Research shows you don't need to resort to chemicals to give those annoying path weeds the brush-off. Here garden expert Ken Thompson gives his advice on how to get rid of them for good.
Weeds in paving are annoying, aren’t they? In fact, because it’s supposed to be one of the tidiest bits of the garden, weedy paving somehow seems more offensive than weeds elsewhere. And although weeds in paving are easy enough to control with herbicides, an increasing number of gardeners don’t like to use them.
Official interest in non-chemical control has been stimulated by the European Water Framework Directive, which aims to reduce pesticide residues in water. The Flemish government has agreed to phase out completely the use of herbicides on public pavements, so the search for “green” alternatives is urgent. A major Belgian research project, reported in the journal Weed Research, has some useful lessons for gardeners.
First of all, not all paving materials are the same. Pavers made of porous material allow water to drain away from the surface, and this lack of surface water inhibits germination and growth of weeds. So, all things being equal, porous pavers are the best material for reducing weed problems. For the same reason, pavers laid on a coarse, free-draining layer are less weedy than those laid on finer, less permeable material.
Joints between pavers should be narrow, but don’t assume that what you fill them with doesn’t matter. Compared with ordinary sand, a special sand enriched with sodium silicate drastically reduced weed growth. In fact, as long as it remained reasonably free of dirt and dust, the enriched sand more or less completely prevented weed germination and growth. The cause is excess sodium, which increases the osmotic pressure of the soil water, preventing seeds and roots from absorbing water and effectively killing emerging seedlings by drought.
The brand the researchers used was Dansand, which is widely available in Britain (from many branches of Wickes, see dansand.co.uk). But it is expensive. Sea sand is a much cheaper but less effective alternative (but still better than ordinary sand).
Eventually, however, you’ll still get some weeds. So how do you kill them? The clear winner among brushing, hot water, hot air and flaming was hot water, with significantly fewer treatments required to maintain an acceptably weed-free state than with other methods. The Belgian researchers had a fancy machine that detected weeds and then squirted them with near-boiling (98C) water, but there doesn’t seem to be a domestic equivalent.
There’s nothing to stop you using a kettle, but wear suitable footwear and gloves to protect you from steam. Gardens are dangerous enough places already, without adding another source of avoidable injury.
The researchers also found that susceptibility to control methods was species-specific; different methods work best on different weeds. So even if you settle on a particular treatment that works for you, the optimum strategy is to occasionally throw in a different treatment, to prevent a build-up of whichever weeds happen to be most tolerant of your chosen method.
A final thing to bear in mind is the importance of cleanliness. The effectiveness of any weed-control strategy is reduced as the joints fill up with organic muck, and even Dansand, completely weed-proof when clean, eventually allows some weed growth. Don’t underestimate the preventive powers of the regular application of a good stiff broom.
Read more: How to get rid of weeds forever
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