From a botanical perspective, the question of whether marigolds are perennials or annuals is fundamentally about their life cycle strategy and how it interacts with the environmental conditions of your specific USDA zone. The answer is nuanced because it depends on the species of marigold and the climatic triggers it has evolved to respond to.
Botanically, an annual plant completes its entire life cycle—from seed germination to flowering, seed production, and death—within a single growing season. Its survival strategy is to invest all its energy into producing a massive number of seeds to ensure the next generation. A perennial plant, in contrast, lives for more than two years. It invests energy into developing structures like bulbs, tubers, or woody roots that allow it to survive a period of dormancy (like winter) and regrow in subsequent seasons. Most marigold species commonly grown in gardens, such as Tagetes erecta (African marigold) and Tagetes patula (French marigold), are true annuals by nature. They originate from warm, frost-free regions in Mexico and Central America and have not evolved the necessary survival mechanisms to withstand freezing temperatures.
Your USDA Hardiness Zone is a critical factor because it defines the average annual extreme minimum winter temperature for your area. For the true annual marigolds, any temperature at or below freezing (32°F or 0°C) is fatal. The plant's cells, full of water, freeze and rupture, causing irreversible damage. Therefore, in any zone that experiences a frost (which is nearly all zones in the continental US), these marigolds will be killed by the first hard freeze of autumn. They cannot survive the winter, regardless of whether you are in zone 4 or zone 8. Their status as an annual is fixed by their physiological intolerance to cold, not by the zone number itself.
It is important to distinguish between the common garden marigolds and a less common species, Tagetes lemmonii (often called the Mexican marigold or Copper Canyon daisy). This species is a true perennial shrub. It develops a woody root system and structure that allows it to survive winter dormancy. However, its perennial nature is still constrained by temperature. Tagetes lemmonii is typically hardy only in USDA zones 8 through 11, where winter temperatures rarely dip below 10°F to 20°F (-12°C to -7°C). In these warm, frost-free or light-frost zones, it will behave as a perennial, dying back after a frost but resprouting from its base in spring. In colder zones, it would be killed outright and thus would function as an annual if planted there.
A phenomenon that often leads gardeners to believe their marigolds are perennial is self-seeding. As prolific annuals, marigolds produce an abundance of seeds. These seeds fall to the ground in late summer or autumn and can lie dormant in the soil over winter. The following spring, when soil temperatures warm, these seeds germinate and new marigold plants emerge, often in the same location as the previous year's plants. While it creates the appearance of the original plant returning, these are genetically new individuals. This is not perennial growth from the same root system but rather a successful annual life cycle strategy playing out over two seasons.
Understanding this botanical reality informs your gardening practices. For the vast majority of gardeners, marigolds should be treated as annuals. This means you will plant new seeds or transplants each spring after the danger of frost has passed. You can collect the dried seed heads in the fall to sow the next year, harnessing their annual nature. If you live in a very warm zone (8-11) and wish to grow a perennial marigold, you would specifically seek out Tagetes lemmonii. For all other common marigold varieties, their beauty is a single-season event, a brilliant burst of life programmed to end with the cold, ensuring its continuation through seed.
