The thinleaf sunflower brings sunlight into your garden. This is what matters when planting and caring for it.
Profile of thinleaf sunflower:
Scientific name: Helianthus decapetalus
Plant family: daisy family (Asteraceae)
Other names: forest sunflower, pale sunflower, roughleaf sunflower, ten-petal sunflower, thin-teaved sunflower
Sowing time: spring
Planting time: spring
Flowering period: June to October
Soil quality: sandy to loamy, calcipholous, nutrient rich, humus rich
These information are for temperate climate!
Use in: flowerbeds, flower bouquets, cottage garden, flower garden, natural garden
Winter hardiness: hardy, USDA Plant Hardiness Zones: 5 (-26 °C / -15 °F)
Bee and insect friendly: Yes
Plant characteristics and classification of thinleaf sunflower
Plant order, origin and occurrence of thinleaf sunflower
The thinleaf sunflower (Helianthus decapetalus) is native to North America, 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 and grows mainly in humid forests along streams. It is a member of the daisy family (Asteraceae). Those who love the bright yellow sunflowers do not have to be satisfied with the annual species – there is a whole range of perennial varieties, also known as perennial sunflowers. Although their sunny flowers are considerably smaller than those of the annual forms, they are all the more numerous. All varieties, once planted, are extremely easy to care for and will give you many years of pleasure.
Characteristics of thinleaf sunflower
Thinleaf sunflowers form numerous, upright growing and rather thin stems on which the numerous flowers sit. The height of growth varies depending on the species and variety – while some perennial species can grow up to 200 centimeters (80 in) high, others reach a height of just 60 to 120 centimeters (24 to 48 in). Typical, however, is the strongly branched and thickened rootstock from which further shoots sprout in most species. Forest sunflowers usually grow in a cluster and form more or less numerous runners.
Helianthus decapetalus has ovate, serrated leaves, the undersides of which are rough, like their annual relatives. The upper leaves are usually alternate, the lower ones opposite and short-stalked.
The flowers of the perennial thinleaf sunflower resemble those of the annual species, but the lemon to golden yellow ray florets are usually much smaller. But the perennial varieties do not only form one, but very numerous flowers. The flower heads have a diameter of 5 to 7 cm (2 to 2.8 in). The flowering time lasts from June to October. There are also double and semi double varieties of the forest sunflower.
The thinleaf sunflower forms – like the well-known sunflower – numerous sunflower seeds, which are gladly accepted by birds as winter food.
Thinleaf sunflower – cultivation and care
A full sunny and protected, warm location in the bed is ideal for the thinleaf sunflower.
On a nutrient-rich, permeable, sandy-clayey, moderately dry to moderately moist garden soil, the forest sunflower can develop optimally. The substrate should ideally be neutral to alkaline and not too heavy. The plant tolerates dryness relatively well.
Planting / Sowing
You can grow young plants from late winter on your windowsill at home and plant them out in the bed after the frosts. The seeds germinate at around 20 °C / 68 °F quite irregularly within seven to 21 days. Always keep the recommended planting distance for both home-grown plants and seedlings – this is about 80 centimeters (32 in). Mix the excavated material with plenty of compost and then water the freshly planted perennials vigorously. It is also advisable to place support rods in the soil to prevent the tall stems from breaking or breaking off.
Care should be taken with sensitive skin, as the plant sap and also the leaves of the plant can cause skin irritation.
Watering thinleaf sunflower
Although thinleaf sunflowers are quite tolerant of dryness, you should not let them dry out. Provide them with plenty of water, especially during hot and dry periods, but avoid waterlogging.
Fertilization with mineral or nitrogen-rich fertilizers is not very useful, as this has a negative effect on the flowering. Instead, supply the perennials with plenty of compost in early spring and a second time in June.
Since some varieties like to seed themselves, you should remove withered flower stems as soon as possible. This also has the advantage that the pruning causes a longer flowering phase. Some species should also be cut back close to the ground in autumn or spring, as they will start to sprout again from the rhizomes.
But remember: If you cut off the flowers there will be no seeds and hence no food for birds.
Helianthus decapetalus should be divided every three to four years in spring to limit excessive spreading and to keep the plant growing for a long time.
Helianthus decapetalus can be propagated either by dividing in spring or by sowing as soon as the temperature is about 15 °C / 59 °F. It is also possible to propagate the plant in spring by cuttings.
Diseases and pests
Especially the new shoots of the thinleaf sunflower are susceptible to snails. Occasionally the leaves are also attacked by powdery mildew.
As a rule, thinleaf sunflowers are winter-hardy, but since the wintering shoots are only just below the ground, the plant should be protected with brushwood if necessary. Alternatively, a layer of compost can be applied to the root area as early as autumn, thanks to which the plant then starts well into spring.
Use in the garden
The thinleaf sunflower is suitable for both mixed and perennial borders, and it also fits perfectly into the countryside garden. In contrast to annual sunflowers, the thinleaf sunflower scores with its longevity and growth. It offers valuable food for insects and is especially popular with bees because of its pollen and nectar.
The variety ‘Meteor’, which is also called ‘Supreme’, has in contrast to the species strongly enlarged flowers. The flowers of the outer wreath are also very broad. They are light lemon yellow. ‘Meteor’ is often used as cut flower for the vase. ‘Soleil d’Or’ is a double, golden yellow variety of Helianthus decapetalus, whose flowers can reach a diameter of up to 12 cm (4.8 in). They are ideal for large flower arrangements. The variety ‘Triomphe de Gand’ captivates with large, laterally inclined flowers and bright yellow, strong florets.