Leaves are a benefit to compost bins

Every fall sees gardeners with garbage bags roaming neighbourhoods and collecting leaves under boulevard trees or picking them up ready-bagged from non-gardening neighbours.

If they were scarce, leaves would cost big bucks because they're rich in carbons, minerals and fibre - brought up to the surface by the long, deep roots of trees. Every autumn, when the leaves fall, they nourish the top layers of soil.

Leaves can be used in general compost heaps, composted separately for leaf mould or spread on garden beds as a mulch. Actually, where the leaf layer isn't hugely thick, fallen leaves can be left on perennial beds where they protect plants against winter cold, compaction of the soil and germination of cool-season weeds.

But leaves do need to be raked up from lawns since they tend to smother grass. Grass benefits from light and air during winter since it doesn't hide underground as many perennials do.

No matter where you put them, leaves break down much faster if you rake them, then shred them by running over the pile with a lawnmower. If you plan to do this repeatedly for a fine shred, you may have to rake them between runs or use the lawnmower bag and keep rebuilding the pile.

Time-short gardeners who want to add leaves to a compost bin can mow the unraked lawn, collect grass clippings and leaves together and add them as-is to the compost. It's not perfect, but the nitrogen in the grass and carbon in the leaves roughly balances, and the job gets done fast.

Some people keep bags of raked leaves through the winter, then add one bag at a time to compost in the spring when moist, green waste is abundant but brown carbon is hard to find.

Fall or spring, leaves can be safely shredded by being dumped in a garbage pail and attacked with a weed-whacker. It only takes a few minutes for a tubful of leaves to be reduced to a few inches of shredded leaf bits. This reduction in volume is the reason why people who want leaf mould need to gather many, many bags of leaves. Neighbours are often very cooperative - especially in jurisdictions where city governments levy a fee for collecting compostable materials.

For leaf-mould, gardeners really need to shred the leaves well. It's the only way to see the approximate quantity you'll end up with - and usually the only way to ensure the leaves will definitely be decomposed in a year. It's worth the effort because aside from their high nutriment level, leaf mould retains moisture beautifully.

Some leaves need shredding because they're slow to break down. This includes waxy leaves like holly and laurel and large leaves like cottonwood, paulownia and big leaf maple. These need to be shredded even for compost unless they're destined for a compost pile that won't be used for two or three years.

Gardeners claim toxins in black walnut leaves vanish after six weeks of composting. So far I've not tested this out. But leaves of poisonous perennials like lupins, foxgloves and daffodils are safe once they're broken down in the compost.

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