URBAN FARMER:Fertilizer is falling all around you at this time of year but be selective which leaves you use for leaf mould, writes FIONNUALA FALLON

NOW THAT autumn has finally arrived, it seems that every city street and country garden is suddenly piled knee-high with fallen leaves that lie on the ground in deep drifts and wind-formed heaps, while still more fall from the sky like nature’s confetti.

Beech, oak, sycamore, chestnut, down they all tumble and in what exquisite colours they come. Few sights can match the flame-coloured beauty of a mature beech tree in autumn, its leaves almost on fire, or the close-up perfection of a copper-yellow acer leaf. And it’s also only now, as they fall to the ground, that their many different shapes and sizes can also be appreciated, from those of the large, heart-shaped lime tree to the tiny, triangular birch and the almost star-shaped sycamore.

But exquisite as they are, there are other, more practical matters for the canny urban farmer to think about when it comes to autumn leaves, chief amongst them being the making of leaf mould.

For, as most grow-your-own (GYO) enthusiasts know only too well, the secret to a really productive fruit and vegetable patch always lies in the ground.

Only a fertile and friable soil will produce healthy and abundant crops, and so the trick is to help it along in any way we can.

In the OPW’s walled kitchen garden in the Phoenix Park, gardeners Brian Quinn and Meeda Downey are lucky enough to have access to a ready supply of well-rotted manure, which they add to the soil in trailer-loads every year. But other urban farmers, particularly those on a limited budget, can find it difficult to source a steady, adequate supply of manure and must improvise in other ways.

Collecting a few sackfuls of autumn leaves and storing them in an out-of-the-way corner of the vegetable plot to rot gently down is one easy, enjoyable and environmentally-friendly way to make your own soil conditioner for free (there can be few purer forms of recycling). And what’s even better is that with such an abundance of leaves waiting to be collected, you can make as much of it as you wish.

But before you start to collect your autumn leaves, there are a few things to keep in mind. First amongst these is the fact that not all leaves are equal when it comes to making leaf mould – some rot down quickly, some slowly, while others simply aren’t suitable at all.

Beech, oak, alder and hornbeam are amongst the best, rotting down easily into a rich, dark crumbly mixture that makes a wonderful addition to any soil.

Other broadleaf trees, such as the horse-chestnut, sycamore and walnut, need to be shredded or chopped with a garden shears before being added to the leaf mould heap, as they’re tougher and thus slower to break down. And then there are the leathery leaves of evergreen trees and shrubs, like holly, aucuba, griselinia and laurel, that aren’t really worth collecting at all.

Identifying the different trees and shrubs will be easy for some urban farmers, while others may need some help. One useful site is www.rogerstreesandshrubs.com, an online database featuring photographs of the leaves of 9,000 trees and shrubs. What makes this website stand out is the fact that all the photographs were taken by the respected botanist and Trinity College graduate, Martin Rix, and his colleague, the writer and photographer, Roger Phillips.

So, if you don’t know your Aesculus from your Betula, your Fagus from your Carpinus or even your Catalpa from your Liriodendron, then this is the website for you – just click on the “leaf index” button on the toolbar to see photographs of all the different leaves. There’s even an advanced key that helps you to identify any particular tree or shrub based on different searchwords.

But even the most expert and knowledgeable urban farmers will find this website both interesting and useful, for along with the photographs, it also gives advice on planting and cultivation as well as the option (boffins will like this) to “talk trees and shrubs” in the website’s global chat room.

Once you have a clear idea of which and what leaves to collect, there are a few other things to keep in mind. Don’t, for example, collect fallen leaves from heavily-used roads, where they may be contaminated with pollutants (or worse), or from long-established woodlands where you’ll disturb the delicately balanced ecosystem. Instead, collect your leaves from back gardens, parks, little-used side-roads, laneways, or even graveyards – but always get permission first.

Use the time-honoured method of raking them up if you wish, although there are other faster and less time-intensive ways. One that some mechanically-minded urban farmers will favour is the lawnmower – put the blades on their highest setting and use the grassbox to collect the leaves. The advantages of this are: the leaves are already nicely chopped-up (they’ll break down more quickly); and they’ll also be mixed with some grass cuttings, which raises the nitrogen levels and will speed up decomposition.

The disadavantage, for those of you with weedy lawns, is that you run the risk of introducing weed seed into your leaf mould, always an unwelcome addition to the vegetable patch.

Leafblowers are another possibility, although only for the lucky few, and even with these you’ll still need to pick up the piles of leaves that you make. As students at the National Botanic Gardens, we kept it simple when it came to this task – a pair of lightweight, slim wooden boards, roughly 45cm by 30cm, that we used to sandwich and scoop up the leaves, were always very effective.

Once collected, you can store your leaves in a variety of ways, from black refuse sacks to custom-made leaf bins – the former are particularly suitable for the space-challenged urban farmer. Puncture the black refuse sacks with a few holes around the base, to allow air to circulate, before packing them full of damp leaves (if you’re collecting on a dry day, remember to wet them). Then simply tie them up and stack them somewhere cool and shady, for unlike garden compost (which is formed by bacterial activity), leaf mould is formed by fungal action and doesn’t require much heat.

It will take at least a year for the leaves to break down, after which the leaf mould can be used as a soil conditioner or moisture-retentive mulch. Either dig it into the top 15cm-20cm of the soil, where the roots of most plants feed, or use it as a 10cm-deep mulch, spread in autumn or late spring (once the ground has heated up).

If you want to raise the nutrient-levels further, you can even add the chopped-up leaves of comfrey (these are exceptionally rich in plant foods, particularly potash) to the one-year-old mix and leave it again for another few months. Left to its own devices for two years, your leaf mould will even be suitable for seed and potting mixes.

Those with plenty of space could also consider building their own leaf mould bin, using wooden tree-stakes, chicken-wire and U-shaped nails to build a simple wire cage in which the leaves can be stored (again, as above, choose somewhere cool and shady).

Hammer the stakes securely into the ground, positioning them roughly 60cm apart to make a square frame that’s about 90cm tall, before wrapping the chicken-wire tightly around it. Then secure the chicken-wire using the U-shaped nails (it helps a lot here to have a pair of thick gloves and an extra pair of hands).

Finally, pile it high with leaves and wait for mother nature to do her thing. But remember that, when it comes to making leaf mould, it’s definitely not a case of “less is more”, as what seems like a large mountain of leaves will turn into a surprisingly small amount of leaf mould. But having said that, it’s surprising quite how useful even a small amount of leaf mould can be when it comes to improving your soil. And when you think that it’s for such little effort on your part, and for free, well, what’s not to like?

The OPW’s Victorian walled kitchen garden is in the grounds of the Phoenix Park Visitor Centre, beside the Phoenix Park Café and Ashtown Castle. The gardens are open daily from 10am to 4.30pm

Next week Urban Farmer in Property will cover getting your vegetable garden ready for winter.

Fionnuala Fallon is a garden designer and writer