Verbascum thapsus, commonly known as Mullein, is a distinctive biennial plant recognized for its large, velvety, silver-green leaves that form a low-lying rosette in its first year. In the second year, a tall flower spike emerges, often reaching heights of seven feet or more. Mullein is primarily propagated from readily available seed, which is the most reliable way to cultivate the plant. This guide provides detailed instructions for successfully growing Mullein from sowing through the care of established plants.
Preparing the Environment for Mullein Seeds
Mullein thrives in conditions where many other garden plants struggle, preferring poor, disturbed, or rocky soils over rich, cultivated ground. The ideal environment offers excellent drainage, as the plant is susceptible to root rot in soggy conditions. Overly fertile soil can be detrimental, leading to weak, sprawling plants that may not develop the characteristic robust form.
The plant requires full sun—a minimum of six to eight hours of direct sunlight each day—for the best growth and prolific flowering. A slightly alkaline soil pH, around 6.5 to 7.0, is preferred. Planting can be timed for either late fall or early spring, allowing the seeds to benefit from natural cold stratification outdoors.
Mullein seeds can germinate without cold treatment, but a period of cold and moist conditions significantly improves the germination rate and helps break dormancy. Sowing in the fall allows winter weather to naturally provide stratification, resulting in spring germination. If sowing in spring, seeds can be artificially stratified by placing them in a damp paper towel inside a sealed plastic bag in the refrigerator for four to six weeks before planting.
Step-by-Step Sowing Methods
Mullein seeds are tiny and require light to germinate, making planting depth an important detail. Whether planting indoors or directly into the garden, the seeds should never be covered with soil, as this prevents successful sprouting. Germination typically occurs within 10 to 20 days under suitable conditions.
For direct sowing, prepare the area by raking the soil surface to expose the seeds to light and provide good seed-to-soil contact. Broadcast the seeds thinly over the surface, then gently press them into the soil using a hand or a board, ensuring they remain on top. The planting area must be kept consistently moist until the seedlings emerge, after which watering can be reduced.
Starting seeds indoors allows for a head start and better control over the growing environment. This process should begin six to eight weeks before the final expected frost. Fill trays with a sterile, well-draining seed-starting mix, and then scatter the seeds across the surface.
A clear plastic dome or a layer of plastic wrap placed over the tray helps maintain the high humidity necessary for germination. The trays should be placed under bright grow lights or on a sunny windowsill, as the light exposure triggers sprouting.
Caring for Seedlings and Established Plants
Once seedlings develop their first set of true leaves, they must be thinned to allow adequate space for development. Mullein plants require significant room, so thin direct-sown seedlings to approximately 18 inches apart. This spacing prevents overcrowding and ensures each plant can form a full, healthy rosette.
For plants started indoors, transplanting to the garden should be done after the danger of frost has passed. Before placing them in the ground, seedlings must be hardened off by gradually exposing them to outdoor conditions over about a week. It is important to minimize root disturbance during this process. Mullein develops a long, deep taproot early and responds poorly to root damage once established.
Established Mullein plants are drought-tolerant due to their extensive taproot system and rarely require supplemental watering. Overwatering is the most common cause of plant failure, particularly in poorly draining soil. Fertilization is almost never necessary; the plant is adapted to poor soils, and adding fertilizer may encourage excessive leaf growth at the expense of the flower stalk.
Mullein follows a biennial cycle. The low-growing rosette is the first year’s growth, requiring exposure to cold temperatures (vernalization) to trigger flowering. The tall flower stalk, known as bolting, occurs in the second year, followed by seed production and the death of the plant. To manage prolific self-seeding, the flowering stalk should be cut down before the seed capsules mature and dry out.