Coleus is prized for its colorful, velvety foliage, offering vibrant patterns without needing flowers. Starting this tropical annual from seed indoors allows gardeners to begin the growing season well before the last spring frost. This indoor start offers control over environmental factors, which is useful for accessing unique or rare varieties not commonly found as garden starts. Initiating growth inside ensures the young plants are robust and ready to thrive once outdoor temperatures are consistently warm.
Essential Supplies and Setup
Success begins with sterile equipment, which prevents common problems like damping off disease. Use shallow seed-starting trays or cell packs that have drainage holes, and fill them with a sterile, fine-textured seed-starting mix. This medium is typically peat-based, light, and well-draining, which prevents waterlogging while providing a clean environment for root development. Proper sanitation of all tools and containers before use is a foundational preparation step.
Coleus requires warmth for optimal germination, ideally between 70°F and 75°F. A waterproof seedling heat mat placed beneath the trays provides consistent bottom heat, significantly improving the germination rate and speed. This specialized warmth ensures the seeds break dormancy efficiently.
A dedicated artificial light source is necessary because a windowsill often does not provide sufficient intensity for strong seedling growth. Fluorescent shop lights or modern LED grow lights should be set up before sowing. Providing light immediately after germination prevents the seedlings from becoming stretched and weak, a condition known as legginess. The entire setup should be prepared and tested before the sowing process begins.
The Sowing Process
Coleus seeds are tiny and should be sown indoors approximately 8 to 12 weeks before the final frost date in your region. After moistening the seed-starting mix thoroughly, gently scatter the seeds thinly across the surface to avoid overcrowding. The soil should be moistened by misting or bottom watering before sowing to prevent displacing the seeds.
Coleus requires light to germinate, making surface sowing necessary. Press the seeds lightly into the medium to ensure good contact, but avoid covering them completely with soil. If any covering is used, it should be a very thin layer of fine vermiculite, which helps retain surface moisture without blocking light.
To maintain high humidity levels, cover the planted trays with a clear plastic dome or plastic wrap. Place the covered trays under the grow lights or in a location with bright, indirect light. Once the sprouts emerge (typically 10 to 14 days), the humidity dome must be removed to allow for air circulation.
Post-Germination Seedling Care
Once the dome is removed, immediate and consistent light exposure is necessary to prevent seedlings from stretching toward the light source. The grow lights should be positioned close to the young plants, typically just 1 to 2 inches above the top leaves. Maintaining a light duration of 14 to 16 hours daily promotes compact, sturdy growth.
Watering should be done carefully to keep the soil consistently moist but never saturated, which helps prevent fungal issues. Bottom watering, where the tray is placed in a shallow pan of water until the medium wicks up moisture, is the preferred method to strengthen roots and keep the stems dry. Once the seedlings develop their first set of true leaves, they should be thinned out, removing the weaker plants to give the stronger ones adequate space.
After the seedlings are established and thinned, the temperature should be gradually lowered slightly from the initial germination warmth. A daytime temperature range of 65°F to 75°F is suitable for continued growth, with slightly cooler temperatures at night. This reduction in warmth and increase in air circulation helps condition the young plants and prepare them for the next stage of growth.
Hardening Off and Transplanting
Hardening off is the process of gradually acclimating the indoor-grown plants to outdoor elements like direct sun, wind, and fluctuating temperatures. This step must begin 7 to 14 days before the final planting date, which should only occur after all threat of frost has passed. Skipping this step can result in severe leaf burn or transplant shock.
Begin by placing the seedlings outdoors for just an hour or two in a sheltered, shaded location, returning them inside each evening. Over the next one to two weeks, incrementally increase the duration of their outdoor stay and slowly expose them to more direct sunlight and breezes. This allows the plant tissue to thicken and adapt to the ultraviolet light intensity.
Once fully hardened, transplant the Coleus into their final outdoor location, ensuring the new hole is large enough to accommodate the root ball. To encourage a dense, bushy plant instead of a single tall stem, pinch the main growing tip after transplanting. Removing the top set of leaves redirects the plant’s energy, causing it to branch out laterally and create a fuller form.