How to Grow and Care for Sunflowers

Sunflowers (Helianthus annuus) are a treasure of summertime. They typically begin blooming in the mid-summer and can persist into early fall. The flowers, which stretch around 3 to 6 inches across on average, have a broad central disk surrounded by short, yellow petals. Sunflowers grow on a hairy, sturdy, upright stem that can be several feet high. The stems can hold a single flower or be branched with multiple blooms. Rough, hairy, oval to triangular leaves grow along the stem. These flowers are annual, meaning they complete their growth cycle in one year. You should plant them in the spring after the garden soil has warmed to at least 55 degrees Fahrenheit.

Sunflower Care

The only mandatory requirements for sunflowers are a sunny location and well-drained soil. Select an area sheltered from strong winds that might topple these tall plants if possible. It can be helpful to plant sunflowers in groups to support each other against wind and rain. Growers often have to stake their sunflowers, especially the tall varieties, to keep them growing upright. The plants can become very top-heavy when in bloom. Planting sunflowers along a fence is the easiest way to stake them. Bamboo stakes are also strong enough to keep them upright. Use care when inserting the stakes so you don't damage the plant's roots.

Sunflowers don't like to compete with weeds, so keep the garden clean. Mulching around your sunflowers will help maintain soil moisture and weed suppression. Furthermore, wildlife is often attracted to sunflower seedlings. So it's best to protect seedlings with row covers or screening, removing the cover once the plants are 1 to 2 feet tall.

Light

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.

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.

Water

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.

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. 

Fertilizer

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.

Types of Sunflowers

• Helianthus annuus 'Russian Giant'
• Helianthus annuus 'Teddy Bear'
• Helianthus annuus 'Giant Sungold'
• Helianthus annuus 'Autumn Mix'
• Helianthus annuus 'Italian White'

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.

Propagating Sunflowers

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:

1. With pruning shears or a sharp knife, cut a 4- to 6-inch stem with no flowers or buds.
2. Remove the lowest leaves to expose nodes, and remove the top 1/2 inch of the cutting, leaving two terminal leaves behind.
3. Remove dust from the leafless section of the cutting and apply the rooting hormone.
4. Place the cutting in a sand and peat moss mix, ensuring that the leafless stem section is buried below the medium.
5. Keep the cutting in a warm place with light shade and cover it with a plastic bag.
6. Check the cuttings for roots after two to four weeks.
7. Relocate the cutting outdoors into light shade for a week before transporting into full sun.

How to Grow Sunflowers 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.

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.

Potting and Repotting Sunflowers

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.

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.

How to Get Sunflowers to Bloom

Sunflowers bloom with vibrant yellow petals and a sweet, woody scent in the summer. Perennial sunflowers bloom for 8 to 12 weeks, from early summer through late autumn. You can encourage bloom by giving your sunflower ample full sun and watering regularly. Handle the flowers very gently while they bloom. It may be helpful to deadhead your sunflowers until the end of the season.

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.