Soil Solarization for Weed and Disease Control in Specialty Crops

OBJECTIVES
The project team has two objectives:

1. To evaluate solarization (or cooking) the soil as a method for weed control in specialty crops where few, if any, herbicides are registered
2. To evaluate multiple-bed solarization (two beds + one furrow) for weed control in crops that are double-row planted (on each side of the bed)

ABSTRACT
To prepare to lay plastic mulch, Mike and Sandie Smith disked and bedded up 5 acres of their farm for the solarization trial. On 4 acres, they pre-irrigated in furrows with siphons then laid the clear plastic mulch, 1.5 mils thick and 84 inches wide, with a plastic-laying machine borrowed from their county small-farm advisor. On the 1-acre block, they laid the plastic first, then irrigated.

The Smiths, who farm on 40-inch rows with two lines of vegetables per row, planted 1.5 acres of parsley, 1 acre of Swiss chard, 1 acre of beets and 1.5 acres of cilantro. The Swiss chard and parsley were slated for multiple harvests. Two rows at a time were covered with plastic. In previous trials, they noticed an "edge effect" on the double-row crops, so they figured that covering multiple beds would help reduce the effect.

In both blocks, the beds were solarized (covered with plastic) for four weeks and the moisture was 70-75% at the beginning of solarization.

SPECIFIC RESULTS
In the 4-acre field, planted to cilantro, Swiss chard and beets, the Smiths saw good weed control, and the field remained fairly clean through harvest. Watergrass (barnyardgrass) was a problem, but other broadleaves and grasses were controlled. They estimated $3,500 saved in weeding costs, compared with the cost of the plastic of just under $2,000 for the 4 acres (97 cents a pound at around 500 pounds an acre).

In the 1-acre field, planted to parsley and cilantro, weed control was unacceptable. The weeds outside and next to the plastic flourished, a problem that could crimp their goal of keeping that ground in organic production.

"For some unknown reason, the solarization did not do so well in this area, and the parsley crop we planted was a total loss," say the Smiths.

They learned several lessons from the project. The soil moisture must be adequate, and the plastic must be applied at peak heat to kill the dormant and germinating weed seeds. Puncture-free plastic is also important for good solarization, which means keeping animals off the beds. Also, the plastic can only be used once, and disposal becomes a problem because of the fee the rubbish company charges to deliver extra dumpsters.

POTENTIAL BENEFITS
On the plus side, producers using this system can reduce weeding and herbicide costs. But for a small farmer with limited ground, having the ground out production for two-and-a-half months for pre- and post-solarization work can be a problem.

DISSEMINATION OF FINDINGS
The results were presented at the 2001 California Weed Conference in Monterey, Calif. A field day was announced over the Hmong radio station, prompting six people to visit the site and learn about the process. Further discussions about the project results were planned at various meetings of the local farm advisor.

FARMER REACTIONS
The Smiths are hoping the solarization works better this year, when they apply more plastic with a few changes. They acknowledged with thanks the help of the SARE grant program.