Helianthus annuus, commonly known as the sunflower, is an iconic plant widely recognized for its large, bright yellow flowers and tall stature. In the United States, it is often referred to simply as “sunflower,” but it also goes by other names such as common sunflower and annual sunflower. This plant is valued for its aesthetic appeal as an ornamental species, its utility in agriculture as a source of sunflower oil and seeds, and its ecological role in supporting wildlife. Sunflowers are also popular among hobbyists for their ease of growth and the cheerful ambiance they bring to gardens.
I. Appearance and Characteristics
The common sunflower (Helianthus annuus) is a species of large annual for the genus Helianthus. It is commonly grown as a crop for its edible oily seeds. Apart from cooking oil production, it is also used as livestock forage (as a meal or a silage plant), as bird food, in some industrial applications, and as an ornamental in domestic gardens. Wild H. annuus is a widely branched annual plant with many flower heads. The domestic sunflower, however, often possesses only a single large inflorescence (flower head) atop an unbranched stem.
Helianthus annuus (common sunflower) belong to the dicotyledonous category, distinguishing them from monocots. As dicots, sunflowers possess embryos with two veins, known as cotyledons, a characteristic feature that sets them apart. The veins in their leaves exhibit a net-like pattern, in contrast to monocots, which typically display parallel leaf veins.
The plant has an erect rough-hairy stem, reaching typical heights of 3 meters (10 feet). The tallest sunflower on record achieved 9.17 m (30 ft 1 in). Sunflower leaves are broad, coarsely toothed, rough and mostly alternate; those near the bottom are largest and commonly heart-shaped.
They plant flowers in summer. What is often called the “flower” of the sunflower is actually a “flower head” (pseudanthium), 7.5–12.5 centimeters (3–5 in) wide, of numerous small individual five-petaled flowers (“florets”). 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 spirally arranged flowers in the center of the head are called disk flowers. These mature into fruit (sunflower “seeds”).
The prairie sunflower (H. petiolaris) is similar in appearance to the wild common sunflower; the scales in its central disk are tipped by white hairs.
The plant was first domesticated in the Americas. Sunflowers grow best in fertile, moist, well-drained soil with heavy mulch. They often appear on dry open areas and foothills. Outside of cultivation, the common sunflower is found on moist clay based soils in areas with climates similar to Texas. In contrast the related Helianthus debilis and Helianthus petiolaris are found on drier, sandier soils.
The precise native range is difficult to determine. According to Plants of the World Online (POWO) it is native to Arizona, California, and Nevada in the present day United States and to all parts of Mexico except the Gulf Coast and southeast. Though not giving as great of detail, the Missouri Botanical Garden Plant Finder also lists it as native to the Western United States and Canada. The information published by the Biota of North America Program (BONAP) largely agrees with this, showing the common sunflower as native to states west of the Mississippi, though also listed as a noxious weed in Iowa, Minnesota, and Texas. Regardless of its original range it can now be found in almost every part of the world that is not tropical, desert, or tundra.
II. How to Grow and Care
Sunlight
For the best flowering and sturdiest stems, plant your sunflowers in full sun, meaning at least six hours of direct sunlight per day. Because they are heliotropic (their flower heads follow the sun), ample sun exposure will help sunflowers remain straight rather than bending toward the direction where the light is strongest.
Temperature and Humidity
Optimal temperatures for growing sunflowers are between 70 and 78 F. Still, they tolerate high heat as long as their moisture needs are met. Sunflowers can handle somewhat chilly but sunny environments. They also tolerate high humidity but must have well-draining soil and good air circulation to prevent root rot and other diseases.
Watering
Although tolerant of dry conditions, watering sunflowers regularly promotes blooming, as flowering often reduces during periods of drought. Allow the top 1 to 2 inches of soil to dry out between waterings. If your sunflowers are drooping and the soil is dry, that’s often a sign they need more water.
Soil
Sunflowers will grow in almost any soil, including poor, dry soils. However, they thrive in well-drained soil that contains a good amount of organic matter.
Fertilizing
Sunflowers appreciate a fertilizer high in phosphorus and potassium. If you have rich, loamy soil, you likely will not need to supplement with fertilizer. But if you have poor soil, apply a slow-release fertilizer starting in the spring, following label instructions. Be careful not to overfeed your plants, as it can cause the stalks to become spindly.
Pruning
Typically sunflowers don’t require pruning but, if grown in clusters, can benefit from occasional trimming. You can trim perennial sunflowers twice a year with pruning shears, once in late spring and once in summer. You should cut annual sunflowers to the ground once they show signs of browning and drying.
Propagation
Sunflowers can be propagated by taking cuttings and rooting them. The main benefit of propagating from cuttings is to create genetic uniformity in your garden. It’s best to take cuttings before the season’s growth has matured. Still, propagating with seeds is easier. Here’s how to propagate sunflowers from cuttings:
- With pruning shears or a sharp knife, cut a 4- to 6-inch stem with no flowers or buds.
- Remove the lowest leaves to expose nodes, and remove the top 1/2 inch of the cutting, leaving two terminal leaves behind.
- Remove dust from the leafless section of the cutting and apply the rooting hormone.
- Place the cutting in a sand and peat moss mix, ensuring that the leafless stem section is buried below the medium.
