Figure 3. Cumulative degree days and precipitation between 1 June and 31 October, 2001 (dashed lines) and 2002 (solid lines). Degree days calculated in metric using the single sine method with a 10 circC cutoff.

The RCI in mixtures was always positive for one crop and negative for the other (Figure 5), meaning that one crop faced less competition in the mixture than in pure stands, and the other faced more. Brussels sprout plants faced less competition in mixtures than in pure stands; basil faced more competition in mixtures; and tomato faced more competition when paired with Brussels sprout, but less when paired with basil (Figure 5). The sum of RCIs for both crops in a mixture tended to be positive in 2001, but negative in 2002 (Figure 5), suggesting that net inter-specific competition was less than net intra-specific competition in 2002 only.

Basil. Basil yields averaged 1.4 and 3.4 kg m-2 in 2001 and 2002, respectively. The contribution of differing resource levels to this difference was confounded with the effect of reduced harvest frequency, allowing the basil canopy to fill in more in 2002 than in 2001 (Figure 4). Individual basil plants yielded more in pure stands than in beds with Brussels sprout companions (Table 2). Basil plants with tomato companions produced less biomass than plants in monoculture in 2001, but differences were not significant in 2002 (Table 2). Basil yields were five times greater in pure stands than in mixed plantings (Table 2).

Relative basil leaf chlorophyll content, measured in SPAD units at season end, was correlated with basil plant weight at final harvest when years were pooled (r = 0.91, d.f. = 20, P<0.001) and when data from 2001 (r = 0.90, d.f. = 8, P<0.001) and 2002 (r = 0.82, d.f. = 11, P<0.002) were analyzed independently (Figure 6). Relative leaf chlorophyll content and basil yield were both higher in 2002 than in 2001. In both years the highest values were found in pure stands and the lowest were found in Brussels sprout mixtures (Figure 6).