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.