Semester

Spring

Date of Graduation

2004

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Horticulture

Committee Chair

Linda Butler.

Abstract

Companion planting is a small-scale intercropping practice often associated with organic or biodynamic gardening. Two garden-scale studies tested popular companion planting claims by comparing garden beds devoted entirely to one of three or more test crops (monocultures) to all possible two-crop mixtures (dicultures) of the same species. A third study evaluated effects of planting density and crop ratio in three dicultures using a novel experimental design to create gradients in both factors. All studies incorporated basil ( Ocimum basilicum L.), Brussels sprout (Brassica oleracea L.), and tomato (Lycopersicum esculentum Mill.). A preliminary study also included snap bean (Phaseolus vulgaris L.), radish (Raphanus sativus L.) before Brussels sprout, and dicultures of tomato and Brussels sprout with a white clover ( Trifolium repens L.) living mulch. Double blind taste tests over three years showed no consistent preference for tomatoes grown with companions over those grown in monoculture. An apparent inhibitory effect of companion planting on some pests of Brussels sprout (e.g. imported cabbageworm, Pieris rapae L.; striped flea beetle, Phylollotreta striolata Fab.) in the first study was reversed in the second study when earlier planting of Brussels sprout allowed it to compete more effectively with its companions. Relative yield indices calculated for a range of densities (1.1--47.2 plants/m2) and crop ratios indicated advantages (x¯ = 20%) to planting either tomato or Brussels sprout with basil companions, but no advantage to planting tomato and Brussels sprout together. The highest yields in tomato, basil, and Brussels sprout monocultures occurred at inter-plant spacings of 25, 25 and 40 cm respectively, suggesting advantages to high-density planting. Yield advantages to diculture were most pronounced at the highest densities tested, and in dicultures incorporating the highest proportions of basil. Canopy light absorption and soil moisture content were inversely correlated, and the use of light and water resources was correlated with plant density and biomass production. I conclude that garden-scale intercropping can offer advantages over monoculture, but these are not achieved simply by combining certain compatible companion species. Crop density, ratio, and relative planting times all affect the way that companion species interact with one another and their environment.*.;*This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Adobe Acrobat; Microsoft Office.

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