Thinleaf sunflower (Helianthus decapetalus) is indigenous to central and eastern North America. A multitude of butterfly species use this plant to feed their larvae, including the silvery checkspot and the painted lady. Muskrats feast on the stems and leaves and gather the stems to use them for building their lodges.
I. Appearance and Characteristics
Helianthus decapetalus, known by the common names thinleaf sunflower and thin-leaved sunflower, is a perennial forb in the family Asteraceae. It is native to the Eastern and Central United States and Canada, from New Brunswick west to Iowa, Wisconsin, and Ontario, south as far as Georgia and Louisiana. It produces yellow composite flowers in late summer or early fall.
The name decapetalus means “with ten petals”, referring to the 8-12 ray florets on the flower heads, resembling petals.
The smooth slender stem of H. decapetalus is 60 to 200 cm (2 to 7 ft) tall and branched near the top. The ovate or lanceolate leaves are borne on 2- to 5-cm-long petioles and have serrated edges. They are 7 to 21 cm long and 4 to 10 cm wide. It has three to 10 flowerheads; each flowerhead is composed of 21 to 50 disk florets, and eight to 12 ray florets, which are 2.0 to 2.5 cm long. The bracts are typically 11 to 16 mm long, surpassing the flower disk by at least half their length. The fruit are 3.5- to 5.0-mm-long cypselae with a pappus of two scales.
In Virginia, it grows in habitats such as floodplain forests and riverbanks. Its native habitats include woodland and forest edges. The presence of this species is dependent on appropriate habitat, and it may be eliminated from an area by development, changes in land use, or competition with invasive species.
The flowers attract many kinds of insects, including bees and butterflies, some of which, such as the painted lady and the silvery checkerspot, use the plant as a larval host. The seeds provide a source of food for birds. Muskrats eat the leaves and stems and use the stems in the construction of their lodges.
II. How to Grow and Care
Sunlight
The seedlings, leaves, and flower heads of sunflowers all exhibit very strong phototropism. Sufficient sunlight will make the seedlings grow strongly and healthily. During the middle period of their growth stage, sufficient sunlight can promote the growth of stems and leaves and the differentiation of flower buds. If sunflowers receive sufficient sunlight during their entire growth period, the stems and leaves grow healthily and the flowers are vivid and bright. However, if there is continuous rain or they are grown for a long time in a partially shady spot they may grow poorly and become weak. The stems will not be straight, the leaves will be soft and droop in yellow green, and the flower head will be small and irregular.
Temperature
Sunflowers were originally native to North America, where it is cold in the winter and hot in the summer, meaning there is a significant annual variation in temperature. Sunflowers like a warm environment. The most suitable temperature for their growth is 18 to 30 ℃, and they can happily grow as long as the temperature is not lower than 10 ℃. Within this suitable temperature range, the higher the temperature is, the faster sunflowers grow.
At different stages of their growth, sunflowers require different amounts of water, although in general they are drought-resistant. However, their soil should not be too moist since waterlogging soil can kill them. During the budding and flowering period, sunflowers should be regularly watered, to ensure better blooming.
Watering
Thriving in its native habitat of moist meadows and along streams, thinleaf sunflower has adapted to environments with ample water availability. This species exhibits a preference for consistent moisture and can tolerate brief periods of drought. For optimal growth, it requires watering every week. As an herbaceous perennial commonly grown outdoors, thinleaf sunflower benefits from mulching to retain soil moisture, which is particularly crucial during its active growing season.
Soil
Sunflowers can grow well in all types of soil. However, the soil they most prefer is sandy loam or loamy soil, with a pH of between 5.8 and 6.5. Good soil drainage is crucial for the growth of sunflowers. If sunflowers are planted in the garden, it is recommended that loose and fertile loam is used, which can be mixed with decomposed compost as a base fertilizer. Quick-acting fertilizer can also be added. Soil should be dug over prior to fertilization and sowing to improve soil structure and drainage.
If sunflowers are grown in pots, their soil should be made up of peat, vermiculite, and sand in the ratio 2: 1: 1; organic fertilizers can be mixed in at the same time. Please note that sunflowers should not be continuously planted in the same soil or in soil where other plants of the same family have been planted, otherwise they will grow poorly. It is recommended that the soil is replaced once a year, or that sunflowers are grown in a different area.
Fertilizing
For optimal growth, fertilize thinleaf sunflower with balanced nutrition fertilizers in spring and switch to high phosphorus fertilizers before flowering to boost blooms. Apply fertilizer once a month, using amounts recommended by the product, to enhance vigor and health.
Over-fertilization can damage thinleaf sunflowers, so adhere strictly to guidelines and reduce feeding in fall. Seasonal adjustments ensure thinleaf sunflower receives necessary nutrients without excess, supporting its lifecycle. Always water thinleaf sunflower after applying fertilizer to prevent root burn and distribute nutrients effectively.
Pruning
To ensure the robust growth of the main stem, the lateral buds of sunflowers should be removed so as not to distribute nutrients too widely and affect the growth of flowers on the main stem. If planted in a border, the top bud of the main stem can be picked off, so that branching occurs to give 4-5 blooms and the plant remains quite short and bushy. For sunflowers planted to provide cut flowers, the apical bud should be removed to facilitate the growth of lateral stems. After such treatment, several stems will be produced, and 7-10 flowers can be harvested from one plant. For sunflowers grown in pots, it is recommended to keep the primary stem and remove the lateral buds, since in this case it is better to have just one flower.
Propagation
Belonging to the Asteraceae family, thinleaf sunflower is a perennial that thrives with adequate sunlight and well-drained soil. Propagation is commonly achieved through division, a reliable method for gardeners to multiply these hardy plants. Carefully separate the root clump in early spring or late fall, ensuring each division has ample roots and at least one shoot. After separating, immediately replant the divisions to avoid drying out. With consistent watering, these divisions will establish themselves quickly, resulting in new, vigorous growth.
Transplanting
The prime time to transplant thinleaf sunflower is from the awakening of early spring to the cusp of summer, ensuring robust root establishment. Choose a sunny spot with well-drained soil, and during the process, keep roots moist to ease the transition.
III. Harvesting and Storage
For sunflowers planted to be used for cut flowers, they can be harvested when the outermost ligulate flowers bloom. It is best to harvest them in the early morning when the temperature is low; they should not be harvested at midday or in the afternoon, when the temperature is high. Gardening scissors should be used to cut the inflorescences obliquely at the bottom of the stem; this can increase the area for water absorption.
The cut inflorescences should be placed in a container of clean water as soon as possible to avoid water loss. If the flower cannot be placed into water immediately, the bottom of the stem should be cut again before it is placed in water to ensure sufficient water absorption. The vase life of sunflowers is 6 to 8 days in the summer and 10 to 15 days in the winter.