How to fight garden pests without poisonous chemicals

Instead of relying upon what’s increasingly seen as an old-school approach to controlling garden pests and diseases that uses a nasty arsenal of poisonous chemicals with a multitude of unpleasant consequences for garden wildlife, much of modern gardening is about doing what we can to help nature maintain a healthy balance.

So for example, I no longer worry about the clusters of aphids that often appear on the soft tips of new growth at this time of year as I’ve learned they’ll soon be greedily devoured by the aphid’s natural predators – birds, parasitic wasps, ladybirds and the larvae of hoverflies and lacewings – that form a part of any healthy garden’s biodiverse ecosystem. Using the very best sort of natural, planet-friendly pest control, everyone (bar the aphids) wins. How sweet is that?

Likewise I’ve learned that, along with good garden hygiene, careful soil preparation and occasional handpicking/squashing, most slug damage on plants can be prevented not by using slug pellets but instead by providing the right kinds of habitats – wild corners, hedges, garden ponds – for slugs’ natural predators, including birds, hedgehogs, frogs and beetles.

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Aphids will be greedily devoured by natural predators, including ladybirds. Photograph: Getty

Natural predators aside, another simple but brilliantly effective way to help vulnerable plants resist slug attack without resorting to harmful chemicals is to give them a natural liquid plant feed made from the leaves of young nettles. Just leave the freshly picked leaves to ferment in a bucketful of water for a couple of weeks (cover it with a lid). Then dilute that oh-so-stinky brown liquid to the colour of weak tea (make sure to wear gloves and old clothes) and apply it with a watering can or sprayer. You’ll be astonished by the results.

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Natural resistance

Of course, providing plants with optimal conditions for their healthy, resilient growth is key to boosting their natural resistance to disease as well as encouraging that all-important balance between pests and predators. By supporting your garden or allotment's soil health with the regular addition of well-rotted organic matter (manure, garden compost, proprietary organic mulches such as Enrich (enrich.ie), Envirogrind (envirogardenandhome.com) and Gee-Up (gee-up.ie) and/or the use of green manures as well as organically approved soil additives such as Soil Renew (soilrenewireland.ie), seaweed-based soil amendments (semegrow.com)and Biochar (probiocarbon.ie), you'll do a huge amount to create these conditions. Cork-based Fruithill Farm also carries a range of nature-friendly, herbal-based plant tonics (Herfosec, Folisec, Tercol) that stimulate growth and help repel attack by insect pests.

Conversely, by encouraging too much soft, sappy growth with the use of cheap synthetic fertilisers, you risk throwing this natural balance abruptly out of synch. The same goes for plants given the sorts of growing conditions that simply don’t suit them – for example, shade lovers in full sun, moisture-lovers in a dry, stony soil or ericaceous plants in a garden where the pH is just too high – which gradually weakens them to the point of making them incapable of healthily fighting off pests and diseases.

Optimal growing conditions aside, bear in mind that in many cases damage can be easily prevented by something as straightforward as the use of a physical barrier. Many common garden pests can be thwarted with the use of modern crop covers such as netting, Bionet, enviromesh and horticultural fleece (fruithillfarm.com), especially if these are combined with good crop rotation and a timely sowing/planting schedule that takes account of pests' annual life-cycles. For example, if you hold off sowing carrots until June, then the plants have a good chance of avoiding the first wave of attack from the emerging adult carrot fly. Sow oriental leaves in early autumn and not only will they grow better but even without crop cover they're also less likely to be aggressively attacked by flea beetles, populations of which are most active in spring and summer.

Careful timing is also hugely important when it comes to interrupting the lifecycle of one of the most destructive pests of Irish gardens, the vine weevil (Otiorhynchus sulcatus), whose small, pale, fleshy, legless, subterranean larvae have a voracious appetite for the root systems and stem bases of a wide range of ornamental and edible species and can quietly eat a plant to death before you know it. Classic symptoms include poor growth and sudden wilting/collapse of leaves and stems, a sure sign that root systems are so compromised that they can no longer deliver sufficient water to the plant. Another classic sign of damage is the distinctive “ticket-collector” notches left by the adult weevils feeding on the foliage at night.

Vulnerability

Container-grown plants are especially at risk, particularly those grown in a peat-based compost, while certain plants are particularly vulnerable to attack including primulas, heuchera, sedum, saxifrage, bergenia, pinus cyclamen, rhododendron, azalea, viburnum, euonymus, heathers, prunus, yew (taxus) and fragaria (strawberry). Gently tip a badly infested plant out of its pot in late spring and you’ll find their distinctive C-shaped, chocolate-headed larvae hidden among the compost, feeding on what remains of the damaged root system. Over the coming weeks these will begin to pupate before emerging as adult vine weevils to feed upon foliage and then lay yet more eggs (just one adult can lay up to 1,000 eggs over the course of her lifetime).

How best to manage them? Along with practising good garden hygiene by examining any new plant purchases or gifts for signs of possible infestation, thoroughly washing the root systems and containers of infested plants completely clean of compost before carefully and quickly repotting them, and disposing of any possibly contaminated compost, one of the very best ways is with the use of a naturally occurring parasitic nematode (a microscopic worm) that feeds on vine weevil larvae. Available to pre-order as the product Nemasys from good garden centres and online suppliers, including mrmiddleton.com, this needs to be dissolved in water and should be applied as a plant drench quickly after purchase.

It’s also really crucial that its use is carefully timed to coincide with both the rise of soil temperatures in late spring (these must be above five degrees) and the larval stage of the weevil’s life-cycle. In a normal year April-early May is the ideal time for use but given this year’s late, cold spring, an application any time in the coming fortnight should be effective as long as the larvae are still present in the compost or soil. August-September is also a suitable time for application.

A final few words of gentle reassurance, which are not to confuse the vine weevil’s microscopic, dark brown eggs with the granules of slow-release fertiliser often added to the compost when plants are being potted up for sale. The latter look like small, bright yellow-orange, peppercorn-sized balls but pose absolutely no threat to plant health.

THIS WEEK IN THE GARDEN

Given the very harsh late frosts of recent springs, it’s a good idea to keep some horticultural fleece to the ready if you have young tender plants such as tomatoes, cucumbers, peppers, chilli peppers, sweetcorn or courgettes planted outdoors or under cover of a polytunnel or glasshouse. The same goes for any tender bedding. When and if low single-figure night-time temperatures are forecast, throw a layer or two over them to protect them , making sure to remove it first thing in the morning so that they’re not left shivering under dew-wet fleece.

“Pinching out” is the horticultural term used to describe the technique where the soft, growing tips of different kinds of young plants are cut off or nipped out (typically using your thumb and index finger) just above a pair of leaves to encourage lots of healthy, bushy growth. While this method isn’t suitable for all species, it’s an excellent way to encourage many kinds of fast-growing annual bedding plants and tender perennials to produce plenty of flowering stems. Species suitable for pinching out in this way include cosmos, fuchsia, tagetes, petunia, amaranthus, antirrhinum, dahlia, and pelargonium.