Regular weeds are a problem in any garden, but really pesky species can be tricky to shift

YOU MIGHT think that when it comes to preparing an overgrown or new site for a fruit and vegetable garden, things should be quite straightforward. You dig, you weed, you manure, you rake, you sow and then you harvest, right?

Well, no, actually. In fact, much depends on the particular site, the inevitable restrictions of time, money and labour, and of course how strictly you want to adhere to organic principles.

Take as an example the OPW’s walled Victorian kitchen garden – now a beautiful and wonderfully productive place but prior to its recent restoration, an overgrown and neglected wilderness. Abandoned and uncultivated since the late 1970s, the 2.5-acre site had become infested with an unlovely assortment of persistent perennial weeds, but in particular with what some gardeners would say is the “grandaddy” of them all, Polygonum cuspidatum or Japanese Knotweed – a plant which has been nominated for the list of 100 of the World’s Worst Invaders on the Global Invasive Species Database (issg.org).

In an irony not lost on OPW gardeners Meeda Downey and Brian Quinn, this very same plant was once highly prized by Victorian gardeners for its ornamental qualities (as well as by beekeepers for its abundant nectar) and was tenderly, lovingly cultivated until the realization dawned, too late, that it was in fact a thuggish weed almost impossible to contain.

Reproducing vegetatively through its root system or through the dispersal of very small pieces of plant material (as little as 1cm in length), and capable of horribly quick re-growth, Japanese knotweed forms three-metre-tall thickets with a dense leaf canopy that completely excludes other plants, while its roots or rhizomes can extend to a depth of two to three metres and up to seven metres away from any parent plant.

Let unchecked, it can penetrate tarmac and concrete and as a result can do great damage to house foundations, paths, roads and walls – including those historic brick walls that enclose the OPW’s walled kitchen garden.

In fact, so damaging is Japanese knotweed that in the UK, any material from the plant is classified as controlled waste and only authorized personnel in possession of an environmental permit are allowed to transport or bury it outside its original site.

So the OPW gardeners were faced with a horrid dilemma. Their aim was to maintain the walled Victorian kitchen garden according to organic principles, once it was restored, but was it realistic in the very early stages of the restoration process to use organic-only methods (repeatedly digging out roots and cutting back re-growth, over a long period of time) to try to clear the overgrown walled garden of Japanese knotweed?

Given the scale of the infestation, the maturity of the colonisation, the time and design constrictions imposed on the project, the risks posed by moving infested soil in and out of the garden and their desire to start with a clean, weed-free site, they decided that it wasn’t.

Instead, the OPW gardeners plumped for spraying the garden with a glyphosate-based systemic herbicide by the name of Roundup. While this did the job, it earned them much criticism from organic gardeners, who argued that even the once-off use of such chemicals in a garden that was subsequently to be managed organically ran contrary to everything the organic gardening movement stands for.

So were the OPW gardeners right or wrong in making the decision to use such a powerful chemical against such a formidable weed? I’ll leave it up to you to decide. What the story does illustrate for me is the importance that the OPW gardeners, like all good gardeners, placed on starting with a clear, clean and relatively weed-free site. But then, as the saying goes, there are different ways to skin a rabbit.

Both the organic gardener Klaus Laitenberger and his successor, Dermot Carey, managed to control (but not eradicate) Japanese knotweed in the walled kitchen garden at Lissadell, using organic-only methods.

Here, the gardeners dug out as much of the plants’ roots as they could before putting the worst infested areas into grass, a time-honoured way of gradually weakening the root-system of many persistent perennial weeds through repeated mowing (it also works brilliantly for ground elder for example).

In his book, Vegetables for the Irish Garden, Laitenberger mentions yet another time-honoured but also time-consuming organic method of clearing an overgrown site, which is to cut back the existing foliage and then cover the ground with manure followed by thick black plastic sheeting.

