Incredible Mullein seeds: The life of a biennial

How many seeds does a single Mullein plant have: 50? 500? 5,000? The answer may very well surprise you!

Verbascum thapsus, more commonly known as Mullein, is a herbaceous plant used to treat respiratory problems[1]. As with other biennials, its life cycle lasts two years. In the first year the shoot apex of the plant develops and the reproductive organs appear in the second year.

In general, the seeds germinate in spring or summer. They must be exposed to light and temperatures between 10 and 40 degrees to germinate. During the first year the young plant grows a deep taproot and a fairly large leaf rosette which can grow up to 40-50 cm. The rosette contains the reserves necessary for the survival of the plant.

During the cold temperatures of winter in the first year, “vernalisation” provides the plant with the ability to flower the following year. In order for this process to take place, a certain level of vegetative development is necessary. In the case of Verbascum Thapsus, the rosette must measure more than 15 cm in diameter.

In the spring of the second year the plant produces a single stalk that can reach a height of 2 metres with a cluster of several hundred hermaphrodite flowers. Each five petal flower contains five stamens, of which two are shorter than the others, and a pistil made up of two fused carpals. Flowering, which generally lasts from June to August in Europe, starts at the bottom of the stalk and progressively moves up to the top. Although this cup-shaped, nectar-rich plant is easily accessible, the access time is limited. It opens early in the morning and closes in the middle of the afternoon. During this time, direct pollination takes place through bees who transfer the pollen from the stamen to the stigma. If the flowers are not fertilized during the day, they detach from the stalk leading to self-pollination.

After fertilization, the wilted, fertilised flowers turn into dry fruit in the shape of a 6mm oval capsule. Each capsule contains numerous seeds which are dispersed by the wind at the foot of the plant (mainly at a distance of less than 5 meters). When flowering has finished, the plant begins to dry out to leave straight stalks in the winter of the second year. The stalk persists for several months before wilting and decomposing. The dry capsules split in two during bursts of wind to allow the seeds to fall.

So, coming back to the original question: How many seeds are we talking about? On average, each plant produces 200-300 capsules containing 500-800 seeds: an incredible total of 100,000-240,000 seeds per plant! This quantity of seeds allows the plant to colonize friendly habitats. The seeds are highly resistant and can remain viable for up to 100 years. Quite incredible!

For information on how plants convert natural resources into food, see "How do plants feed themselves...and us?"

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[1] Turker, Arzu & Camper, N. (2002). Biological activity of Common Mullein, a medicinal plant. Journal of ethnopharmacology. 82. 117-25.