URBAN FARMER:Spend more time on the maths and you won't have to palm off extra courgettes on friends, writes FIONNUALA FLANAGAN

NOBODY ever warns you when you start your own fruit and vegetable garden that it helps an awful lot if you’re good at maths. Of course, green(ish) fingers are also very useful, as is a strong back and the understanding that Nature and not you, is the head gardener, but “computational skills” are surprisingly handy too. That’s because, very quickly, the urban farmer finds himself or herself in the garden, measuring-tape in hand, furiously adding, subtracting and multiplying in order to calculate exactly what quantity of seed potatoes, onion sets, carrot seeds or courgette plants will be needed to fill the plot. Length, width, perimeter and area (square metres or feet, depending on whether youre metric or imperial) determined, it’s back to graph-paper, scale-rulers and Google we go, to painstakingly work out the “how much?” and “how many?”.

Which is why it’s so strange that when it comes to estimating how many vegetables we’d actually like to eat (rather than grow), it’s another story entirely. Any effort at calculating quantities suddenly goes out the window. “Eight courgette plants, one square metre per plant- that sounds about right . . .”, we mutter vaguely to ourselves, without really stopping to consider quite how many courgettes will be produced (somewhere between 100 and 160 if picked when they’re about 10cm long).

Then harvest time comes round. “Not another glut of courgettes,” we groan, before quickly calculating how many friends/neighbours they can be palmed off onto. It’s the same with lettuce – they’re sown, they’re grown and then, all at once, there are twenty plump, purple heads of “Lolla Rossa” sitting there reproachfully, just begging to be eaten.

Even in the OPW’s walled kitchen garden, gardeners Meeda Downey and Brian Quinn are now dealing with a glut of cauliflower, cabbage and calabrese, which comes hot on the heels of a glut of summer berries. The difference here is that much of the fruit and vegetables produced in the walled garden goes straight to the next-door Phoenix Café, where it’s quickly used up. Not so for the home-grower.

In defence of those who over-sow and overgrow (which is probably most gardeners), part of the problem is that there isn’t exactly a glut of available information when it comes to calculating how much to grow of any particular fruit or vegetable. That’s one of the reasons professional gardener Klaus Laitenberger’s new book, Vegetables For The Irish Garden, is very useful (we hope he’ll bring out an equivalent on fruit).

Take potatoes, for example, a staple crop for most gardeners yet one that many have difficulties with when it comes to calculating yield. While he points out that it all depends on how much you like them, Laitenberger suggests that an area of 40 square metres should provide “more than enough potatoes for a family of four from July until April. One square metre may yield 5 to 7kg of potatoes”.

Annual spinach? If you love it, successionally sow one square metre every three weeks, he suggests. Onions? Working on an average of five a week, Laitenberger points out that youll need to have stored away 140 onions to see you through from September to the following March (28 weeks). Lettuce? Sow 15 seeds every fortnight, he suggests, enough to give you five heads a week while allowing for losses from slug damage etc.

He’s good, too, when it comes to the lesser-known vegetables, such as the tuberous oca (“just a few to see if you like them”), asparagus (“ten plants are sufficient for a generous weekly helping for the six-week harvesting period”), and Jerusalem artichokes (“5 plants are more than sufficient as each tuber will yield around 2kg of artichokes”).

All of which is very useful advice for next year, but what to do with this summer’s gluts? Serious GYOers know that part of the answer lies in having a chest freezer, which goes a long way towards turning a summer glut into tasty winter dinners. Here also, Laitenberger’s book gives plenty of advice as to technique (always blanch first in boiling water) and as to which vegetables are best suited to freezing (asparagus, broad beans, runner beans, peas, calabrese, sweetcorn and Florence fennel amongst others).

There’s also advice on other storage methods, such as “clamping” and storing in boxes of sand, both of which techniques are best suited to root vegetables. But what about courgettes, which traditionally form the “gluttiest” of all the vegetable gluts? Laitenberger advises that these are best eaten fresh.

I know, however, that another GYOer of long experience, the Irish Timescolumnist and original "Good Lifer", Michael Viney, has an alternative solution to the courgette glut. He, or rather his wife, Ethna, uses them to make ratatouille, the traditional French Provençal dish that is a stew of courgettes, aubergines, onions, tomatoes, peppers, herbs and garlic, which she bags into handy portions and then freezes. All of which tells you that, along with being good at maths, being an excellent cook is also very helpful (even, perhaps, necessary) when it comes to the art of growing your own. Of course, having an excellent cook to live with can be very useful too.

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 4.00pm

Vegetables For The Irish Garden, by Klaus Laitenberger, is available online from cultivate.ie, theorganiccentre.ie and scealeilebooks.ie for €14.95 plus pp

Hans Wieland, of The Organic Centre in Rossinver, Co Leitrim, (theorganiccentre.ie) will be giving a one-day course titled ‘Store and Preserve Your Vegetables, Herbs and Fruit’, on September 4th, 2010 (Price €90)

Next week Urban Farmer will advise on growing climbing beans

Fionnuala Fallon is a garden designer and writer

What to: sow, plant and do now

Sow: Beetroot, broccoli raab, carrots, endive, kales (as CCA), kohl rabi, Florence fennel, komatsuna, land cress, lettuce, mibuna, mizuna, mustards, pak choi, spring onions, Chinese cabbage, radish, rocket, spinach, Swiss Chard, turnips, winter purslane

Plant: Sprouting broccoli, chicory, French beans, Brussels sprouts, cabbages, calabrese, cauliflowers, kale, leeks, second-cropping potatoes (eg. Carlingford).

Do: Continue harvesting, continue sowing seed and pricking out/ thinning seedlings, weed/ hoe beds net young brassicas, soft fruit and fruit bushes; cover carrots with Bionet, spray non-blight resistant potatoes with Dithane to protect against blight, pinch out side and basal shoots and stake tomatoes, feed tomatoes, celeriac, celery, pumpkins, watch out for garden pests.