Rosemary, a fragrant perennial herb botanically classified as Salvia rosmarinus, is prized for its needle-like leaves and strong, aromatic oils. While propagating rosemary from stem cuttings is common, starting the herb from seed offers a unique gardening challenge. Successfully growing rosemary from its tiny seeds requires understanding the seed’s characteristics and specific sowing techniques.
Understanding Rosemary Seed Viability
The primary reason for planting multiple rosemary seeds is the herb’s naturally low and unpredictable germination rate. Rosemary seeds often exhibit a viability range between 15% and 50%, meaning a significant portion may never sprout. This low success rate necessitates over-seeding to ensure at least one plant establishes itself in each desired location.
Rosemary seeds also have a slow germination period, demanding patience. Under optimal conditions, sprouting can take 15 to 25 days, or sometimes longer. Maintaining a consistent soil temperature is a significant factor, with a warm range of 65–70°F proving most effective for germination.
The extended time the seeds spend in the soil increases their susceptibility to issues. If the soil is too wet during this prolonged waiting period, the seeds can rot before they sprout. This inherent delay and uncertainty strongly influence the recommended planting density.
Sowing Density and Seed Depth Guidelines
The number of rosemary seeds to plant per container depends directly on the size of the pot or cell being used. For standard small containers, such as seed-starting cells one to two inches wide, sow three to four seeds to increase the probability of success. If planting directly into a larger, four-inch diameter pot, a slightly higher density of five to eight seeds is advisable to account for the low germination rate.
The planting medium should be a sterile, well-draining seed-starting mix to prevent excess moisture and fungal issues. Rosemary seeds require light for germination, so they must be sown on or very near the soil surface. The appropriate planting depth is extremely shallow, about one-eighth of an inch, or just enough to barely cover the seeds with a fine layer of mix or vermiculite.
Pressing the seeds gently into the surface of the pre-moistened medium helps secure them without burying them too deeply. This shallow depth ensures the seeds receive necessary light exposure while maintaining contact with the moist medium. Proper spacing minimizes future root disturbance, and this initial density provides a buffer against the seed’s unreliable nature.
Managing Seedlings After Germination
Once the first sprouts appear, usually two to four weeks after sowing, focus shifts to supporting vigorous early growth. Maintaining high humidity is beneficial during the initial phase, often accomplished by covering the container with a plastic dome. This cover should be removed immediately once seedlings emerge to allow for better air circulation and prevent fungal diseases like damping off.
Adequate light is necessary for the health of young rosemary plants, which can quickly become thin and weak, or “leggy,” without it. Seedlings require a full-spectrum light source for 10 to 12 hours a day. Indoor growers will likely need to use supplemental grow lights positioned a few inches above the plants. The soil should be kept consistently moist but never waterlogged, allowing the surface to dry slightly between waterings to mimic the herb’s native, well-drained conditions.
The most important step after germination is thinning, performed once the seedlings develop their first set of true leaves. Since the goal is one strong plant per pot, select the single, most robust seedling and remove the others. The preferred method is to snip the unwanted seedlings at the soil line with small scissors. This prevents disturbing the roots of the chosen plant, allowing it to fully utilize the available space and nutrients.