Recent warm weather and rain has been a boon for weeds, which means plenty of work for gardeners

MOONFIRE, Happy Flame, Fire Ice . . . With names like these, it’s no surprise to discover that many of the dahlias being grown in the OPW’s walled kitchen garden are in the fieriest shades of yellow, orange, vermillion and coral red.

Five fat sacks of these colourful plants’ sausage-shaped tubers were delivered to OPW gardeners Brian Quinn and Meeda Downey last week, and have already been given a home in the freshly-prepared side panels that run along the garden’s old brick walls.

“We enriched the soil with barrow-loads of compost earlier this week so the ground is perfect for planting, says Brian Quinn. “Dahlias are notoriously greedy feeders – you have to give them a nicely moist, fertile soil or they just won’t do well.”

With the walled garden’s soon-arriving honey bees in mind, he and fellow gardener Meeda Downey also made certain that some of the dahlia varieties chosen are single-flowering types.

“The more showy the flower shape is and the more petals that it has, the less likely it is to be a good source of pollen or nectar – the two things that the honeybee is after,” says Downey, as she helps Quinn to bury the dahlia tubers 25cm deep in the soil. “Some of the double or multi-petalled dahlias produce barely any pollen at all while the nectaries [the part of the flower that produces the nectar] can be so hidden amongst the petals that it’s hard for honey bees or pollinating insects such as hoverflies to find them.”

Having said that, the OPW gardeners aren’t being overly strict. “We’ve chosen two single- flowering types, ‘Moonfire’ and ‘Happy Flame’, along with some semi-doubles (‘Bishop of Llandaff’ and ‘Fire Ice), and one really elaborately flowering pompon dahlia called ‘Franz Kafka’,” says Quinn, adding that they’re being grown in solid blocks of colour rather than mixed.

Now planted in the walled kitchen garden, the dahlia tubers will root quickly, producing tender green shoots that should appear above ground within the next few weeks.

“Soaking the tubers in lukewarm water for an hour or so before planting also helps them to get established,” says Quinn. “After that, you need to watch out for slugs and late frosts – the young shoots are very vulnerable when they first appear.”

The ever-present risk of late spring frosts is also the reason why the OPW gardeners have been so chary up until now about sowing or planting much outdoors. “Apart from planting the dahlia, sown outdoors in the walled garden so far are carrots and parsnip seed. But we’ve sown loads of stuff indoors over the last month or so, and now it all badly needs to be pricked-off and potted on,” says Downey worriedly. Indeed, a quick visit to the OPW gardeners’ nearby glasshouse reveals tray after tray of tiny young seedlings, including celeriac, calabrese, cabbage, cosmos, basil and cucumbers. There’s no sign as yet, however, of the much-anticipated banana seeds germinating.

“And there might not be,” shrugs Downey with resignation. “We came in the other morning and found the compost in the pots had been disturbed by something. We think a mouse might have got at them so we’ll just have to wait and see.”

But the vulnerable baby vegetable and flower seedlings in the glasshouse aren’t the only things that Downey and Quinn have to worry about, for just as they predicted, the recent rainfall, combined with a rise in temperatures, has resulted in a sudden flush of weed seedlings in the OPW’s walled kitchen garden.

“We have to get the weeds now, either by hoeing or hand-weeding, otherwise they’ll quickly take over,” says Quinn, as he and Downey use their favourite oscillating hoes to slice the young seedlings away from the ground before carefully raking them into neat piles and barrowing them away to be composted.

Regular hoeing like this is a weekly ritual for the OPW gardeners at this time of year, when the thousands of weed seeds lying dormant on the surface of the soil spring into sudden growth. Left to their own devices, these weeds would quickly smother any young vegetable plants, robbing them of nutrients, water and light while also sheltering pests and encouraging disease.

That said, even the experienced OPW gardeners have discovered that the thorny question of what is a weed?, is sometimes harder than you might think to answer.

Take the recent reappearance in the walled garden of Italian ryegrass – originally sown in 2009 as a useful green manure crop but now a classic example of the definition of a weed as “a plant in the wrong place”.

“We had no idea it would be so invasive,” says Downey with frustration. “Now we just can’t seem to get rid of it. It hides amongst the box hedging along the edges of the beds where it’s almost impossible to get at, and we have to hand-weed it wherever it reappears.”

Along with the Italian ryegrass, occasional tentative reappearances in the walled kitchen garden of the horribly persistent Japanese knotweed are yet another reminder to Quinn and Downey that a plant introduced as a friend can quickly become foe.

But foe to whom is the question – for just as the OPW gardeners are keen to grow nectar and pollen-rich plants in the walled garden, so were their Victorian predecessors. And one of the reasons why the now-dreaded Japanese Knotweed was once so popular in Victorian horticultural circles was the fact that its flowers are a wonderful source of nectar for honeybees and other pollinators at a time (September) when it is badly needed. The resulting honey (a monofloral honey known as Bamboo honey) is famed for its dark colour and rich, earthy sweetness.

So, an invasive weed that honeybees love – it seems that Downey and Quinn might have something of a dilemma on their hands. Friend or foe? Beauty or beast? As the British nature writer Richard Mabey points out in his recent book Weeds, plants such as Japanese Knotweed (and Italian Rye for that matter) are reminders of the fact that when it comes to these kinds of questions, there is no definitive answer. “Life,” as Mabey wryly puts it, “is not that tidy.”

* 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 4pm.

* Next week, Urban Farmer will cover sowing seeds in the walled garden

* Last week we gave the the address of Ask About Ireland as askaboutireland.com. The correct address is askaboutireland.ie.

* FIONNUALA FALLONis a garden designer and writer

WH AT TO: sow, plant and do now:

Sow in a heated propagator for greenhouse or tunnel cropping:French beans, peppers, tomatoes, sweetcorn, cucumbers, early courgettes and melons, basil, early calabrese, Alpine strawberries (Reugen best) and tender single flowers such as tagetes, French marigolds and nicotiana attract beneficial insects to help with pest control and pollination both under cover and out in the garden.

Sowin gentle warmth for planting outside later: celery, celeriac, coriander, dill, Greek oregano and Florence fennel. Sow in modules, in a seedbed for transplanting, or in situ where they are to crop: asparagus, globe artichokes, beetroot, broad beans, carrots, endive, all varieties of peas, parsnips (early April), summer and autumn cabbages, red cabbage, savoy cabbage, Brussels sprouts, all varieties of sprouting broccoli including calabrese, cauliflowers, leeks, salad onions, shallots, pak choi, Hamburg parsley, landcress, lettuces, kohl rabi, kale, radishes, rocket, salsify, swiss chards, spinach, seakale, white turnips and swedes, claytonia, lamb's lettuce, salad mixes and hardy herbs. Rhubarb can also be sown from seed (Unwins early red and Glaskin's perpetual are both good varieties). Asparagus peas, cardoons and New Zealand spinach can be sown outside from mid-April. Also some single hardy annual flowers such as limnanthes (poached egg flower), calendula, Californian poppies, convulvulus tricolour, phacelia, and sunflowers will attract bees, butterflies and other beneficial insects, which will in turn also attract wildlife such as birds and bats.

Plant outside:Seed potatoes, onions, shallots, cabbage plants.

Do:Water/prick-off/pot on young seedlings and plants and protect against slugs; earth-up early potatoes; continue hoeing and handweeding; and keep glasshouses/polytunnels well-ventilated during warm, sunny days.

* Sowing details courtesy of Nicky Kyle, nickykylegardening.com