Starting herb seeds indoors extends the growing season and allows for a greater selection of varieties than those available as nursery starts. By controlling the early environment, you bypass unpredictable spring conditions and produce robust seedlings ready for transplanting. Success hinges on precise timing and a controlled indoor setup, ensuring the seedlings develop the strength needed to thrive.
Essential Materials and Setup
The foundation of successful indoor herb starting is the proper growing medium. You must use a sterile, soilless seed-starting mix, typically composed of fine-textured materials like peat moss, vermiculite, or coconut coir. This light texture retains moisture without becoming waterlogged, preventing the compact density of regular potting soil that can impede root growth and lead to diseases like damping-off.
Containers should have adequate drainage holes, whether using trays, small pots, or biodegradable peat pots. Standard window light is insufficient for strong growth, so supplemental light is necessary to prevent seedlings from becoming tall and spindly. Full-spectrum LED or T5 fluorescent grow lights are effective choices and should be positioned close to the plants to maximize light intensity.
For optimal light delivery, T5 fluorescent lights should be placed approximately 4 to 6 inches above the container surface. LED grow lights, which run cooler and are more energy-efficient, are typically positioned slightly further away, around 8 to 12 inches from the seedlings. Optional tools like a heat mat placed under the trays can significantly increase the germination rate by providing a consistent soil temperature, ideally between 65°F and 75°F.
Sowing Techniques for Herb Seeds
Before sowing, the sterile seed-starting mix must be pre-moistened until it feels like a wrung-out sponge, ensuring consistent moisture for germination. Once the containers are filled, the sowing depth is determined by the specific herb seed size. A general guideline is to plant the seed no deeper than twice its thickness beneath the surface, though tiny seeds are exceptions.
Fine seeds, such as thyme and oregano, require light to germinate, so they should be surface-sown and pressed gently onto the soil without being covered. Larger seeds, like basil and dill, benefit from a light covering of the mix, about one-eighth to one-quarter inch deep. After planting, immediately label each container with the herb name and the planting date to avoid confusion once the seedlings emerge.
After sowing, covering the trays with a clear humidity dome or plastic wrap helps trap moisture and heat, encouraging rapid germination. The trays should be placed in a warm location, often on a heat mat, until the first sprouts appear. Use a fine mist spray bottle for initial watering to avoid displacing the small seeds.
Managing Light and Water After Sprouting
The moment the first seedlings emerge, the clear dome must be removed to allow for air circulation and prevent fungal diseases like damping-off. This is also the time to move the containers directly under the grow lights to deliver the high light intensity necessary for photosynthesis. The lights should be on for an extended period, generally 14 to 16 hours per day, to mimic the long days of summer.
Insufficient light causes the seedlings to stretch, resulting in weak, elongated stems; this is corrected by moving the light source closer. For T5 fluorescents, this means maintaining the distance of 4 to 6 inches, raising the lights as the plants grow. Proper watering is managed by allowing the soil surface to dry slightly between applications, preventing the roots from sitting in saturated conditions.
Bottom watering is preferred, where the tray is filled with water that the pots wick up from below. This encourages deeper root growth and keeps the foliage dry, reducing the risk of disease. Once the seedlings develop their first set of true leaves and appear crowded, thinning is necessary; snip off the weaker plants at the soil line, leaving only the strongest plant in each cell.
Indoor temperatures during the day can range from 65°F to 75°F, but allowing the temperature to drop slightly at night, perhaps to 60°F, can help produce stockier growth. Consistent air circulation is also beneficial, which can be provided by a small fan set to a low speed, gently moving the seedlings and helping to strengthen their stems. This controlled indoor environment builds a strong foundation for the seedlings before their eventual transition outdoors.
Acclimating Seedlings for Outdoor Planting
The process of preparing indoor-grown herbs for the outdoor environment is called “hardening off,” which is essential to prevent transplant shock. This transition should begin one to two weeks before the planned outdoor planting date, typically after the last expected frost in your area. Seedlings must gradually adjust to direct sunlight, wind, and fluctuating temperatures, as they have only been exposed to consistent, sheltered conditions.
Begin the hardening off process on a mild, overcast day by placing the seedlings in a protected, shaded outdoor location for just one hour. The initial exposure must be gentle because the tender leaves can easily scorch from sudden direct sun or be damaged by strong wind. Over the next seven to 14 days, the duration of outdoor exposure should be increased by an hour each day.
The location should be slowly shifted to allow for increasing amounts of morning sun, which is less intense than afternoon sun. After a few days of adjustment, the herbs can be exposed to wind and slightly cooler temperatures. If temperatures drop below 45°F to 50°F, bring the seedlings back inside to prevent cold damage.
During this period, reduce watering slightly to slow growth, encouraging a more robust, hardened structure. The seedlings are fully hardened off and ready for permanent planting when they can tolerate a full day and night outdoors without showing signs of stress.