You don’t have to be an organic grower to seek organic tools for fighting weeds. Synthetic herbicides come with a cost to growers and the environment, so more and more farmers are seeing the value in employing organic tools with or without other means.

Which tools work the best in which situations? That’s the million-dollar question UC Davis weed scientists help growers answer.

Mulches – Researchers test them all – plastic, bark, wood chips, other porous material, even what they call live mulches like clovers and fava beans. Mulches block light, which weeds need to grow. “I think mulches can be the best organic option for fighting weeds, especially for vines and trees,” Lanini says. Mulches are also a key weed-fighting component in organic strawberry and many other crops.

Flamers – These propane-fueled devices quickly raise the temperature of the weed to more than 130 degrees, rupturing its cell membranes. Grasses are hard to kill by flaming because the growing point is protected underground. Flamers require quite a bit of fuel, which can be costly.

Organic sprays – Coverage is the key. “No matter what type you use – oils, soaps, acids, etc. – if you don’t spray-to-wet, 100 percent coverage, the weeds will grow back,” Lanini says. Temperature and the age of the weed matters too. Apply in temperatures above 75 degrees when weeds are very young – about a week old – for best results. Broadleaf weeds are easier than grasses to control with sprays.

The most effective organic spray Lanini has found so far is good old-fashioned vinegar, the kind you use to make pickles. The trouble with that is, the FDA has yet to approve it for controlling weeds. “You can eat it, but can’t spray it on your weeds,” as Lanini says. There are herbicides with vinegar as their active ingredient, but they are much more costly than household vinegar.

Weeding and thinning by tool or machine is a time-honored solution, but the labor costs can be prohibitive. With new technology and the recent rise of precision agriculture, a concept that recognizes the variables within an orchard or field, cultivation has become more popular and efficient. UC researchers are evaluating machine vision technology in which camera guidance which allows faster and more precise cultivator operation. Newer models even allow in-row weed removal.

Solarization has become an important method of weed and disease control in organic desert vegetable crops. In this system, four to six weeks of solar heat under clear plastic film will kill weed seeds and pathogen propagules.

Cultural practices are extremely important in all vegetable crops especially in organic crops. Crop rotations result in shifting environments that do not favor any one weed. Use of preplant irrigation followed by shallow tillage, or flaming is a very effective method of reducing the potential weed infestation during the crop cycle.

What’s precision agriculture?

Orchards and fields, especially the 10,000-square-foot variety, are not uniform. So treating them uniformly with fertilizers, pesticides or herbicides is a waste of money and other resources. Precision agriculture attempts to take in account those variables and treat them accordingly.

Weeds, for example, are not uniformly distributed in a field or orchard. You’ll find more on the edges, where the infestation begins. How do you measure those variations? You could count and map every weed in the field, but then there would be no time left for farming (or sleeping or eating, for that matter).

“What we’ve done is map the distribution of weeds based on seedlings, 99 percent of which fall from their parent plant,” Lanini says. “We look at the seedling population – sample-per-acre after irrigation and other events. Using geostatistics, we can map an entire field based on sample seedling populations and determine areas of high, medium and low densities.”

Seedling sampling takes a lot of time, so the practice, on its own, hasn’t been adopted much by growers. But when combined with other tools, weed mapping shows great promise.

Example: Ag engineers have designed a machine with a camera that can distinguish a crop from a weed, then using syringes, apply herbicide to just the weeds. In areas with high-density weeds, there is so much overlap between weed and plant that it’s hard for the camera to tell the difference between the two.

“But, in low and medium density areas, there is enough separation for it to work quite well,” Lanini says.

New forms of soil disinfestation

Methyl bromide is a soil fumigant that has been critical to crop production for the past 40 years. Methyl bromide works systematically in the soil to control a wide range of plant pathogens and pests, like nematodes, diseases rodents and weeds. The trouble is, it destroys the ozone layer, as well.

“The government is phasing out the use of methyl bromide because it’s an ozone depleting chemical,” Hanson explains. He and Fennimore and others are working on alternatives, including superheated steam.

A bit of background: The first time crops and orchards are planted in virgin soil, they grow beautifully. But the plants themselves change the biology of the soil so subsequent plantings are less pest-free. Farmers have been dealing with that for centuries with means such as rotating crops, leaving land fallow and heating soil under a plastic tarp in the sun. Soil fumigation has, by far, been the most effective means of cleansing the soil but more regulations and fewer effective fumigants have left farmers looking for more. Enter UC scientists.

“Injecting superheated steam into the soil may be one way to replace chemical fumigants in agriculture,” Fennimore says. “Steam technology has been around for more than a century in California, but it took a back seat when methyl bromide – which was a much cheaper alternative back then – came along. We asked, ‘Why not see if we can make steam fumigation work on a field-scale level?”

Working with a company in Kingsburg, CA., Fennimore imported a machine that is used in Italy to prepare soil for greenhouse basil plantings. The machine, manufactured by Ferrari Constructione, has a 100-by-74-inch platform fitted with 99 ten-inch spikes that inject steam into the ground.

“It’s surprising how fast it heats the soil,” Fennimore says. “Within two minutes, it will take 60-degree soil in the surface 8 inches and heat it to 200 degrees. It’s like a microwave.”

However, there are drawbacks – cost, for one. The machine must crawl eight feet at a time up and down the field, each time pushing the spikes into the ground and leaving them positioned there for six minutes while the steam does its work. With one steam machine, it takes about 30 hours per acre.

Fennimore calculated that operating the steam machine, labor and fuel cost $3,848 per acre. Currently, chemical fumigation in California costs $2,700 to $3,300 per acre. Applying steam to raised beds rather than the entire field could cut steam cleaning expenses to about $3,000 per acre.

“We tested a small machine that was easy to ship and most accessible,” Fennimore said. “It is not the final design by any means.”

Another concern is air quality. The Italian machine runs on diesel fuel. In some parts of California, the operation of diesel engines is regulated to reduce air pollution.

“We’re concerned about the air quality issue and people point out all the time that we’re burning lots of carbon,” Fennimore says. “That’s true, but you can only solve so many problems at once. We’re trying to develop a practical system and we’re interested in switching to propane. We have a joint project with the Propane Education and Research Council (PERC) to develop practical heat-based soil disinfestation treatments for California crops. Currently for strawberry and tree crops we have over $1 million in recently awarded funding from the USDA NIFA methyl bromide transitions, USEPA, PERC, CDFA and California commodity organizations to develop mobile steam heat soil disinfestation systems to replace soil fumigants like methyl bromide.”

Fennimore is encouraged by the success Italians have had with the steam system.

“The Italians have not been able to use methyl bromide for a while now,” Fennimore says. “Yet, they have a number of the same industries that we have and they’re able to keep going.”

Using steam to treat land before replanting orchards and vineyards is another issue. The 10-inch depth of the steam injectors wouldn’t be sufficient for trees whose root zones reach four to six feet into the ground such as walnuts. As a gas, methyl bromide and other fumigants readily move through tiny gaps in soil and with proper application can treat deep down under the soil surface.

Hanson and Fennimore started preplant steam trials using an auger-based injection system for almond and peach trees last fall. Preliminary results will be available in 2012.

This article is brought to you by UC Davis.