After 30 years of organic farming, we are pretty good at controlling weeds without herbicides, and mercifully now spend little time on our hands and knees. Success depends on a persistent strategy: we prevent weeds from setting seed (‘one year’s seeding; seven of weeding,’ so the adage goes); create stale seedbeds, by germinating then killing the weeds near the surface of the soil before our crop is planted; heat-treat manure through composting, to kill any seeds within (we still have a bit to learn about this); grow crops from plants rather than seeds, to give them a head-start over the weeds; perform efficient, timely mechanical weeding; rotate our crops; and choose strong crops that can fight off the competition. On the horizon are nimble swarm robots, able to recognise a weed and punch, electrocute or laser it to death.
Despite all this, we have a frightening 5,000 weeds per square metre emerging ahead of our first salad crops. Early crops always tend to be weedy: rising soil temperatures awaken dormant seeds, and spring cultivations bring a new batch of seeds within striking distance of the surface, where a flash of light or increased diurnal temperatures tell them the time is right to make a dash and claim their place in the sun.
We will try a thermal strike on pre-emergent weeds today using a giant tractor-mounted gas grill. All being well, the crop will emerge tomorrow surrounded by withering weeds. As the season progresses we are able to take more weed strikes (shallow cultivations), and progressively exhaust the weed seed bank in the top 80mm of the soil, creating our ‘stale seedbed’. By June, weeding costs have fallen tenfold and we can get off our knees.
My organic farming neighbours in France have a less subtle approach; they sterilise the soil with giant diesel-fired boilers that inject steam into the top 100mm, heating it to 70 degrees, which kills all weed seeds and plant pathogens (plus all the good bugs). I watch with horror as they burn 0.5 litres of diesel to sterilise one square metre of soil, which will grow perhaps 3kg of salad leaf.
It may be pesticide free, legally organic and economically rational, but it is the sort of environmental insanity our planet cannot afford; we will never do it, and I gently nag them every time we meet. There are far better methods that still (mostly) save you from crawling on the ground.