The problem with this technique is that it takes up to a year for it to be effective and even then, it wont be entirely so against persistent weeds such as Japanese knotweed or bindweed.

In contrast, raised beds (they must be at least 20cm high and the boards 5cm thick) are a very time-efficient (but relatively expensive) way to create a new garden on a previously overgrown site. Just build them on top of the existing surface (again, start by cutting any existing foliage down to the ground), and then cover the ground with a thickish layer of newspaper, followed by well-rotted manure and then soil. While this method works well, particularly in a poorly drained site, be warned that the root systems of the most persistent perennial weeds will still survive.

Ditto single digging, where the ground is dug in a series of narrow parallel trenches to a depth of 20cm, with the sod lifted and flipped upside down before being used to refill the previous trench. An excellent way of clearing a smallish site, it’s far quicker than black plastic (the ground is usable in about six weeks) but is labour-intensive. And yet again, deep-rooted persistent perennial weeds such as Japanese knotweed and bindweed will continue to be a problem. Of course, weeds are always going to be a problem in any garden. But how much of a problem is down to how willing and physically able you are to repeatedly roll up your sleeves and do battle.

Many years ago, my gardener grandfather became so obsessed with the bindweed that was invading his city vegetable plot from his next-door neighbour’s garden that he sank sheets of corrugated iron six feet down into the ground to try and block its invasive roots. It didn’t work. Which tells you that when it comes to the most persistent weeds, even the most persistent gardener occasionally has to accept defeat.

Finally, in an Urban Farmer column earlier this year on gardener Nicky Kyle and how to grow early potatoes in a polytunnel, readers were told that they would find it impossible to buy Mayan Gold seed potatoes here in Ireland. I’m very glad to tell you that this was incorrect. Brian Paterson, owner of The Secret Garden Centre near Newmarket, North Cork (thesecretgardener.com), emailed me to say that they have taster packs of Mayan Gold from Taylor’s Bulbs in stock, which are retailing at €2.99 for a pack of 10.

WHAT TO: sow, plant and do now

BEGIN TO SOW :

In modules under cover, for planting out later in the vegetable garden: Asparagus, broad beans, globe artichokes, beetroot, Swiss chard, early Brussels sprouts, summer cabbages, cauliflowers, red cabbage, carrots, parsley, chives, Welsh onions, spring onions, leeks, bulb onions, shallots, parsnips, early peas such as Kelvedon wonder, radishes, kohl rabi, white turnips, landcress, rocket, mizuna and salad leaf mixes. Also sow hardy annuals such as limnanthes, calendulas, convulvulus tricolour, etc. These will flower early and attract insects to help with pest control. Remember to keep seedlings frost free once they’ve germinated, but don’t grow them in too much warmth or they will be too soft.

Under cover in a heated propagator to later transplant into polytunnel: early tomatoes, aubergines, calabrese, celery, French beans, sweet peppers and chilli peppers, to grow on using a heat mat before hardening off and transplanting into polytunnel in late spring.

In a polytunnel (either direct-sow or sow into modules, using fleece at night for frost-protection): broad beans, carrots, Ragged Jack kale, ruby chard, mangetout and early peas,

(Sowing details courtesy of Nicky Kyle, nickykylegardening.com)

PLANT:Very early, chitted potatoes into 2L pots indoors, to move to cool, frost-free spot before planting in polytunnel using fleece at night for frost-protection; garlic (in individual cloves, direct outdoors or into modules to transplant later, Jerusalem artichokes.

DO:Start planning this year's vegetable plot; chit potatoes; order seed (catalogues or online), weed polytunnel crops, sort through stored vegetables and discard any rotten/frost damaged ones.

* The OPW’s Victorian walled kitchen garden is in the grounds of the Phoenix Park Visitor Centre, beside the Phoenix Park Cafe and Ashtown Castle. The gardens are open daily from 10am to 4pm

* Next week: growing onions from seed and selecting potato varieties

* FIONNUALA FALLONis a garden designer and writer