The question of whether Bidens plants are perennials or annuals is a common point of confusion in horticulture. The answer depends on the specific species and the climate where it is grown. The genus includes true annuals, true perennials, and ornamental varieties that are biologically perennial but sensitive to cold. Understanding the difference between the plant’s biological classification and its practical application is necessary to determine its life cycle.
Understanding the Genus Bidens
The plant known commonly as Bidens belongs to a large genus within the Asteraceae family, which also includes daisies and sunflowers. This genus comprises approximately 230 species distributed across tropical and warm temperate regions worldwide. These plants are generally herbaceous, meaning they have non-woody stems, and can range in height from 10 to 150 centimeters.
The genus name comes from the Latin words bi (two) and dens (tooth), referring to the fruits. These fruits, or achenes, typically feature two to four barbed awns that facilitate dispersal by clinging to clothing or animal fur. This characteristic earns them common names like beggarticks, Spanish needles, and bur marigolds. Ornamental varieties are primarily hybrids of species like Bidens ferulifolia, featuring abundant daisy-like flowers often in bright yellow, white, orange, or pink.
The Crux of the Matter: Annuals, Perennials, and Tender Varieties
The biological life cycle of a plant is defined by its ability to survive multiple growing seasons. An annual plant completes its entire life cycle, from germination to seed production and death, within a single year. A true perennial lives for more than two years, typically going dormant in winter and regrowing from the same root system each spring.
The Bidens genus includes species that fit both definitions, particularly among the wild varieties. For example, Bidens pilosa and Bidens tripartita are often classified as true annual weeds that complete their life cycle quickly. However, the popular ornamental cultivars, such as those derived from B. ferulifolia, are biologically classified as tender perennials.
A tender perennial naturally survives for multiple years in its native, warm climate but lacks the hardiness to survive freezing temperatures. Ornamental Bidens varieties originate from regions like Mexico and the Southwestern United States, where frost is rare. These plants will die when exposed to temperatures below approximately 40°F.
Because most temperate regions experience winter frost, gardeners must treat these tender perennials as annuals. The plants are enjoyed until the first hard frost kills them, requiring new plants to be purchased the following spring. This highlights the difference between a plant’s inherent biology and its practical cultivation reality outside of its native range.
Practical Cultivation Based on Climate Zones
A gardener’s decision on whether to treat Bidens as a perennial or an annual is dictated by the local USDA Hardiness Zone. Ornamental varieties will reliably survive the winter only in the warmest regions, specifically USDA Zones 9 through 11. In these zones, the plant behaves as a true perennial, maintaining its root structure to return vigorously in the spring.
In Zones 8 and colder, where freezing temperatures are common, the plant must be considered an annual for in-ground planting. Gardeners who wish to save a specific cultivar can choose overwintering, bringing the potted plant indoors before the first frost. The plant must be kept in a bright, sunny location with reduced watering until temperatures warm up the following spring.
Some weedy Bidens species, even if technically annuals, mimic perennial growth through prolific self-seeding. Species like B. pilosa produce thousands of viable seeds that germinate readily. This creates an appearance of continuous growth from year to year, even though the original plant died.