Shiso (Perilla frutescens) is an aromatic herb popular in Asian cuisine. This member of the mint family typically appears in deep-purple (red) or bright green varieties, each offering a distinct flavor. While growing shiso provides a rewarding yield of edible foliage, its seeds are difficult to germinate successfully due to natural dormancy, which requires specific preparation steps.
Essential Preparation Before Sowing
The primary reason shiso seeds fail to germinate is seed dormancy, a protective natural mechanism that prevents premature sprouting. To ensure optimal germination rates, growers must simulate a period of cold and moisture, a process known as cold stratification. This mimics the seed overwintering in the soil, signaling that conditions are right for growth.
A common method is to mix the seeds with a small amount of moist material, such as sand or a damp paper towel, and place them inside a sealed plastic bag. Store this packet in a refrigerator for one to two weeks, maintaining a cool, moist environment without freezing. This chilling process helps soften the seed coat and break the dormancy.
Alternatively, soak the seeds in room-temperature water for 12 to 24 hours just before planting. This prolonged exposure to moisture helps to hydrate the seed embryo and initiate the germination process. Regardless of the method chosen, the seeds must remain consistently moist, as drying out can halt the process entirely.
The Sowing Process and Light Requirements
The optimal timing for sowing shiso seeds indoors is approximately six to eight weeks before the last expected frost date. Use a sterile, fine-textured seed starting mix in a clean tray, which should be thoroughly moistened before sowing. This medium helps prevent disease and provides an ideal environment for the small seeds.
Shiso seeds are photoblastic, meaning they require exposure to light to trigger germination. Therefore, the seeds should be surface-sown and pressed gently onto the top of the soil mix instead of being buried. If covering them, use only the thinnest dusting of soil or vermiculite, ensuring light can still reach the seed surface.
To encourage rapid sprouting, maintain the soil temperature consistently at 70°F (21°C), often requiring a seedling heat mat for steady warmth. Immediately after sowing, cover the tray with a clear dome or plastic wrap to maintain high humidity. Keep seeds continuously moist but not waterlogged; germination should occur within seven to twenty-one days.
Nurturing Seedlings to Maturity
Once the shiso seeds have sprouted, remove the humidity dome to allow for air circulation and prevent fungal issues like damping off. The young seedlings require strong light immediately to prevent them from becoming long and spindly. Consistent moisture remains important, but allow the soil surface to dry slightly between waterings.
When seedlings develop their first set of true leaves, they are ready to be thinned out. Thinning involves carefully removing the weaker seedlings so the remaining plants are spaced one to two inches apart in the tray. This prevents overcrowding and ensures the strongest plants receive sufficient light and nutrients.
Before transplanting outdoors, the plants must undergo “hardening off” to acclimatize them to outdoor elements. Over seven to ten days, gradually expose them to increasing periods of direct sunlight, wind, and cooler temperatures. Once all danger of frost has passed, transplant the shiso into the garden or larger pots, spacing them about 12 inches apart. Shiso thrives in rich, well-draining soil and performs well in full sun or partial shade, though afternoon shade is beneficial in very hot climates. Regularly pinching back the growing tips encourages branching, resulting in a bushier habit and greater leaf yield.