To Rake…or Not to Rake

Our gardens are little ecosystems, little biological communities of interacting organisms in a particular physical environment. We shape and care for them. And as we do, our choices impact local populations of plants and animals. They impact lifecycles and biochemical cycles. Let’s think about leaf litter and fall clean up with respect to those cycles.

We all know that plants in general and trees in particular are really important for life on this planet. They provide the air we breath, regulate the movement of water and cycle elements like carbon and nitrogen. There is a complex interplay between these biochemical cycles and the lifecycles of many organisms. Falling leaves are part of that great complexity.

Plants remove carbon (in the form of carbon dioxide CO2) from the atmosphere during the process of photosynthesis and release oxygen. Woody plants sequester that carbon, using it to build trunks, roots and leaves, and store it as compounds like lignin and cellulose. At the same time, they are cycling Nitrogen. Nitrogen (in the form N2) is deposited from the air on soil surfaces where microorganisms help transform it into compounds like nitrate (NO3−) and ammonium (NH4+). These are chemicals that plants can absorb. Once absorbed through the roots, nitrogen like carbon is used to build compounds essential for plant growth. Many other elements are extracted by plants from the soil and air. Over time, these elements are cycled back to the earth.

One part of this cycling can occur in fall. In northern climates as days shorten and the air cools, trees drop their leaves and herbaceous plants collapse. They do this to conserve resources and survive winter conditions. Fallen leaves and plant material form a protective layer above the plant roots. These plants are essentially creating their own mulch to conserve moisture and moderate temperatures. When we remove leaf litter, we are reducing this natural winter protection. We are also preventing essential elements from returning to the soil. While plants transfer some of the more mobile nutrients to maturing seeds or relocate them to their roots in autumn, the rest remain in the fallen plant matter to be recycled.

Dozens of elements are stored in leaves. One study measuring plant nutrients in leaf litter collected from a mature northern hardwood forest found 14 g/m² were returned to the soil each fall: Nitrogen (N ), Calcium (Ca) > Potassium K > Manganese (Mn) > Magnesium (Mg) > Sulphur (S) > Phosphorus (P) > Zinc (Zn) > Iron (Fe) > Sodium (Na) > Copper (Cu), (Gosz, 1972). Now that may not seem like a lot, but along with other elements from the air and soil, it is all the trees need to thrive and grow. Over time, if leaves are continually raked up and removed, macronutrients and micronutrients are lost and the soil becomes depleted of organic matter and of life. For plant litter is a major source of carbon, nutrients and energy for many organisms.

Dropped leaves become part of soil organic matter (SOM) in the process of pedogenesis – soil building. Detritivores feed on the dead leaves, which are a form of detritus or dead organic matter. Beetles, millipedes, slugs, woodlice and worms are among the hundreds of creatures that feed and fragment plant litter. Then decomposers like bacteria, protists, and fungi continue to break down the pieces producing humus, that nutrient rich dark coloured portion of healthy soils. Microbes feeding in these soils release inorganic nutrients (a process called mineralization) making elements available for tree and plants. All the while, these populations of decomposers and detritivores are food for predators. (Explore THE SOIL FOOD WEB https://web.extension.illinois.edu/soil/SoilBiology/soil_food_web.htm). By allowing leaves to remain in the garden, you are cycling elements, producing soil and supporting many organisms necessary for plants and people to thrive. Healthy soil microbial populations are indicators of healthy soils.

Farmers have learned that removing plant residue, such as stems, leaves, and roots, after crops are harvested damages soils and has negative impacts on water and air quality. Current best practice is to leave crop residues where possible. This results in improved soil organic matter and soil biological activity. It also improves the soils ability to receive, retain and release water. What farmers have learned can be transferred to the home gardener. Leaving plants and leaves to decompose in the garden will enrich soils, increase microbial biomass while helping to conserve water.

In addition to improving soil health, fallen leaves and other detritus provides shelter or habitat for many species. Some like squirrels and birds gather the leaves for nests. Others like mining bees (Andrena spp.) may burrow under the leaf litter taking advantage of the protective layer, while leafcutter bees and mason bees tuck into the dead, dry, hollow stems of other plants. Plant “litter” is essential for many species to complete their lifecycles.

Now, it is true that pests and pathogens can also live in dead plant material. So at times, it is important to remove litter to break a disease cycle or reduce a particular pest. But beneficial predators and healthy soil microbia can be diminished in the process, so care should be taken in making that choice. For instance, tar spot (Rhytisma spp.) may be unsightly on leaves, but causes little harm. Removing leaves is unlikely to reduce infection rates in areas where the disease is prevalent, and thus removing them may do more harm than good.

It is also true that a heavy layer of leaves can block light and reduce air circulation for underlying plants. Quickly degrading leaves, like those of ash (Fraxinus spp.) can typically be left in place. But certain species like oak (Quercus spp.) produce leaves rich in lignin and tannins that can be slow to degrade. Thinning or shredding such leaves with mulching mower can benefit lawns and ground covers. Alternatively, gardeners can redesign their garden ecosystems to work better with natural cycles and allow for natural leaf decomposition. It is interesting to note that leaves often decompose faster when left under the parent plant. This is called the “home-field advantage,” and is due to the build up of specialist decomposers in the soil around their preferred species. (There are some complicated issues surrounding the leaf litter of introduced species that can alter soil chemistry and microbia, but that discussion is best left for another day.)

There is so much happening in our gardens. We may find it difficult to balance our desire to grow certain plants, create beautiful designs and support healthy ecosystems. But understanding how our gardens cycle water, carbon and nitrogen while supporting the lifecycles of many species, can help us choose sustainable gardening practices. So when you can, let leaf litter remain in your gardens to build healthy soils and continue the cycle of life.