How do gardeners know when to begin to lift potatoes?

THE chitted potato seed went into the ground almost exactly four months ago, just a few days before a snowstorm. Back then, the OPW’s walled kitchen garden was still in the chilly grip of an icy spring and the soil was cold and cloddy.

Late frosts then singed the emerging foliage, while more recently, both the near-drought conditions and the new, earlier, more aggressive strains of blight have threatened hopes of a bumper harvest.

But potato plants are stubborn, tenacious creatures, hardwired like all plants to try to produce “offspring” no matter what, and so this week OPW gardeners Meeda Downey and Brian Quinn proudly lifted the first of the walled kitchen garden’s earlies.

The pale creamy tubers of the British Queens emerged from the ground like so much buried treasure, with each plant giving a yield of between 10-20 potatoes. Some, admittedly, are as small as a golf ball (but no less tasty for it) while others are the size of Meeda’s fist.

“But you dont expect earlies to be very big,” explains Brian as he carefully scrutinises the freshly-lifted tubers for any sign of disease. “They’re perfect, no sign of any damage,” he says with relief, rubbing away the paper-thin skin to reveal the luminous pale flesh beneath.

Just like anyone who grows their own potatoes, Meeda and Brian were worried about when exactly to start harvesting. Do it too early and the potatoes won’t have reached a decent size, while if left too late, the skin thickens and you lose out on the wonderful tender flavour of summer’s new potatoes.

So how did the OPW gardeners know when to begin? “With most of the early potatoes, you have to wait until the flowers have fully opened,” explains Meeda. “So we did that, and then we poked around the roots,” admits Brian with a grin. “That was a couple of weeks ago and the potatoes were still tiny. Then we noticed that the ground was bone dry, even when we dug down quite deep. So we decided to put the sprinkler on them for a full day and it made all the difference. The tubers needed water to swell in a very dry soil, they just wont grow to any decent size.”

While the potatoes themselves are just perfect, the foliage is another matter. Despite the gardeners recent and very reluctant decision to spray the crop with a preventative fungicide (due to the threat of the newly-identified Pink 6 and Blue 13 strains of blight), the leaves of the vulnerable British Queen variety are already showing the dark brown spots that are one of the distinctive signs of blight.

“We sprayed with Dithane rather than copper sulphate, or bluestone as it’s more commonly known, because we were worried by recent research which has shown that copper (a heavy metal) can accumulate to dangerously toxic levels in the soil over time. But even after spraying with Dithane, the leaves of the British Queens are already infected, and so we’ll have to cut the foliage right back to the ground to stop the fungal spores from washing down into the soil and infecting the tubers,” adds Meeda.

“So what will the gardeners do with the infected leaves and stems, or “haulms” as they’re more properly called? “You need to burn them, to stop the disease from getting a hold elsewhere in the garden – the same fungus, for instance, can also infect tomatoes. Ideally, then we should leave the potatoes in the ground for a couple of weeks before we lift them, to be sure that any remaining blight spores left on the surface of the soil have died. But as the British Queens are going to be harvested and eaten fairly quickly, I think it should be okay.”

Other urban farmers interested in learning how to identify the symptoms of potato blight will find much useful information on the website of the British Potato Council, www.potato.org.uk, as well as a useful photographic key (click on the link for allotments/ gardens at the top of the page).

Alternatively, advice for growers of organic crops can be found under the “Growers Advice – Fight Against Blight” link. Try, also, to keep watering of potato crops to a minimum, as it creates ideal conditions for the spread of blight. Finally, for more information on the risks of using copper sulphate as a fungicide, google the words “copper sulfate cornell”, which gives the results of research carried out some years ago by Cornell University in collaboration with several other American universities. It makes for interesting reading.

WHAT TO: sow, plant and do this week

Sow:Beetroot, broccoli raab, carrots, autumn and mini cauliflowers, chicory, endive, kales , kohl rabi, komatsuna, land cress, lettuce, mibuna, mizuna, mustards, pak choi, spring onions, peas, radish, rocket, spinach, Swiss Chard, turnips, winter purslane

Plant:Sprouting broccoli, Brussels sprouts, cabbages, calabrese, cauliflowers, kale, leeks, second-cropping potatoes (e.g. Carlingford).

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

• 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 will cover harvesting the first of the summer's carrots

• Fionnuala Fallon is a garden designer and writer