Q: I’d like to try “leaving the leaves” this year. How do I start?
It can be as simple as leaving any fallen tree leaves where they’ve landed, except for sidewalks and driveways. If too many leaves smother areas of lawn, rake or blow them off the grass and onto garden beds or fallow vegetable beds – any areas with exposed soil or which need to be mulched to prevent erosion or discourage weeds.
Mowing leaves on a lawn chops them into small pieces that will fall between the grass blades and disappear. However, keeping the leaves whole provides more ecosystem benefits.
Studies by Dr. Max Ferlauto, a former UMD researcher and current state entomologist with the DNR, found that, over the long term, the soil retains more carbon when leaves are left on the ground to decompose. Releases of stored carbon contribute to climate change. Intact leaves also help insulate overwintering insects and provide shelter from weather and predators.
Shredding leaves hastens decomposition and settling. The leaves take up less space in the landscape, but at the cost of destroying hidden arthropods and making the end material less useful for other organisms. It isn’t always desirable to accelerate the decomposition of material covering the soil surface, as exemplified by the soil degradation caused by high populations of invasive jumping worms, which break down the natural mulch of the forest floor too quickly for some native plants to tolerate.
Not every gardener will have the room (or neighbor tolerance) to leave several large trees’ worth of fallen leaves in the yard. That’s OK – start small and keep leaves where you can, and rake extras into a pile in an out-of-the-way spot, where they will still provide some ecosystem benefits. If necessary, composting extra leaves to return their nutrients to the soil is still better than having them hauled away. You can share a leaf surplus with other gardeners, though I wouldn’t transport them too far, lest they accidentally spread invasive species with the debris.
How thick a leaf layer is too much for perennials being buried? I don’t think anyone knows on a species-by-species basis, but rest assured that plants adapted to forest conditions probably won’t get smothered. Tree roots are definitely adapted to dealing with a covering of the tree’s own leaves. Limit how much you cover species that grow naturally on more exposed ground further from the canopies of deciduous trees, such as moss phlox (Phlox subulata) or little bluestem (Schizachyrium scoparium). Eastern columbine (Aquilegia canadensis) and alumroot (Heuchera americana) grow in some shade in the wild, but primarily in rocky outcropping crevices that don’t accumulate many leaves. Otherwise, I would not be too concerned.
Q: I used straw to cover grass seed a few weeks ago. Do I leave it in place or remove it?
It’s fine to leave it, because it will decompose over the winter. This adds organic matter and nutrients to the soil, to the benefit of turfgrass roots. Besides, trying to remove the straw by raking it out runs the risk of tearing up the young grass, which might not have enough roots yet to withstand that.
We occasionally hear from gardeners who have weedy grasses growing in lawn areas that were covered in straw. If you find undesirable grasses sprouting in the lawn next spring, they are probably cereal grains whose seed contaminated the straw. In that case, don’t worry, since those grasses are not very tolerant of mowing, and after a few trims as the turfgrass starts growing again, those cereal grasses will die out on their own. Harder-to-control perennial weeds, like goosegrass, quackgrass, or nutsedge, could also be straw contaminants, and in that situation, try to identify them early so they can be removed before they establish and reproduce.
