The genus Cosmos, including popular species like Cosmos bipinnatus and Cosmos sulphureus, is widely cultivated for its cheerful, daisy-like blooms. These annual flowers display color throughout the summer and fall seasons. Cosmos are highly effective attractors and providers for a broad array of pollinating insects. Their structure and resources establish them as a valuable addition to pollinator habitats.
Why Cosmos are Pollinator Magnets
Cosmos flowers offer a dual resource—nectar and pollen—in an accessible format. Nectar is a high-energy carbohydrate source, fueling the flight and metabolic needs of adult insects like bees and butterflies. Studies indicate that Cosmos bipinnatus ranks highly in the volume of nectar it produces per flower.
The flowers also deliver substantial, protein-rich pollen, which is important for female bees to provision their nests and feed their developing larvae. The extended blooming period of Cosmos, often lasting from mid-summer until the first frost, ensures a continuous supply of these resources.
This season-long availability makes them a dependable foraging option when other seasonal flowers have faded. The simple, open architecture of the traditional Cosmos bloom allows insects of various sizes and mouthpart lengths immediate access to the central disk. This design minimizes energy expenditure for foraging insects.
The Specific Pollinators Attracted to Cosmos
The open, disk-like shape of the Cosmos flower attracts a diverse range of insects beyond the typical honeybee. Many species of native bees, including small solitary bees and larger bumblebees, are frequent visitors that easily access the pollen and nectar in the central disk.
Butterflies, such as the Monarch during migration, rely heavily on the accessible nectar for long-distance flight energy. Their long proboscises are well-suited to extract the sugary liquid from the shallow nectaries found in the central florets.
The flowers also serve as a resource hub for beneficial insects that aid in garden pest control. Syrphid flies (hoverflies) feed on the nectar and pollen as adults, and their larvae are predatory, consuming pests like aphids. Other helpful species, including parasitic wasps and lacewings, are also drawn to the accessible resources.
How Flower Structure Affects Accessibility
The effectiveness of Cosmos is directly linked to its floral structure, characteristic of the Asteraceae family. The flower head, known as a capitulum, is composed of two distinct types of smaller flowers: the colorful ray florets and the central disk florets. The ray florets are the showy, strap-shaped “petals” that primarily function to attract pollinators from a distance.
The actual reproductive and resource-producing units are the tightly clustered disk florets in the center. Each disk floret produces both pollen and nectar, opening progressively from the outer edge toward the center of the disk. This sequential opening provides a sustained supply of fresh resources over the life of the bloom.
The breeding of ornamental varieties can compromise this structure, especially in double-flowered types. In these cultivars, genetic mutations cause the stamens to convert into extra petals, creating a dense, pom-pom appearance. This transformation physically obscures or eliminates the central disk florets, making the flower difficult or impossible for most insects to access. For maximum benefit, gardeners should seek out the traditional, single-form varieties where the central disk is fully exposed.
Selecting and Siting Cosmos for Maximum Ecological Value
To maximize the ecological benefit of planting Cosmos, selecting the flower form is the most important consideration. Gardeners should choose single-petaled varieties, such as the classic C. bipinnatus or C. sulphureus, which retain the flat, open central disk. Avoid highly-bred cultivars labeled as “double,” “pom-pom,” or “crested,” as their complex petal arrangements hinder access to resources.
The placement of the plants also influences their effectiveness, as pollinators are more attracted to large patches of color. Planting Cosmos in dense mass groupings increases their visual signal and reduces the flight time required for insects to move between food sources. This strategy conserves the pollinator’s energy.
Maintaining a continuous bloom period is achieved through regular removal of spent flowers, a process known as deadheading. Removing faded blooms prevents the plant from setting seed prematurely, redirecting energy into producing new flowers and extending the resource supply. Allowing some flowers to remain and set seed, however, is beneficial for seed-eating birds and encourages self-seeding.