against mulching is getting stronger. In Maryland there are over 430 species of native bees, many of which are ground-nesting. The majority of these bees are solitary, non-aggressive bees, and they have been diligently pollinating tree, shrub, and perennial blossoms throughout the seasons in our landscapes.

Some bumble bees and solitary bees spend the winter in underground burrows. If you have dry, sandy soil (as I do), a fresh layer of mulch can prevent overwintering and also prevent use of the area for early nesting in spring. That's not such a good thing.

This would be particularly ironic if you are mulching a garden that has been designed for pollinators, right? So before you mulch, look around your landscape. Is there any other bare ground nearby that would be suitable habitat for solitary ground-nesting bees? If not, you may be removing a valuable winter hibernation spot and, later, a spring nesting spot near the pollinator plants, increasing the distance the bees have to travel to get from nest to food.

If the mulching task is hired out to a landscape maintenance company and/or is accompanied by "cleaning up the beds," the negative impact is magnified. If you are gardening to support pollinators, you don't want to cut all the dead stalks back in the fall, since some pollinators use those stalks to shelter or lay their eggs for the next generation to hatch in spring.

If you must cut things back (hey, I understand, maybe it's the bed by the front door or something, I get it), then neaten them up by cutting only partially, leaving the stems at 15" high (Heather Holm, 2017). Also, unless the plants were diseased, leave behind what you've cut off, as a light mulch on the ground beneath the plants.

If you decide not to mulch, but you are still concerned about weeds and soil health, I suggest a different strategy: using an evergreen or semi-evergreen groundcover to help reduce weeding and protect the soil over the winter months. More on this topic in the next newsletter!

Happy gardening!
Chris