Variations in the weather, the soil, pests and diseases can all affect your crop

SEASONED GYOers often say much of the addictive appeal of vegetable gardening lies in its unpredictability and in the discovery that although you may have successfully grown flawless onions, blight-free potatoes or monster-sized pumpkins this year, it should oh-so-not be taken as a guarantee that you’ll always (or even ever) do so again. The biggest reason for this is that growing conditions vary hugely from one year to the next – rainfall amounts, soil moisture levels, light levels, minimum and maximum temperatures, late frosts and cold/strong winds are just some of the variables plants have to contend with during their life cycle, and they can have a dramatic effect on their growth and vigour.

Then there is the vast range of pests and diseases (themselves equally vulnerable to the whims of the weather) that plants and gardeners must do battle with, from sap-sucking aphids, slugs and root-devouring larvae to mysterious viral, fungal and bacterial infections. As if this wasn’t enough, for some, such as OPW gardeners Brian Quinn and Meeda Downey, there’s even the worry of gluttonous squirrels or hungry badgers. No wonder then, that even the most successful GYOers are always wary of resting on their laurels (as the poet Shelley once drily pointed out, nothing wilts faster than laurels that have been rested upon).

A case in point is the OPW gardeners’ pumpkin crop, which was one of the glories of the walled kitchen garden last autumn. This year, however, it’s another story entirely. Brian is calling the crop the worst ever, with the plants being stricken both by disease and a suspected nutrient deficiency or imbalance in the soil. “By mid-July, the leaves of both the pumpkins and the butternut squash plants were covered in the white, almost sugary-looking growth that’s the classic symptom of powdery mildew,” sighs Brian. “It’s a nasty fungal disease that’s often a problem with courgettes, pumpkins and squash in warm, humid weather, when the soil gets dry. It’s something we didn’t have to worry about last year because of the cool, wet weather, but this summer, growing conditions in the walled garden have been completely different.”

A study of Met Éireann’s monthly data for 2010 (met.ie) confirms that difference, with rainfall amounts from January to July of this year coming to a total of just over 330mm in comparison to just over 520mm for 2009. Put another way, in the first seven months of this year, Dublin has had only about 63% of the total rainfall that it had during the same period last year. Mean temperatures for June and July (the pumpkins were planted in early June) are also above average this year, in contrast to last year’s, which were the complete opposite.

“We don’t have the time to regularly water crops other than when the plants first go into the ground in late spring and early summer, so the pumpkin plants had to take their chances. On top of that, we didn’t get around to feeding them as regularly as we should have (pumpkin plants are very greedy feeders). The powdery mildew has made things even worse, of course, because the disease stunts growth. But on top of that, something else is wrong with the plants and were not quite sure what it is,” says a perplexed Brian.

“We sprayed the plants with copper sulphate to treat the powdery mildew, and it seems to have worked quite well. But the leaves are showing the classic signs of a deficiency, with yellowing or chlorosis between the veins”, explains Meeda. “As well as that, the plants just aren’t growing as well as they should. The leaves haven’t spread and the fruit hasn’t swelled as much as they did last year. But we grew sunflowers in this exact same part of the garden last summer , and those plants didn’t do well either – there was poor growth and the leaves were all yellowy. So we think it’s a deficiency in one of the plant micronutrients – maybe iron or manganese. We know it’s not magnesium, because we sprayed the pumpkin plants with a solution of Epsom salts (rich in magnesium) and it didn’t have any effect. Other than that, it might be a virus or something in the soil, but we just don’t know.”

So now the OPW gardeners have to play both scientists and detectives as they work out what exactly is wrong with their plants. It’s not an easy task, because the relationship between the different plant nutrients is a complex and interdependent one while the symptoms of their deficiencies can be confusingly similar. For example, too much potash (K) can mean too little magnesium (Mg), as least as far as the plant is concerned. But one of the symptoms of magnesium deficiency (yellowing between the leaf veins) is the same as that for iron (Fe) and manganese (Mn) deficiency, as commonly seen in lime-hating plants.

Plant viruses can also cause yellowing or mottling of the leaves.

“We’re planning to get a complete soil analysis this autumn after the pumpkin crop has been lifted, which should tell us a lot about its nutrient levels and whether there’s anything to worry about. Until then, we can’t be sure,” shrugs Meeda resignedly.

In the meantime, and despite the OPW gardeners’ disappointment with the crop, the walled garden’s ripening pumpkins are still drawing gasps of admiration from visiting children who whoop with glee to see these colourful giants of the vegetable world.

“It’s always the same at this time of year,” grins Meeda. “There’s something about pumpkins that children love.” Adults love them too, of course, which is why the business of growing giant pumpkins is taken so very seriously. But competitive pumpkin growers will need to have very green fingers (and no worries about nutrient deficiencies or powdery mildew) if they’re to have any hope of toppling the present Irish record, held since last year by Michael Byrne of County Louth. His giant pumpkin weighed in at a whopping 936lbs and was so big that you could eat your dinner off it.

And I’ll happily admit that, all grown-up as I am, the sight of it (see virginia.ie for photographs) always makes me smile.

• 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

• Rabbits in the walled garden – next week in Urban Farmer

• Fionnuala Fallon is a garden designer and writer