Autumn provides a treasure for your soil

Gathering up fallen leaves and leaving them to decompose may mean a wait of up to 12 months, but the end result will have huge benefits for your garden, writes Fiann Ó Nualláin.

AM all for a bit of thrifty gardening but I’m more for a bit of ecological application — leaf mould is both. It’s a free resource, literally lying all about the place this weekend and no one will mind you bagging it up for yourself and taking it away before it becomes a slippy nuisance in few weeks’ time. It’s a natural way to improve the structure of your soil or to make some potting mixes next year.

There are two ways to make it — the bag method and the bin method — before we look at both let’s look at what’s so good about it and why gardeners call it brown gold. In essence, leaf mold is the end product of simply letting gathered up autumn leaves sit and decompose to a dark brown to black crumbly texture — similar in texture and earthy aroma to a good well-cooked home compost. And like compost it is humus rich.

Humus is the organic component that increases soils water-holding capacity and increases earthworm and bacteria populations and other beneficial aspects of the soil web. Standard subsoil can hold around 20% of its weight in run off and soak down rain water; standard topsoil can maintain around 40% while good quality topsoil or previously cultivated soil comes in at around 60% — sounds great until we look at leafmould which can retain over 300% of its weight in water.

Leaf mold is also consider far superior to compost as a soil conditioner — ok it is low in nutrients as they are absorbed back into the tree before its autumnal shedding . There will be some trace elements and also both potash and phosphorus, which is great for roots. But what is also left behind is fiber — especially a type called lignin.

Lignin also helps improve the soil’s holding capacity of inherent nutrients — it acts as a buffer to prevent leeching out and improves soil structure and this enhances earthworm movement, ease at which beetles burrow leaf litter down — all creating a better nutrient supply and a better root reach.

Lignin is slow to break down and while that may be a complication in composting it is a bonus in it benefits the soil web that feed off it or derive benefits from the amendments it makes to the soil. It is still good to mow or shred your leaves before leafmoulding in order to break down some of the lignin and speed the process to crumbly material.

Leafmoulding is considered as slow process composting — because apart from the lignin, autumnally shed leaves are pretty much all carbon, which takes longer to decompose than nitrogen-rich materials such as hedge trimmings or grass clippings. But the decomposition process averages at six to 12 months — which is no eternity in the gardener’s universe. Some gardener like to mix a sprinkle or two of grass cuttings with the leaves to increase both the decomposition speed and enhance the nitrate content of the leafmould.

The reason why leaf mould is made as a separate type of compost is because of how leaves breakdown- which is different to how the standard material thrown into a compost heap decomposes. Leaves break down by the activity of fungi, which is non-heat dependent while kitchen waste and garden clippings of home compost require heat generating bacteria. Mixing leaves with home compost in a small amounts is a good carbon sources but keeping separate is a better option.

So how to...

The bag method: 

Simply rake up and bag the leaves from the side of the road/path — making sure to not include any people litter (cans, cigarette butts, food wrappers) — Or mow the leaf shrew lawn/patio and tip the mowers’ collection bag into a bin liner. Add enough water to moisten the leaves (decomposition needs a moisture layer to really fire up the fungal activation) – I have in the past recommended a sprinkle of urine to kickstart both types of compost heap and those subsequent letters still keep me up nights.

Once moistened, seal the bag with a knot or tie at the top. Place in a corner of the garden, puncture a few holes bag for airflow. Some ‘experts’ will say just let sit until spring – and you may be lucky but to make your own luck — give it a shake or an invert every month – to get air in and keep breakdown potential going, we don’t want a soggy bag but we don’t want a dry zone either so you might want to spray into some of the holes on the day you turn/shake.

The bin method: (a plastic free option).

A bin is a neat way to organize a decomposing pile.

Ok there is a tiny bit of construction involved but it can be thought of as an extension or annex to the compost heap or to your wildlife hotel. The easiest version is as simple as four garden stakes and some chicken wire to make your leaf bin. I like a bit of tarp or ply over the top to manage moisture levels. Again what you want is moist - not sodden later on with winter rain and sleet.

Hammer in the stakes, staple, nail or tie in the chicken wire walls and pile in your gathered leaves – a quick squirt of the hose and it is on to the waiting game. Now you can speed the process with a turn or two in autumn and again in winter (unless it’s just become a home to some overwintering hedgehogs). Adding a few handfuls of grass clipping will assist breakdown now end.

It depends on how cold the winter was, how much moisture was present in the pile, how finely shredded or not the foliage and so on as to how quick you get to an end product – for most that’s between 6 months to a full year. You can let it go to 18 months for a super fine structure.

Beech, birch, oak, horse chestnut, sycamore and hornbeam are high in lignin and so slower - while low lignin foliage falls from ash, cherry, elm, Japanese maples, linden, poplar and willow. Don’t be tempted to add some evergreen leaves from a hedge trim as most non-deciduous hedging can take over three years to breakdown. Don’t be tempted to add in the desiccated Christmas tree in January as that will only acidify the pile – unless of course you want an ericaceous compost for blueberries, azaleas, camellias, Pieris and heathers.

How to Use Leaf Mold:

The tradition is to use Leaf mold as a top dress mulch or as a dug in amendment but it also suitable to make up into mixes for containers and treated ion the same manner as garden compost.

Fiann’s tips

Autumn provides a treasure for your soil

Early November is also the best time to plant bare-rooted apples, pears, cherries and plums.

Asparagus crowns can go in now and any perennial veg can be divided.

Traditionally we can thin spurs on orchard trees and garden espaliers/cordons etc.

Do not prune cherries, plums, apricots, peaches, nectarines or almonds — best wait until spring to avoid any potential winter establishing diseases.

Prevent winter moth damage to your fruit trees by utilising grease bands around the trunks.

Check your stored fruit and veg and discard any showing the slightest sign of rotting so as not to lose your entire store.

Develop a practice of checking in on your stores regularly.

Spread manure, compost or last year’s leaf mould over vacant cultivated beds.

Cover can be applied this month to prevent weeds taking over empty bed and stop soggy soil happening and slowing the heat up come next spring.


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