Microalgae are a rich source of plant hormones, vitamins, and other substances that can influence plant physiological metabolism, which in turn affects plant development, biotic and abiotic stress resistance, and yield. This study aimed at testing microalgae Planktochlorella nurekis clones obtained by co-treatment with colchicine and cytochalasin on four plant species to check their potential use as biostimulators in agriculture. The results are valuable for breeders, farmers, and microgreen producers. Eleven clone extracts in 1%, 5%, and 10% concentration were tested on four plant species: lettuce, wheat, broccoli, and radish. Germination and seedling characteristics (leaf and root length, fresh weight) were measured for each species. P. nurekis extracts show both a stimulating and inhibitory effect on tested plants, depending on the tested concentration, plant species, and algal clone tested. Co-treatment with colchicine and cytochalasin may be a good source of clones for potential use in agriculture as biostimulators and herbicides.