The Science Behind Thermal Weed Control

Well actually “YES IT DOES”, but there is a little more to it than it sounds.

Universally weeds are a hassle for farmers, gardeners, property owners and landscape managers. Hand digging and mulching is the ‘go to’ method I use in my own garden but this can be problematic for commercial crops and public open space. With the wealth of information now in the public domain about the health and environmental issues related to chemical herbicides for killing weeds, there is a major shift to adopting non-toxic weed control methods.

Steam weed control falls under “Thermal Weed Control” category on non-toxic methods. These can be classified into the following Hot Dry and Hot Wet categories.Thermal Weed Control Methods

Hot Dry includes controlling weed with infrared radiation, flame and hot air. They all use propane/ LPG as their heating source.  There are a number of small flame units available which are comparatively portable and are used by home owners, land-care groups and some cities.  There are also larger ones such as HOAF and Sunburst with ride on or tractor versions.
There are some obvious limitations of using a blast of open flame to cook weeds.  Dry leaf litter and grass easily catches alight and the flame melts rubber soft fall in playgrounds and rubber paving, and irrigation lines melt on contact.

My observational experience of using flame is that it doesn’t really penetrate the crown of the plants. It cooks off the top pretty effectively but regrowth from the crown is observed within a few days after treatment.  This leads to having to treat frequently.

Hot Dry Weed Control

Let me explain why this is the case. I remember when I was in science lab at high school and we had Bunsen burners alight. My classmates and I would see how slowly we could pass our fingers through the flame. Despite feeling the heat, we didn’t end up with any blisters.

That is because as we passed our fingers through the flame we were exposed to dry heat and dry heat doesn’t linger. Whilst the temperature of the flame was in the vicinity of 1300° C / 2400° F the heat transfer of hot air and flame is quite slow and air cools as soon as the flame or other radiant heat source moves on, or in the case of our fingers, they moved to cool air.

When using a flame to cook weeds, in order to get better penetration and a consequently longer die back, the flame must be held in place for a period of time long enough for the soil around the root to heat up.  The problem with this is that it increases the chance of fire and damage to the assets you are trying to protect by weeding. I have observed better results in early morning when dew is present or after rain.  For a start its safer, but I also reckon the water boiling on the leaves is retaining the heat for longer and transferring the heat into the leaves more efficiently.

Hand held and push type flaming and radiant heat apparatus can be useful because of their portability but they come with high risk of off target damage.

Of the hot dry methods available flame and radiant heat from ceramics or metal plates heated by flame are the most effective.

Hot Wet methods of weed control include steam, hot water, hot water and insulating foam and saturated steam and boiling water.

Some hot wet methods are regarded as having superior control over weeds than hot dry method.  The reason for this is easily explained by comparing the Bunsen burner at 1300 °C/ 2400 °F to almost boiling water at 95 °C/ 203 °F.  The devastating burns that occur when a cup of freshly brewed tea is spilt onto skin is a perfect example showing how the water transfers heat so effectively and intensifies the damage as it stays on the skin.  Hot oil is even more effective as it doesn’t evaporate and heats to higher temperatures than water. People that work in kitchens learn soon enough that the worst burns are the hot wet oil and water scolds, rather than the more localised burns from touching a hot dry pan or oven rack.

Hot wet methods of weed control enable more rapid transfer of lethal heat into cell structure of weed. In hot wet methods, deeper penetration into the crown of the plants is achieved.  This is where the meristematic cells that are the central growing point of many weeds are.  Most common soft wooded, annual and perennial weeds by their nature are plants that have migrated or been introduced from environments such as grasslands, prairies and open forests.  In their natural environment they are grazed upon by ruminants and occasionally exposed to fire.  They are adapted to growing back from the meristematic cells, the crown of the plant that sits at about or just below soil surface.  They draw on the stored energy of their roots to regenerate new stems, often multiple stems, and a new flush of leaf growth to start the process of replenishing their root system and stored energy. Hot Wet Weed Control

What makes hot wet methods so effective is that they are unlike any process that the weeds are adapted to. The heated fluid penetrates and damages the growing tips, delaying the usual rapid regeneration that is experienced with dry thermal or mechanical cutting methods.

Have you ever hand weeded a garden bed, thinking that you have left the soil weed free, job done, only to have a veritable vegetative carpet of seedling weeds appear after the first moisture?

That’s the seed bank being triggered to life by the warming of the soil, previously shaded from the sun by the weeds you removed, perhaps some exposure to light and certainly some available moisture. So, what I have seen time and time again in weeding with boiling water is that the combination of adding heat and water to the soil surface germinates some species in seed bank. Some species of seeds can’t tolerate the heat and are de-natured, they die and decay in the soil, whilst others love it, as it splits the hard seed coat and allows their first embryonic roots and shoots to emerge. Either way it’s a bonus.  It’s a bonus because we can now deplete the seed bank which means that we can control next year’s weeds this year.

Seed Bank Diagram

I won’t go into the environmental contamination and potential health effects caused by chemical methods of pre-emergent seed bank control, because seriously we can manage the seed bank with just water.  Really, really hot water!

 – Excerpt of an article written by Jeremy Winer

If you still have an itch to learn more, feel free to read Jeremy’s Holistic Vegetation Management Report.  This paper further explores methods of thermal weed control and cites scientific research into weed management practices from around the globe.

Holistic Vegetation Management Report

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