Alyssum shoots averaged 31 g per transplant in the replacement
treatments and were 12 g larger than the average 19 g per
transplant in the additive treatments (F1,35.5 = 57.2, P < 0.001)
(Fig. 2A). Compared with A100, alyssum shoot DM was lower in
the additive treatments but not in the replacement treatments
(Dunnett’s test, P 6 0.001). Based on the overlapping confidence
intervals, alyssum shoot DM did not appear to differ between the
additive treatments (L100 + A100, L100 + A30) indicating that alyssum
growth was affected by lettuce density but not total transplant
density that was higher in L100 + A100 (26 transplants per
3 m of bed) versus L100 + A30 (40 transplants per 3 m of bed) (Table
1). Although not quantified, the canopies of alyssum and lettuce
within a bed began to overlap at approximately 22 d after
transplanting in the additive treatments compared with 10 d later
in the replacement treatments; this is illustrated with the replacement
treatment L50A50S and the additive treatment L100 + A100
(Fig. 3). This figure and the lower alyssum DM in the additive than
replacement treatments (Fig. 2A) suggest that competition between
alyssum and lettuce in the additive treatments began earlier
in the season and was more intense than occurred in the replacement
intercropping treatments.