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.
Season & Zone
Exposure: Full sun
Direct sow mid-April to late May. Starting indoors is possible, but does not gain much time. If sunflowers are allowed to get root-bound, they will lack stability at transplant time, and will require staking. Optimal soil temperature for germination: 21-30°C (70-85°F). Seeds should sprout in 10-14 days.
Sow seeds 5mm-1cm (¼-½”) deep. Space dwarf varieties 15cm (6″) apart, but give the giants lots of room at 1m (3′) between plants.
When the back of the flower head turns yellow, it is ready to be cut from the plant.
Sunflowers planted near rows of corn are said to increase yields. Aphids may be herded by ants onto sunflowers without causing the plants any harm. Sunflowers can be used to draw heavy metals, toxins, and even radiation from soil, but should be burned at the end of the season if grown for this purpose.
More on Companion Planting.