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. When the plants are 30cm (12) tall, dissolve 5ml (1 tsp) of borax (for boron) in 350 ml (12 fl oz) of water and spread the solution over 5m (15′) of row. Be careful not to over-apply this solution. This application of boron will produce big seeds and flower heads. Do not apply boron to other garden plants. Annual flowers that can grow to incredible heights in just one season.