Thermal weed control methods use a variety of energy sources to generate the heat needed to kill weed seeds and seedlings.
Flame weeding
Applying heat such as flaming can help encourage weed seed germination by heating the soil. Emerged weeds can then be killed off with either shallow tillage or flaming. It is essential to make contact with foliage when weeding by flame. Flame weeding has greatly improved over earlier models. Flame weeding uses burners fuelled with liquefied petroleum gas (LPG). Flame weeding is an effective option when soil is too wet for mechanical weeding. Wet soil flaming destroys sprouts in the upper layer of soil.
Flame weeding applications are most effective when weeds were at the cotyledon stage. More advanced weed growth leads to less successful weed control. The same has been found for weed species whose growing points are below the soil surface, such as brassica weeds.
Timing
The timing of flaming is critical for good weed control.
Infrared radiation
Infrared radiation uses radiated heat directed towards weeds. The radiated heated is produced from burners fuelled by propane/butane which heat ceramic and metal surfaces.
Freezing
Two common freezing treatments rely on liquid nitrogen and carbon dioxide snow (dry ice) to destroy plant tissue. Both treatments can be applied to emerging weeds with a tractor mounted device. Findings indicate that liquid nitrogen is more effective than solid carbon dioxide for killing weeds however neither method outperformed flaming.
Freezing however could be an option in high fire danger situation.
Steaming
Steaming involves applying steam pressure directly onto crop beds and killing weed seeds at soil temperatures ranging from 70-100 degrees Celsius. This method kills most seeds to a minimum depth of 10cm. Problem seeds are hard shelled legumes and clover. It is important not to cultivate soil past the depth of steam application as this could bring deeper laying seeds to the surface. Steaming has shown to be effective for at least two seasons.
Direct heat
Direct heat uses equipment which passes cultivated soil through chambers heated to 68-70 °C by a diesel-fired burner. The soil is then returned back onto the ground in a ridge of weed free soil. Depth of cultivation ranges from 10cm for shallow rooted crops to 25 for deeper rooted crops such as potatoes.
Microwaves
Microwave weeding is a form of thermal weeding in which weed seeds in the soil are destroyed by heat.
Solarisation
Soil solarisation can only be carried out while the ground receives sufficient sun to create a hydrothermal process whereby moist soil is heated for around six weeks. This weakens any regrowth making them easier to remove.