Information: Thermal Weeding

2010 March update - please also see the article Thermal weed management for crop production (621KB pdf file) written for eOrganic (USA). Please note that the article states that steam weeders are not widely available. The article was written for a USA audience and PhysicalWeeding does not yet have a presence in North America, however, if you are interested in steam weeders they can be purchased direct from us.

Introduction

Thermal weeding covers all physical weeding techniques that kill weeds by heating them up or cooling them down. The most common form is flame weeding, but many other approaches have been tried, including microwaves, UV (ultraviolet) light, microwaves, electrocution, focused sunlight, lasers, carbon dioxide 'snow' and liquid nitrogen.

Thermal weeding has a long history. The first liquid-fuel flame ‘cultivator’ was patented in 1852 and by the 1940s reliable gas burners had been developed. However, the 1940s also saw the arrival and massive uptake of herbicides which resulted in the rapid demise of flame weeding until the 1970s and its re-uptake, predominately by organic producers.

Thermal weeding works by exposing weeds and/or weed seeds to temperatures higher or lower than they can survive thereby killing them. In practice only heating is used, because cooling techniques have been found to be impractical. Heating techniques fall into two broad approaches, ‘soil heating’ and ‘plant heating’. Soil heating, often misnamed ‘soil sterilisation’, because in practice it only pasteurises the soil, uses very large amounts of energy e.g., 7,000L diesel/ha. Soil heating is mostly used to manage soil borne pests and diseases in very high value crops, however, it will also kill any growing weeds and also weed seed if sufficiently high temperature and treatment durations are used.

The aim of the plant heating form of thermal weeding is to heat the above ground parts of weeds sufficiently to kill all the meristems (buds) or kill a ring of phloem at the stem base. The reason the buds have to be killed is they are the only parts of plants that can grow, so if unharmed they will produce new shoots and leaves, thereby resuming growth. This means that while weed biomass may be reduced by thermal treatment, i.e., the leaves are destroyed, new shoots, produced by the buds, will quickly replace the lost leaves and the weeds will continue to grow and be a problem. The exception to this is if a ring of phloem around the stem based is killed. Phloem is living tissue, unlike xylem, and if it is killed it can no longer transport nutrients to the plants roots causing the plant to die, similar to ring barking a tree. However, in agricultural situations targeting just a ring of phloem at the stem base, especially of small weeds, is difficult to achieve. As the aim of plant heating is to heat the weeds not the soil, energy consumption is a fraction of soil heating, with a typical amount being 80L diesel or LPG consumed per hectare.

The term 'flame weeding' can conjure up images of weeds burnt to a cinder, however, this is incorrect as 'hot' thermal weeding,including flame weeding, only needs to raise weeds' temperature to about 70 to 80°C / 160 to 175°F to kill them. Indeed, to use thermal weeding to actually burn weeds would be very inefficient. However, efficiently getting heat into weeds growing in fields is not as easy as it first may appear… Also while just about every means of thermal weeding have been tested, excluding the use of ionising radiation (radioactivity), all of them have been found to be impractical, or at best, of use in only very restrictive situations. The only practical means of getting heat into weeds is to use flames or steam.

Plant heating thermal weeding has received only limited scientific research and there has been even less crossover of information between engineers who understand the physics of thermal weeding, biologists who understand the biology of plants and farmers and growers who understand the day-to-day practicalities of growing crops. This information gap has resulted in thermal weeders that are less than optimal in some or even many aspects of their designs.

PhysicalWeeding overcomes these issues by combining a deep appreciation of farming, with a thorough knowledge of the physics of heat and an in-depth understanding of plant physiology to produce plant heating thermal weeders that are practical, effective and efficient. PhysicalWeeding produces both flame and steam weeders as there are advantages and disadvantages for both and we aim to match the thermal weeder to the production system. For more information see our steamweeding, flameweeding and flame vs. steam comparison pages. The HDRA in the UK also have a good weed management website with other information on thermal weeding

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