Soil-Steaming and Steam-Boiler Blog

Bad news for vegetable and salad growers: Poisenous ragwort is further spreading throughout Germany. Just within a couple of decades this plant which is mainly original to the southern regions has developed large populations in south-west Germany coming from Belgium.

For this reason ragwort grows into a threat for the organic and conventional, heavily automated horticulture.
Ragwort contains substances which have a harmful effect on liver and nerves. After consumption they might even cause death. If parts of ragwort get into the harvest, as it became public in 2009, growers might face incalculable consequences.

Growers hardly have any options to fight the danger. There is no herbicide which provides 100% protection. Furthermore the leaves of ragwort seedlings look almost exactly like arugula, which makes a later automatic or manual extraction after harvest almost impossible.

The responsible government office of the German federal state Rhineland-Palatinate (DLR Rheinlandpfalz) is presently reassessing existing methods to fight ragwort. Soil steaming is also among them (please see report from May 2010). The results of the investigation will be published shortly.

The regional board of Freibug / Germany has finished its research on the control of Japanese knotweed with mere hot steam without chemicals which was started in autumn last year. The last treatment process was conducted to distroy the plant’s rhizomes in deep soil layers.

Already in september 2009 the experiment with hot steam was started. Large areas on the shore of the river Rench which were highly contaminated with Japanese knotweed were excavated down to 20-30 cm depth and steamed completely. The success oft he treatment of more than 95% was already visible at the end of the year 20009 and even more at the beginning of 2010. On the treated area only scattered sprouts appeared as expected.

Counter measures against these sprouts were taken by depth treatment with steam. High pressure lances made by the company MTM-SPINDLER were first driven into the soil down to about 1 m depth. The soil was broken and loosened with induced highly compressed air. Then steaming lances were applied and host steam at 200°C induced for about 15 min.

Already in May 2009 a similar test was made together with a regional government organization in the southwest of Germany to sucessfully kill all surviving germinable root residues after a first superficial treatment of contaminated area.
Hot steam denatures all germinable plant parts in the soil and harms them sufficiently that invading germs weakens the plants to a degree that no offspring can be produced.
Later on the positive results will be further supervised and checked if there is a sustaining positive effect.

On the 4th of May 2010 the service center Rheinlandpflaz in Schifferstadt conducted a bigger field test on weed control. A testing area of 2.000 m² was segmented into several parcels on which different methods of weed control were applied and compared.

Ragwort in focus
Growers in the region were particularily challenged by ragwort in 2010. Hence the testing area was in particular contaminated with ragwort.

Besides the application of chemical means also steaming was considered as alternative method.

16 parcels were steamed with steaming hoods. Testing conditions were suboptimal since the soil was wet and overly compacted due to rain on the previous day. Nevertheless steam penetrated the soil well and heated it up to 99°C down to 5 cm depth within 6.5 minutes. The steamed area was covered with a light sheet afterwards. 10 minutes after steaming temperatures of 95°C could be maintained in 5 cm depth. In 10 cm depth 82°C could still be reached.
It is expected that the steamed area was completely freed from weeds.
Hence the culture can be harvested without weeds and ragwort afterwards.

One of the leading bio medical research institutes in Switzerland, the Friedrich Miescher Institute, has conducted comprehensive research on the killing effect of hot steam on different weed seeds.
Scientific research was necessary, since the institute had to master the task to sterilize large areas which were cultivated with genetically modified plants.
All germinable plant parts and seeds had to be completely destroyed.

Soil steaming was the first choice
In order to proof the success of the measure at different locations of the contaminated area small bags of seeds were put into the soil before the steaming process. The seeds were examined after steaming on their germination capacity.
The result was 100% convincing: All seed bags exposed to steam, also those which were put in deeper soil layers down to 20cm, were completely killed. No shooting was discovered.