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Love thy enemy

It might seem counter- intuitive, but most weeds should always be composted or recycled.

Over the years, I've met hundreds, possibly thousands, of folk who chuck buckets of weeds into their rubbish or council waste bins. Yet, by doing this, they're impoverishing their garden soil, throwing out all the nutrients the weeds used for growing.

Dumping weeds this year is even crazier than usual. Hoeing in the rain holds little appeal, so many weeds have survived. Some have grown tall and lush, and the larger they get, the more nutrients they've absorbed. When you get round to weeding you'll find half the bed is stuck to the roots, so you could be consigning precious soil to landfill as well.

Weeds use different survival strategies. Short-lived annuals spread throughout the garden by seeding; the more resilient perennials spread vegetatively, growing a subterranean root system. Annuals, such as groundsel (senecio vulgaris), produce million of seeds. Undisturbed, one groundsel plant grows to 30cm and disperses at least 25,000 seeds throughout a bed. But the average number of seeds per plant is only estimated at 1200-1700, so it's best to whip out these plants as soon as they appear. Even if lifted at the flowering stage, it's reckoned that one-third of the plant's potential seed could germinate.

Groundsel's lifespan averages five to six weeks – a blink of the eye compared with perennials like ground elder or bishop's weed (Aegopodium podagraria). Ground elder only produces a little seed that isn't viable for long. Instead, it uses its rhizomes to colonise the garden. This is a frightening story. Between two and five rhizomes form at the base of each tuft of leaves. The shallow rhizomes have scale leaves every 4-5cm with a bud in each axil. Each bud could develop into a branched rhizome. In no time, one plant could produce an infinitely more impressive network than our railways. Although ground elder has the decency to die down in winter, it's the first to emerge again at the beginning of spring.

Because of this, some gardeners are feart to compost any weeds, certain they'll be digging millions of groundsel seeds back into the ground. And they are terrified of planting lots of ground elder root cuttings into a bed. Luckily, this nightmare needn't happen.

By making compost, you're taking advantage of a perfectly natural system: plants die, break down in the soil and enrich it by returning their nutrients. By providing heat and moisture in the compost bin you simply accelerate the process. Annual plants rot down quickly – within weeks they won't be recognisable and after a year they'll become soil-like.

As the cautious gardener knows, the seeds will be in the compost, all ready to spring into life as soon as they detect sunlight. Under ideal conditions, if you remove weeds from the beds before they start to flower, you'll be perfectly safe. More realistically, you won't be on the case quickly enough.

You can kill seeds by composting at high temperatures – from 55C-60C for a fortnight – but it's hard getting that kind of heat in a home composter. You can, at certain times of year, reach these temperatures. Your compost unit must be in full sun and you should put in lots of small pieces of material – shredded or finely chopped. If possible, mix in freshly mown grass clippings; they'll make the bin go like a furnace. Just before writing this, I stuck a thermometer in my New Zealand box and recorded 56C. I also test the heat in plastic compost boxes at our demonstration compost area at Woodside Walled Garden, near Jedburgh. A good pile of fresh material, including grass, readily reaches 60C in the summer.

You may not kill all the seeds but, if your bin is working well, you'll significantly reduce the number. Of course, a few will survive, but some will be in the soil anyway. And you'll have weed seed, however "pure" your compost. Seed arrives in birds' droppings and on your boots, while willow herb parachutes in. However, a bit of gentle weeding is relaxing and lets you check out your plants at close quarters, which is never a bad thing.

You need to be more cautious with perennials. It's best not to compost them if you plan to use the finished compost within a year, but it's perfectly safe to do so if using a New Zealand box. When kept well topped up, this bin works at a high temperature over two or three years and most roots will have broken down by then. Be careful never to compost Japanese knotweed, horsetail, lesser celandine or bindweed.

Alternatively, you could drown perennials. Stick the roots in a bucket of water, weigh them down with a brick or stone, cover and leave for five weeks or so. By then, the roots have been drowned and are ready for composting safely. You can then use the liquid as a feed. n

Visit www.askorganic.co.uk. Email your gardening queries to da@askorganic.co.uk

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