Helianthus annuus. Inconspicuous Giganteus sunflower seeds produce absolutely huge plants in one short season. This is the giant among sunflowers, with towering stems up to 3.6m (12') tall, as thick as a man's leg. At the top grows a monumental flower head up to 45cm (18") across, heavy with edible seeds. The seed heads tend to nod forward at maturity, facing down. Be sure to get the seeds planted early so the seed heads can mature before the wet weather arrives in September. It's a good idea to direct sow (as opposed to transplanting) these enormous plants so their root growth is not interrupted. They need a solid anchor in order to keep them standing upright.
Sunflowers will grow in just about any well-drained soil, and they are drought tolerant. To grow sunflowers from seed, to grow really big plants with really big flowers, enrich the soil with organic matter and incorporate 2 cups of complete organic fertilizer per 3m (10′) of row, and irrigate regularly.