The fragrant herb rosemary (Salvia rosmarinus) is a desirable addition to any garden or kitchen, prized for its woody scent and flavor. While it is possible to grow rosemary from seed, it presents a higher degree of difficulty and a significantly longer timeline compared to other common garden herbs. Starting rosemary from seed is a demanding process, requiring more patience and environmental control than most home gardeners are accustomed to providing.
The Challenges of Starting Rosemary from Seed
The primary hurdle when growing rosemary from seed is the consistently low germination rate, which can range from 15% to 50%, and sometimes as low as 5% for older seeds. This low viability means gardeners must sow many more seeds than they ultimately wish to grow to ensure a reasonable yield. The seeds are also slow to sprout, typically taking between 14 to 21 days just to show the first signs of life.
Even after germination, the initial growth of the seedlings is notoriously slow, making the entire process a significant time commitment. This prolonged slow growth leaves the young plants highly susceptible to issues like “damping off,” a fungal disease that causes seedlings to rot at the soil line. Rosemary seedlings demand sustained and specific environmental conditions, and slight variations in moisture or temperature can quickly lead to their demise.
The seeds require sustained warmth to initiate growth, ideally needing a soil temperature between 70°F and 80°F, which is warmer than what is readily available in many homes. A precise balance of moisture is also necessary; the seeds must be kept consistently damp but never waterlogged, as excess moisture will cause them to rot before they sprout. This delicate interplay of specific temperature and moisture requirements makes seed starting an unreliable method for this particular herb.
Successful Germination Techniques
Gardeners who choose to accept the challenge of starting rosemary from seed can employ several techniques to maximize success. One helpful step is to pre-treat the seeds by soaking them in water overnight before planting. Another method is cold stratification, which involves exposing the seeds to a cold, moist environment, such as a refrigerator, for 10 to 14 days to break the dormancy that naturally inhibits germination.
When sowing, press the seeds gently onto the surface of a sterile, well-draining seed-starting mix rather than burying them. Rosemary seeds are photoblastic, meaning they require exposure to light for germination to occur. The planting container should be placed on a heat mat to maintain the necessary soil temperature of 70°F to 80°F.
Moisture control is accomplished by using a clear plastic dome or plastic wrap over the container to create a high-humidity environment. This covering helps maintain a consistent, moist surface without the need for frequent watering, which can easily oversaturate the soil. Once the first seedlings emerge after two to three weeks, the cover should be removed to allow for air circulation and reduce the risk of fungal disease. Seedlings should then be placed under bright, artificial grow lights or in a south-facing window to receive the intense light necessary for healthy growth.
Propagation Through Cuttings: The Preferred Method
Due to the difficulties associated with seed starting, propagating rosemary through semi-hardwood cuttings is widely considered the easier, faster, and more reliable alternative. This method offers a significantly higher success rate and a much shorter time frame to a mature plant. Cuttings also ensure the new plant is a genetic clone of the parent, guaranteeing specific desirable characteristics like flavor and growth habit.
The process begins by taking a three-to-six-inch cutting from the tip of a healthy rosemary stem, ensuring the cut is made just below a leaf node. Strip the leaves from the bottom two-thirds of the cutting to expose the nodes where roots will form. While optional, dipping the cut end into a rooting hormone powder can stimulate faster and more robust root development.
The prepared cutting is then inserted into a small pot filled with a well-draining, sterile potting mix or placed in a jar of water. Roots typically begin to form within six to eight weeks, which is dramatically faster than the months required for a seed-started plant to reach a similar size. Once the roots are established, the young plant can be transferred to a larger container or planted outdoors after the last frost.