Blanch your weeds

You don't need to spray weedkiller to remove the weeds between your paving stones. Six treatments throughout the summer with either boiling water, steam or flaming will dispatch even the hardiest of unwanted plants. This is the conclusion of a new PhD project from the University of Copenhagen.

Weeds can be killed off with repeated treatments, but it is important that each treatment is dosed correctly.

Basically, every single plant needs to be burned lightly – or blanched – but on the other hand treated more frequently.

However, little is achieved if you just treat the weeds superficially by running quickly over the paving stones. The leaves must collapse completely – and the plant's stem must also be struck by the flames.

Six times a season

Treatment must be repeated up to six times a season. A regime of fewer treatments encourages grass weeds to regrow, while more treatments are not necessarily more effective:

"We have conducted several kinds of experiments since 2004. Controlled field experiments, experiments where we have planted weeds in hard surfaces and then burned them, as well as experiments with 'real' weeds on stones. We have observed that the treatments work. It does not really matter whether you use flaming, steam or boiling water. However, if we only give a few treatments or if we use low doses, the weeds quickly reappear. Very low doses can even stimulate grass growth. This is also the case with many herbicides," says Anne Merete Rask, PhD from Forest & Landscape, Faculty of Life Sciences at the University ofCopenhagen.

Leaves must wilt after each treatment

Using alternative methods of weed control can achieve the same result as with classic herbicides, but it is necessary to pursue a clear strategy.

"It is vital that the weed is starved. This means that the plants must be treated repeatedly, and that the leaves must wilt completely after each treatment. This prevents the plants from storing carbohydrates in their roots, which is necessary for regrowth," says Anne Merete Rask.

In other words, the right dose and the right number of treatments are required, but then even the most obstinate grass weed can be eradicated.