The broccoli and lettuce in these beds are intercropped to increase the yield of each.
From research by Aoki, M., A. Gilbert, A. Hull, M. Karlberg, and M. MacDonald. 1989, University of California, Santa Cruz.
“The yields of broccoli and lettuce grown together at three different planting densities were compared with yields from their conventional monocrops. Results demonstrate that broccoli/lettuce intercropping results in higher yields than can be produced by monocultures of lettuce and broccoli grown on the same area of land, a process known as overyielding. All three intercrop densities produced higher total yields than the combined monocrops, ranging between 10% and 36% greater yield. Lettuce heads in the intercrop were of slightly lower mean weight, but the combined production still exceeded the total that was produced by a combination of monocrops on the same amount of land.”