Warfare in walled garden as gardeners fight pests


URBAN GARDENER:RAIN, rain, rain, rain . . . and then more rain. Planting of the new herb patch at the OPW’s walled vegetable garden in the Phoenix Park has been wisely postponed because of the regular, ground-drenching downpours of the previous few weeks.

“We had to put it off,” says OPW gardener Brian Quinn with a heavy sigh. “The soil was too wet and the young plants would have hated it.”

And so the sun-loving, Mediterranean herbs – like sage, rosemary, basil and thyme – will wait it out in the glasshouses for another week or so. Meantime, OPW gardeners Brian Quinn, Meeda Downey and Declan Donohoe have another challenge on their hands, and it’s not meteorological.

“Sawflies on the gooseberries,” says Brian with a grimace. “Actually, not the sawfly itself but its larvae, which can strip all the leaves off a gooseberry bush in a matter of days. We spotted the green and black caterpillars on the plants earlier this week, and so it was all hands on board.”

It’s at times like this that the resolve of most organic gardeners is sorely tested, and the temptation to turn to chemical warfare becomes almost too much. “It took five gardeners over a day to hand-pick the larvae off all 50 gooseberry bushes,” says Meeda exasperatedly. And then what did they do with them? “We sliced them in half with secateurs,” smiles Brian. In anticipation of further similar infestations, the OPW gardeners have being doing some careful research and have discovered an alternative, and less time-consuming (but still organic) solution. Boiled rhubarb leaves . . .

While cooked rhubarb stems are delicious and entirely edible, the large leaves of this perennial plant are quite poisonous because they contain large amounts of oxalic acid. This naturally occurring chemical has many uses and is an active ingredient in some rust-proofers, wood restorers and household cleansers.

In horticulture, it can also serve as a highly effective insecticide, as first promoted by the pioneering organic gardener, Lawrence Hills (founder of the Henry Doubleday Research Association).

He discovered that a spray, made from the liquid of strained, boiled rhubarb leaves mixed with soapflakes, killed aphids and some other garden pests such as the gooseberry sawfly, but isn’t poisonous to animals, birds, bees and humans.

It also breaks down naturally in the soil, or in strong sunlight. It seems to be the organic answer to the OPW gardeners’ prayers. “We just don’t have the manpower or the time to spend days collecting sawfly larvae,” explains Brian. “So boiled rhubarb leaves it is.”

He and Meeda collect several pounds of rhubarb leaves in preparation – but then disaster strikes. Brian discovers that, for some as-yet-unclear reason, EU regulations appear to prohibit the use of rhubarb spray, despite its long-standing and proven record in organic gardening (it’s still widely used outside the EU). More research is required to discover why, and whether this is still definitely the case. But, for the moment, it’s back to hand-picking the little blighters.


Quiet warfare is now taking place in the Phoenix Park’s walled vegetable garden. As if having to slice up hundreds of sawfly larvae weren’t bad enough, the OPW gardeners now have to build elaborate defences against the carrot fly, a common pest that most vegetable gardeners struggle with.

The female carrot fly takes to the air round about now, leaving its previous host plant, cow parsley, to search for young carrot seedlings/plants on which to lay its eggs. As the maggots hatch, they burrow below ground and into the carrot itself, damaging the vegetable or, in very bad cases, rendering it completely inedible.

“Last year, we grew the window sill carrot, Parmex, which has stumpy, almost round roots. It was really badly affected – half of the crop was destroyed. Just mushy and full of holes,” remembers Meeda with a shudder. Prevention, it seems, is by far the best form of defence. The female carrot fly isn’t the most athletic of garden pests and can’t fly above a certain height, so this year, by erecting a low barrier (about 60cm high) around the young carrot seedlings, the OPW gardeners should be able to prevent the fly from reaching her target. They’re doing this with a double layer of garden fleece, sandwiched tautly into place between short lengths of timber driven into the ground. You could also use clear polythene or a white geotextile membrane (both of these are also stronger and less likely to rip than the fleece), but don’t use dark plastic, which will block off light from young plants. Raised beds and containers also offer some protection.

Brian and Meeda are also experimenting with different varieties of carrot, including Red Samurai, Early Nantes and Flyfree – the last of these is less attractive to the carrot fly because it naturally contains lower levels of chlorogenic acid, the chemical that the young larvae need to survive.

As the female fly is attracted by the smell of carrots, another useful tip is to plant strong-smelling onions nearby.

Finally, thin seedlings only in the evening and remove the thinnings quickly and as far away as possible (not in the compost heap), so the fly doesn’t catch the alluring scent of freshly crushed foliage. “Although we sowed at the beginning of April, we won’t thin the seedlings until late June,” explains Brian. “That way, we hope to avoid the worst, but only time will tell.”


Unlike carrots, leeks are a relatively easy, undemanding crop to grow, so the more tender-hearted urban farmers needn’t trouble their consciences about how best to battle pests or diseases. At the Phoenix Park, the gardeners are growing a reliable, mid-season variety called Musselburgh, a long-time favourite amongst grow-your-own (GYO) enthusiasts that’s been in cultivation since the early 1800s.

Brian and Meeda grew theirs from seed sown into modules in the glasshouse, back in late February, but you can also buy young plants in garden centres. Alternatively, if you really hurry, it’s not too late to sow seeds of later varieties such as Porbella and Pandora. (Technically, these should have been sown by the end of April, but you should get away with it – plant indoors or under cover, and into seed trays or modules for best results).

At the Phoenix Park, Meeda planted out the young leeks a few day ago, using the handle of a hoe to make individual planting holes, 15cm deep and spacing them at 15cm intervals along the row.

Ideally, leave about 30cm between each row. Young plants are ready to go outdoors when they’re about 20cm long and the thickness of a pencil. Some gardeners very gently trim the roots and leaves before planting, but this isn’t actually necessary.

After dropping the plants into their individual planting holes, Meeda watered liberally but didn’t (this is important) backfill with soil. Intuitively, this feels all wrong, and you’ll have to fight the strong impulse to push some soil around the roots, but this kind of deep planting without soil is the best way of blanching the leeks. Over the next few weeks, she’ll be keeping a close eye on the plants and watering them in dry weather until the roots become established.

Soil will slowly begin to naturally fall in and around the planting hole, but this is okay. For even longer blanched stems, very gradually draw dry soil around the roots of well-established plants. Try your best not to let it fall between the leaves, which gives that distinctively horrible gritty taste to some cooked but poorly washed leeks.

Alternatively, the RHS suggests slipping a short length of drainpipe over established plants. Leeks (and carrots) can also be grown in containers as long as they’re sufficiently large and at least 60cm deep – just remember to keep them well watered.

- Next week (fingers crossed), Urban Farmerin Propertywill cover preparing and planting a herb bed.

Fionnuala Fallon is a garden designer and writer