Propane torch is hot weapon for cool-season weeds

While a machine called CultiClean rolled across a test plot outside of Salinas, the front end mulched the soil to a depth of seven inches and shaped the beds, while the back end sent out a propane flame hot enough to kill weed seeds to a depth of two to three inches.

This multi-tasking piece of equipment imported from the Netherlands, is the latest entry to the growing list of candidates to mechanically rid cool-season vegetables of weeds.

"Organic spring mix and spinach are the target crops now, but it will also be useful for other crops. Spring mix and spinach are tricky because they are so fast," said Bob Sutton, owner of Sutton Ag Enterprises in Salinas.

Since Sutton had the manufacturer Struik ship CultiClean from Europe this spring, numerous trials have already shown the machine has some value, but much is still be learned about how to modify it and how to use it for maximum effectiveness.

"There is some value here, but it needs improvement. The results are not black and white. They are so-so. I'm not going to tell you this is a clear-cut home run," said Steve Fennimore, University of California Cooperative Extension vegetable weed specialist.

Fennimore tested the machine this summer in Salinas on romaine lettuce plots that were deliberately infested with sclerotinia lettuce drop as well as purslane, pigweed and rye weeds.

Even under these artificially tough conditions, CultiClean made a small difference in protecting the crop against the range of pests.

"We're seeing a slight reduction in lettuce drop with this machine. It's heating the soil to temperatures that kill pests, weeds and disease pathogens. We've got some reduction in weeds and in pathogens. It's enough that we want to keep looking at it," Fennimore said.

The propane flame heats the soil to a weed seed-killing temperature of 170 degrees, but to be more effective CultiClean will have to be modified to keep that heat in the soil a little longer, he said.

"I think it needs to be modified. The soil needs to be held at high temperatures a little longer. It needs some kind of insulating shroud dragged behind it. With fumigants, you need to cover it for a few days, with heat, it's a few minutes. The shroud is going to have to be fireproof. The guys from Struik said some farmers use an aluminum shroud and it helps," Fennimore said.

Sutton has additional spring mix trials planned for this year at four ranches in the San Juan Bautista area. Those trials will include a look at a shroud to keep the weed-killing heat in the ground.

Earlier, Sutton did a trial on purslane in an organic spring mix lettuce field in the San Lucas area.

"We had fantastic results. To me, it looks fantastic, but I'm the guy selling it. I wouldn't have brought a $90,000 piece of equipment over from the Netherlands if I didn't think it was promising," Sutton said.

No one has yet been able to replicate in the Salinas Valley the weed control results documented by Dutch researcher Pieter Bleeker in his European trials of CultiClean.

"In their testing in England, Germany and the Netherlands, they were getting up to 80 percent weed control. Over time you get more reduction in weeds," Sutton said.

One key to rivaling these results may be slowing the machine to a crawl, to increase the length of time the soil remains hot enough to kill weed seeds.

"Bleeker ran it at a bunch of different speeds. The slower you go, the hotter the soil gets and the more weeds you kill," Fennimore said.

Another management technique would be to heat the soil when it is wet, which would help to spread the heat.

"The guys from Struik said it works best if you irrigate before you use it. If I do it again, I will water the day before," Fennimore said.

Sutton discovered CultiClean almost by accident, as the result of searching in Europe years ago for a mulcher that could work three 80-inch beds with a single pass.

"Struik is a manufacturer of regular mulchers. They make very heavy duty, high quality mulchers. We met them 10 years ago when we were looking for something that would do three 80-inch beds at a time. The model we first got from Struik four years ago maintains the soil structure and leaves you with three inches of fine soil at the top for a good seed bed. It made a beautiful seed bed and held up very well. The way it mulches is one thing; the way it holds up is more important," Sutton said.

Struik developed the 80-inch mulcher-weeder combination shortly after Sutton began doing business with the firm.

"In the process of bringing that machine in, they came up with this machine that also has a propane blower," Sutton said.

Struik representatives visited the Salinas Valley in February to get a close look at the soil, and sent the CultiClean in May. Since then, Sutton said he has found numerous growers willing to cooperate in trying out the machine.

"There's a lot of interest among growers, partly because this is completely new," Sutton said.

CultiClean interests organic spring mix growers, in particular, because they do not have a lot of options for controlling weeds.

"Growers doing organic spring mix and spinach cultivate and hand weed. Outside of the crop cycle, they try to stop the weeds from going to seed. They do a good job of that around here," Fennimore said.

With increasing restrictions on fumigation, an economical way to heat the top of the soil could also be of some interest to conventional growers.

"It's not like methyl bromide or vapam," Fennimore said. "But I think it will be helpful. It can be improved."

(Bob Johnson is a reporter in Monterey. He may be contacted at bjohn11135@aol.com.)