Hand-loaded cell-type carrousel transplant implements used in
this region have several carrousels, each that is loaded by one person
and which plants a single row. Therefore, intercropping treatments
A100 and L50A50D would be the easiest to plant because
plant type is constant within a row, whereas the other treatments
with alternating arrangements within a row would not be practical
on a commercial scale, especially those that alternated every other
plant (i.e. L50A50S and L100 + A100). However, an alternative to
the strip intercropping treatments evaluated here would be to
intersperse alyssum in all rows of lettuce throughout the field.
For example, to achieve a density of approximately seven alyssum
open inflorescences per head of lettuce, as in L50A50S and
L50A50D where 4 percent of the field was alyssum transplants,
the 2667 alyssum transplants per ha could be distributed across
all 196 rows (two rows per bed, and 98 beds per ha) where one
alyssum transplant would occur approximately every 7.3 m or
after every 24 lettuce plants in each row. To implement this, four
alyssum transplants could be interspersed in a transplant tray with
100 cells of lettuce. This approach is currently being used by some
growers in the region as an alternative to strip intercropping. This
alternative approach may have several potential advantages. First,
it could minimize competition between lettuce and alyssum by
spreading alyssum though the whole field thus reducing the number
of small heads. Any small heads would be more evenly distributed
through the field so that they could be conveniently mixed in
boxes with larger lettuce heads to ensure the box still meets the
minimum box weight. Second, it would distribute the alyssum pollen
and nectar for adult hoverflies more evenly throughout the
field rather than in concentrated strips. This may facilitate adult
hoverfly movement throughout the field; however, studies have
not evaluated if there are differences in biological control of aphids
in lettuce fields with concentrated strip versus scattered plantings
of alyssum. Third, it would improve weed management by eliminating
concentrated strips of alyssum that are difficult to hand
weed; concentrated strips of alyssum are difficult to hand weed
because it is difficult to see the weeds between adjacent alyssum
plants within a row. Weed management is expensive in organic
systems in this region and weeds that escape hand weeding can
produce seed that infest future crops and increase weeding costs.
In a long term rotational study with organic vegetables, weed densities
were often higher on beds that have been used repeatedly for
concentrated strips of alyssum such as A100 (Brennan, unpublished
data). Fourth, scattering alyssum insectary plants throughout
a field may minimize post-harvest tillage requirements
because alyssum residue would be more evenly distributed