Combined planting of submerged macrophytes could be an effective way of controlling algal blooms in landscape waters. In this study, the algal inhibition of single and combined planting of Hydrilla verticillata (A) and Myriophyllum spicatum (B) were explored. The optimized combined planting conditions were investigated using the central composite design. The results showed that the combined planting had a synergistic algal-inhibiting effect. Its inhibition (I (K)) is about 10.8% higher than that of single planting with the same density. The synergism of the combined planting may be due to the different ways in which the two plants inhibit the algal growth. H. verticillata inhibited the algal biomass and M. spicatum inhibited the algal specific growth rate. When the density of H. verticillata and M. spicatum were 7.2 g/L and 6.7 g/L, the value of I (K) reached a maximum 92.2%. Although increasing planting density would improve the algal inhibition, high planting density was not beneficial for the growth of plants. Moreover, no further significant improvement was shown with the increasing planting density when the value of I(K) was higher than 90%. Therefore, the cost-effective combined macrophyte density was 11.6 g/L and the value of A/B ranged from 1.05 to 1.07, where the value of I (K) could achieve 90%. This study can provide a practical basis for using macrophytes to control algal blooms.

Knowledge of planting density and lodging resistance is essential to developing effective cropping systems. This study determined (i) the lodging resistance of different genotypes of oilseed flax (Linum usitatissimum L.) under different planting densities, and (ii) the effect of density on lignin content and related enzyme activities in different genotypes in Northwest China. We hypothesized that (i) lignin content and related enzyme activities varied with different planting densities, and (ii) a planting density of 750 × 104 grains ha−1 increased the lignin content and related enzyme activities of stem tissues and improved lodging resistance. The characteristics of oilseed flax from three different cultivars—‘Dingya 23’(V1), ‘Jinya 10’(V2), and ‘Neiya 9’(V3)—were evaluated under different planting densities. Results indicated that increasing plant density increased the phenylalanine ammonia‐lyase (PAL) and tyrosine ammonia‐lyase (TAL) activities but decreased the 4‐coumarate:CoA ligase (4CL) activity and lignin content of a high‐lodging‐resistance cultivar. However, increasing plant density decreased the PAL and TAL activities but increased the 4CL and lignin content of a low‐lodging‐resistance cultivar. We conclude that plant density affected the plant lignin content by regulating the 4CL activity. Flax seed yield increases with density were mainly due to increased lignin content, bending strength, and regulated PAL, TAL, and 4CL activities. The study concluded that optimizing planting density could play an important role in crop lodging resistance and grain yields in the northwest of China.