Seaweed polysaccharides have been proven to achieve excellent results in plant growth parameters, when compared to synthetic commercial fertilizers. When applied to the soil directly or sprayed on the foliage, seaweed poly- and oligosaccharides can improve plant vigor, increase the uptake of soil nutrients, and protect plants against several abiotic and biotic stresses, by stimulating a plant to produce secondary metabolites and manage its defense pathways. In this study, three different polysaccharides (alginate, agar and carrageenan) were extracted from one brown seaweed, Saccorhiza polyschides, and two red seaweeds, Gracilaria gracilis and Chondrus crispus, respectively, with intention to analyze their impact on growth, development and metabolism of turnip greens (Brassica napus L.) plants. Altogether, the turnip plants treated with the carrageenan solutions of Chondrus crispus showed the best results in improving the crop’s productivity, particularly, λ-carrageenan extracted from the tetrasporophyte generation of Chondrus crispus, which had the highest bioactivity and positive effect in turnip plants among all treatments. λ-carrageenan has shown that can improve plant growth, increase plant’s biomass, and root system, enhance photosynthetic activity and essential soil nutrient uptake.