Flamers, or burners, which use propane gas to burst the plant’s membranes, work well for small annual weeds. They will burn back perennials, but not kill them, requiring additional treatment to control perennial weeds.
There are no easy solutions to controlling weeds in an organic orchard or vineyard. While there are several options available for organic growers, most have pros and cons to their use, says a University of California weed ecologist.
Effective and economical weed control is one of most often cited problems by organic growers. The more common organic weed control methods include cultivation, mulch, flamers or burners, biological control, and organic herbicides.
Some work well on annual weeds, but not perennials, said Dr. Tom Lanini, weed ecologist at UC-Davis, who shared his thoughts on weed control options during a telephone interview.
Cultivation—Cultivation is commonly used by organic growers, but is not necessarily the best option in all cases, he said. It requires a lot of diesel to run the tractor multiple times through the orchard or vineyard and can impact the soil structure through compaction.
"It’s not ideal because you are bringing new weed seeds up to the soil surface every time you cultivate," Lanini explained. "Once you start cultivating, you have to continue. It’s an unending cycle."
Mulches—Mulches are a good option for orchardists and vineyardists, he said. They are one of the better methods overall, Lanini commented, as mulches suppress weeds by suppressing light, and they also add organic matter to the soil. "The weeds that do grow are weak and can be easily pulled or flamed," he said, adding that flaming can be done over the mulch after rain or irrigation without chance of ignition (unless the flamer is held in one spot for a period of time). While mulches can eliminate new annual weeds, their downside is that they don’t control perennial weeds, like bindweed.
Flaming—Flaming, which works by passing heat over the weeds to burst the plant membrane, works well for really small annual weeds, he said. Flamers are not meant to be held for so long over the plant that it burns—only long enough to burst the plant’s membrane. Growers can tell if the flamer is moving slow enough by pressing their thumb and index finger over the weed leaf to see if their fingerprint remains. Flamers burn back perennials but don’t kill them.
"The advantage to flaming over cultivation is that you’re not bringing up new seeds and not disturbing the soil," Lanini said. But with today’s high fuel prices, running the flamer (with propane gas costing $2 a gallon) and the tractor, make it an expensive option, he noted.
"Perennial weeds are like icebergs," Lanini said. "With cultivation, you’re only cutting off the top 15 percent, but 85 percent still lies below the surface." Whether flaming or cultivation is used on perennial weeds, the practice must be repeated every two to three weeks to prevent the top weed portion from sending energy back to the roots, he explained, adding that mapping the trouble spots with a hand-held Global Positioning System will allow the grower to hit the perennial hot spots without treating the entire field.
Biological control—Animals like geese or sheep work on both perennial and annual weeds and have the added value of turning weeds into fertilizer, said Lanini. Geese eat perennial and annual grasses, and are especially good at getting down to the roots of troublesome perennials like Johnson and Bermuda grasses. He suggests starting out with two to four geese per acre, using babies instead of adults. Older geese are more difficult to manage than young ones. Day-olds are imprinted by human contact and will be easy to herd, he said. "Babies eat and sleep, whereas adults fight, have sex, and eat in between fighting and having sex."
Geese also do well in orchard and vineyard settings because they like shade. A portable coop and electrified fence can protect the geese from predators.
Sheep are coming into fashion for vineyard weed control, he noted. Although they will graze beyond the ground on vine leaves and shoots, sheep can be trained not to eat the leaves by feeding them leaves treated with lithium chloride. "Sheep are great because they’ll eat any weed down to the nubbins."
Organic herbicides—Organic herbicides work by contact, but most require a high concentration to make them effective, he said, citing four useful organic herbicides: Matran EC (clove oil); GreenMatch EX (lemongrass oil); acetic acid; and Racer (ammonium nonanoate). Matran and GreenMatch work better than most, but both need high concentration rates of 10 percent to be effective, which may be higher than the label rate, he said. Gallonage required for effective control is around 70 gallons or more. Racer is not yet registered for use in California.
"At the higher concentration rates, it means that the cost for organic herbicides is relatively high—around $400 to $500 to broadcast the acre. That’s why you don’t see a lot of organic herbicides used." Spraying only tree rows instead of the entire acreage or using a WeedSeeker sprayer that sprays only green vegetation will reduce the cost. "But if the WeedSeeker is spraying 60 percent of the row or cover, than you’re basically treating the entire acreage," he said.
"What we really need is an organic Round-up herbicide that works as a translocator in the plant," he said.
Weed scientists continue to look for natural products that have residual, pre-emergent, or translocation properties. Lanini is studying mustard meal as a potential fumigant-preemergent weed killer in an almond orchard.