People told to ‘leave the leaves’ on your gardens and lawns. Should you really?

You might have heard that we’re being urged to ‘leave the leaves’, but what is the science behind it?

Unless you’ve been living under a pile of leaves, you’ve no doubt heard about the “Leave the Leaves” movement that’s been gaining in popularity in recent years.

The idea is to avoid sending bagged-up fallen leaves to landfill. Instead, we’re asked to leave them be, allowing them to naturally decompose over the winter into nutrient-rich organic matter that also shelters hibernating pollinators and other beneficial insects.

Done thoughtfully, leaving the leaves is one of the best ways to turn garden waste into free fertiliser, and that’s good for your plants, the environment — and your wallet. But it’s important to consider the types of leaves you’re dealing with and where they’re landing.

Whole leaves should not be allowed to remain on pathways, where they’ll create a slipping hazard, or on the lawn, where they are likely to cause disease.

Although turf grasses can handle a light scattering of leaves, a thick layer would threaten their health. In areas that experience snow cover, moisture would become trapped between lawn and leaves, encouraging mould, mildew and fungal infections. In areas without snow, whole leaves would likely smother the lawn, and block moisture and sunlight from reaching the soil.

The solution many have arrived at is to shred the leaves using a mulching mower and allow the fragments to fall between grass blades, where they break down into a rich soil conditioner. I’ve recommended this myself before realizing that doing so risks shredding up hibernating insects and their larvae. We’ll need those caterpillars (eventual moths and butterflies) and other pollinators come spring, and so will newly hatched birds, which survive solely on insects during their first weeks of life.

So, what to do?

These days, I rake (or blow) leaves off the lawn and into garden beds and spread them to achieve a layer no more than 2 inches deep. To speed decomposition, sometimes I add an inch or so of homemade or well-sourced compost over the leaves. There’s no need to till; just let it sit.

The leaves usually break down considerably by spring and almost entirely by summer. But if they appear matted (again, conditions vary), remove them before spring growth resumes.

Leaves also can be used to make leaf mould, a type of compost made entirely from leaves. Just pile them up in a corner of the garden, sprinkle with nitrogen fertiliser, and water the mound periodically to keep it from drying out. It may take a year or two, but the leaves will break down into a nutritious soil amendment that can be used as mulch or added to planting holes and containers.

Some leaves contain compounds that inhibit the growth of other plants. Black walnut is perhaps the most notorious of offenders, as a toxic chemical in its leaves called juglone adversely affects and sometimes even kills susceptible plants like Asiatic lilies, baptisia, columbines, peonies, hydrangeas, lilacs, petunias, apples, asparagus, cabbage, aubergine, peppers, tomatoes and potatoes.

Avoid mulching beds with especially thick or broad leaves, like those of oaks, because their slow decomposition rates could threaten to block sunlight and water from the soil and, by extension, plant roots. They can, however, be used in leaf mould piles if shredded, which isn’t ideal but is better than setting them out with the rubbish, which wouldn’t help insects, either.

Fallen leaves are nature’s mulch, meant to protect (and build) soil, insulate plant roots and shelter wildlife, as they do on the forest floor. Why waste that precious resource?

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