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.
Anne Merete Rask defended her PhD thesis: ‘Non-chemical Weed Control on Hard Surfaces: An Investigation of Long-term Effects of Thermal Weed Control Methods’ on 20 March 2012.
Anne Merete Rask, PhD, Forest & Landscape, University of Copenhagen, mobile +45 22 58 51 88.
Palle Kristoffersen, Senior Adviser and PhD, Forest & Landscape, University of Copenhagen, mobile +45 21 22 80 09.
Katherina Killander, communication advisor, University of Copenhagen
Mobile: + 45 51 68 04 12
Herbicide-free weed control:
Flaming: The most common thermal method. Many types of gas burner are available for the garden as well as more advanced equipment for mounting on, for example, a tractor.
Steam: The principle behind steam treatment is to heat and vaporise water in order to transport, release and distribute the heat energy effectively. Steam equipment costs considerably more to purchase than gas burners, but on the other hand the method entails no fire risk.
Hot water: Weed control using hot water (at approx. 95°C) is, like steam weed control, a relatively new control method. Equipment is available that consists of a diesel-fuelled heating unit which heats up the water. The unit is designed as a rectangular box which can be transported on, for example, a trailer. A tank of water is also required.
Hot water with foam: Functions like the hot water method, but an extract of plant sugars is added to the water. Compressed air turns the water into foam at the time of spraying, ensuring better heat utilisation because of the foam’s insulating properties.
Hot air: Gas is the fuel, but combustion takes place away from the hard surface and can therefore be described as ‘hot air’. The temperature under the heat shield is 550-600°C.
Freezing: Different studies have been conducted to find out whether liquid nitrogen and dry ice can be used for freezing weeds to death. The method is very energy-intensive and, as far as is known, can only be used to a limited extent.