Common Sunflower

Appearance

The plant has an erect rough-hairy stem, reaching typical heights of 3 metres. The tallest sunflower on record achieved 9.17 metres. Sunflower leaves are broad, coarsely toothed, rough and mostly alternate. What is often called the "flower" of the sunflower is actually a "flower head" or pseudanthium of numerous small individual five-petaled flowers.

The outer flowers, which resemble petals, are called ray flowers. Each "petal" consists of a ligule composed of fused petals of an asymmetrical ray flower. They are sexually sterile and may be yellow, red, orange, or other colors. The flowers in the center of the head are called disk flowers. These mature into fruit. The disk flowers are arranged spirally. Generally, each floret is oriented toward the next by approximately the golden angle, 137.5°, producing a pattern of interconnecting spirals, where the number of left spirals and the number of right spirals are successive Fibonacci numbers.

Typically, there are 34 spirals in one direction and 55 in the other; however, in a very large sunflower head there could be 89 in one direction and 144 in the other. This pattern produces the most efficient packing of seeds mathematically possible within the flower head.

Most cultivars of sunflower are variants of "Helianthus annuus", but four other species are also domesticated. This includes "H. tuberosus", the Jerusalem artichoke, which produces edible tubers.

Naming

There are many species in the sunflower genus "Helianthus", and many species in other genera that may be called sunflowers.

⤷  The Maximillian sunflower is one of 38 species of perennial sunflower native to North America. The Land Institute and other breeding programs are currently exploring the potential for these as a perennial seed crop.
⤷  The sunchoke is related to the sunflower, another example of perennial sunflower.
⤷  The Mexican sunflower is "Tithonia rotundifolia". It is only very distantly related to North American sunflowers.
⤷  False sunflower refers to plants of the genus "Heliopsis".

Predators

One of the major threats that sunflowers face today is Fusarium, a filamentous fungus that is found largely in soil and plants. It is a pathogen that over the years has caused an increasing amount of damage and loss of sunflower crops, some as extensive as 80 percent of damaged crops.

Downy mildew is another disease to which sunflowers are susceptible. Its susceptibility to downy mildew is particular high due to the sunflower's way of growth and development. Sunflower seeds are generally planted only an inch deep in the ground. When such shallow planting is done in moist and soaked earth or soil, it increases the chances of diseases such as downy mildew.

Another major threat to sunflower crops is broomrape, a parasite that attacks the root of the sunflower and causes extensive damage to sunflower crops, as high as 100 percent.