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  • Solarization for Non-native Plant Control in Cool, Coastal California
  • Susan C. Lambrecht (bio) and Antonia D'Amore (bio)

Invasive plants are a worldwide threat to native flora. Owing to the magnitude of this problem, land managers need large-scale control methods with relatively little investment of time. While often used, herbicide is not an option in many situations. For example, the presence of threatened or endangered species that may be harmed by chemicals or landowner policy can limit chemical use. Thus mechanical control of weeds such as repeat mowing and disking is highly desirable, as these methods can be used broadly. Solarization with plastic is another method, despite controversy regarding its effectiveness (e.g., Bainbridge 1990, Marushia and Allen 2009). It has been argued that cool climate regions, such as central coastal California, do not have warm enough temperatures for this method to produce satisfactory results (Smith et al. 2000, Stapleton et al. 2005).

In order to gauge the relative efficacy of mowing twice, disking, and solarization in controlling invasive weeds, we set up experimental plots as part of an ongoing restoration project. The two specific objectives were to test whether solarization could be used effectively for weed control in a cool, coastal region, and to compare this approach with other methods of weed control in terms of effectiveness, cost, and time required for implementation.

Our study was conducted on the Triple M Ranch of the Agriculture and Land-Based Training Association (ALBA) in Monterey County (36.9°N, 121.6°W; elev. 7 m). This 24 ha ranch, located within the Elkhorn Slough watershed, is an organic farm used to train farmers about soil and water conservation practices, as well as habitat conservation for native species. About one-third of the ranch is in cultivation; the remainder is held in conservation easements.

Our study was located within a 1 ha fallow field that had not been cultivated for ten years. Non-native annual grasses dominated the vegetation, primarily ryegrass (Lolium perenne ssp. multiflorum), but also soft brome (Bromus hordeaceus), oats (Avena spp.), and foxtail barley (Hordeum murinum). Mustard (Brassica rapa) and radish (Raphanus sativus) were also common non-native weeds. The site is about 7 km from the ocean, so it experiences a strong maritime influence. Average annual temperature is 13°C with minimal seasonal variation (11°C winter/spring and 15°C summer/fall). Ninety percent of precipitation falls during the winter and spring, but considerable fog contributes to soil moisture during summer. [End Page 424]

The approximately 1 ha study area was divided randomly among three treatments: disking, two mowings, and solarization. Treatment areas were 9 m wide, which is the width of the tractor mowing and disking attachments, and 140 m long, the length of the field. In 2008, to prepare the field for treatment, it was mowed in its entirety April 4 and July 5 to coincide with weed seed set, which occurs during the late spring and summer. Then both the disking and solarization treatment areas were disked by tractor on July 16. The solarization treatment area was watered to a depth of 15 cm to terminate seed dormancy and then covered with 3 mm transparent plastic film (following Duff and Connelly 1993) from August 6 to November 3 (12.5 weeks total).

We sampled plant cover before (spring 2008) and after (September 2009) implementing the treatments to assess the effectiveness of each in controlling weeds without further manipulation. Four randomly selected 1 × 1.5 m plots (≥ 1 m from edge) were sampled within each treatment area. In each plot, we measured relative percent cover in three random locations, using a 0.25 m2 frame with a grid of 25 sampling points. We returned to the same locations for posttreatment sampling.

We used a partially nested analysis of variance model with sampling grids nested within plots. To account for potential difference in plant size because of the different sampling times for the before and after measures, we used relative percent cover as our response variable. However, plant growth is minimal during the summer drought months. We used the time × treatment interaction to test for differences in relative cover among the treatments. The...

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