Sunflowers are the largest known family of flowering plants. "If all the known species of flowering plants on Earth were lined up randomly, every fourth one would belong to the sunflower, orchid or legume family," according to Dr. Wayne Armstrong of Palomar College. Sunflower varieties include annuals that bloom once per year and perennials that return to bloom each year. Sunflowers bloom in the warm summer months from July to September. Variation in bloom times is indicated on seed packets for specific varieties.
Asteraceae "Pacino," "Sunspot," "Del Sol," "Dwarf Yellow Spray" and "Double Dandy" are sunflower varieties that bloom in the early months of summer. In warmer western growing zones this is mid-June and in cooler areas it is July. Sunflowers grow in a wide range of soil types from clay to sand. Seeds germinate in temperatures as low as 3.89 degrees Celsius (39 degrees Fahrenheit), but temperatures of 7.78 to 10 degrees Celsius (46 to 50 degrees Fahrenheit) are optimum. They grow and thrive as young plants when temperatures are 21 and 25.5 degrees Celsius (70 and 78 degrees Fahrenheit), but are tolerant of a range between 17.8 and 37.8 degrees Celsius (64 and 91 degrees Fahrenheit). Seeds are planted in spring, covered by 2.5 cm (1 inch) of soil, and germinate in 5 to 10 days.
Most commercially available sunflower seeds and plants bloom in midsummer. The red-orange flowers of "Claret," dark red "Moulin Rouge" and gold "Sunny" variety sunflowers grow to 1.5 m (5 feet) in height. August-blooming "Ring of Fire" and "Lemonade" grow to 1.2 m (4 feet) in height and have light and dark yellow petals. The dwarf double-petalled gold "Teddy Bear" is a 60 cm (2 foot) tall plant that blooms midseason. Sunflower plants need 2.5 cm (1 inch) of water per week, by rainfall or irrigation. As the flowers begin to develop and open, they turn to face the sun during the day. When their stems become woody they stop turning. Their large disc flowers are comprised of hundred of tiny flowers clustered together. After pollination each tiny flower produces a sunflower seed.
Sunflowers that bloom in late summer and early autumn include cultivars such as gold double "Titanic," light yellow "Valentine," bright gold double "Giant Sungold" and the dark red "Chianti." Valentine has a long bloom period with flowers that last until frost. Titanic is often the last sunflower in the garden to bloom. Good soil drainage is important for growing sunflowers and soil enriched with organic matter compost creates optimum growing conditions. They require medium to high levels of the 13 nutrients all plants need to thrive.
Sunflowers and bees
Sunflowers attract "specialist bees", according to entomologists at the University of California Berkeley. Specialist bees forage for pollen on only one plant species, such as sunflowers. They emerge from their nests at the same time sunflowers begin to bloom. Generalist bees visit and pollinate many species of plants and flowers. The sunflower bee (Diadasia enavata) is attracted by the large amount of pollen created by sunflowers, which they carry from plant to plant. Generalist and specialist bees are responsible for pollinating one-third of all plant species.