- Keep the cutting in a warm place with light shade and cover it with a plastic bag.
- Check the cuttings for roots after two to four weeks.
- Relocate the cutting outdoors into light shade for a week before transporting into full sun.
How to Grow from Seed
Sunflowers are usually grown from seed. Seeds can be sown directly into the garden once the threat of frost has passed in the spring, or they can be started indoors three to four weeks before transplanting into the garden. Peat or paper pots that ultimately can be planted directly into the ground are recommended for seedlings started indoors, as sunflowers don’t like to have their roots disturbed.
Plant seeds in the garden 1 to 2 inches deep in a shallow trench about 6 inches apart. Keep them lightly moist (but not soggy) until they sprout, which takes around seven to 10 days. Taller varieties should be thinned to about 1 to 1.5 feet apart. Dwarf varieties can be grown about a foot apart.
Potting and Repotting
To pot a sunflower, select a pot suitable for its size. Fill the pot or container with loose potting mix and organic matter and place it in direct sunlight. Generally, you may use a 7-gallon to 10-gallon pot or plastic container with good drainage.
Overwintering
Sunflowers are sensitive to frost and may need to be overwintered indoors. You can keep your sunflower in a warm, bright, dry location indoors throughout winter. Sunflowers can survive winters, but their flowering will likely be duller in color.
Pests and Diseases
Common Pests & Diseases
Several pests and diseases can affect sunflowers. Birds and rodents are very fond of the seeds. So if you’d like to save seeds for planting, cover the flower heads with netting to protect them from pests. Moreover, the sunflower moth lays its eggs on the plant, and the larvae feed on the flower heads, tunneling and leaving holes in the seeds. Pesticides can help to control the moths.
Similarly, you might have issues with beetles or caterpillars eating foliage. Sunflowers also can be prone to fungal diseases, including powdery mildew and rust. To avoid such conditions, provide adequate air circulation around your plants, and apply a garden fungicide as soon as you spot the first signs of infection.
Common Problems With Sunflowers
While sunflowers are a generally easygoing plant, they are still susceptible to some common issues.
Discoloration
Sunflowers can develop brown and black sores on their stems and leaves, often in humid months. This discoloration, called stem canker, can kill the plant quickly if unaddressed. Begin by applying a heavy-duty fungicide, but if the problem doesn’t resolve, you should destroy the sunflowers by burning them or removing them from the root.
Leaf Spot
Sunflower leaves will sometimes display black or brown spots, called leaf spot. Affected leaves will develop spots and then drop from the plant. You can try treating this disease with garlic spray or diluted apple cider vinegar, but if the treatments aren’t successful, you will need to remove the leaves or the plant altogether.
III. Uses and Benefits
Sunflower “whole seed” (fruit) are sold as a snack food, raw or after roasting in ovens, with or without salt and/or seasonings added. Sunflower seeds can be processed into a peanut butter alternative, sunflower butter. It is also sold as food for birds and can be used directly in cooking and salads. Native Americans had multiple uses for sunflowers in the past, such as in bread, medical ointments, dyes and body paints.
Sunflower oil, extracted from the seeds, is used for cooking, as a carrier oil and to produce margarine and biodiesel, as it is cheaper than olive oil. A range of sunflower varieties exist with differing fatty acid compositions; some “high-oleic” types contain a higher level of monounsaturated fats in their oil than even olive oil. The oil is also sometimes used in soap. After World War I, during the Russian Civil War, people in Ukraine used sunflower seed oil in lamps as a substitute for kerosene due to shortages. The light from such a lamp has been described as “miserable” and “smoky.”
The cake remaining after the seeds have been processed for oil is used as a livestock feed. The hulls resulting from the dehulling of the seeds before oil extraction can also be fed to domestic animals. Some recently developed cultivars have drooping heads. These cultivars are less attractive to gardeners growing the flowers as ornamental plants, but appeal to farmers, because they reduce bird damage and losses from some plant diseases. Sunflowers also produce latex, and are the subject of experiments to improve their suitability as an alternative crop for producing hypoallergenic rubber.
Traditionally, several Native American groups planted sunflowers on the north edges of their gardens as a “fourth sister” to the better-known three sisters combination of corn, beans, and squash. Annual species are often planted for their allelopathic properties. It was also used by Native Americans to dress hair. Among the Zuni people, the fresh or dried root is chewed by the medicine man before sucking venom from a snakebite and applying a poultice to the wound. This compound poultice of the root is applied with much ceremony to rattlesnake bites.
However, for commercial farmers growing other commodity crops, the wild sunflower is often considered a weed. Especially in the Midwestern US, wild (perennial) species are often found in corn and soybean fields and can decrease yields. The decrease in yield can be attributed to the production of phenolic compounds which are used to reduce competition for nutrients in nutrient-poor growing areas of the common sunflower.
IV. Harvesting and Storage
To harvest, cut the whole flower head with about 1 foot of stem attached, and hang it in a warm, dry, ventilated spot away from insects and rodents. Cover the seed heads with cheesecloth or a paper bag to catch loose seeds; poke small holes in the cover for ventilation.
When the seeds are completely dry, they can be easily rubbed off the flower head and collected. Select some of the largest, plumpest seeds with the best chance of germinating, and store them in a dry, cool location until spring planting time.