Growing rosemary indoors offers fresh, aromatic herbs year-round. Starting this woody perennial from seed is challenging because the seeds are notoriously slow to germinate and often have a low germination rate, sometimes falling to 30%. Unlike propagating from cuttings, growing from seed requires significant patience and precise environmental control. Success depends on meeting the plant’s specific needs, especially during the first few weeks of its life.
Preparing Seeds and Starting Medium
Rosemary seeds possess a tough outer coating that impedes water absorption, contributing to low germination rates. To help break this physical dormancy, soak the seeds in lukewarm water for 12 to 24 hours before planting. This presoaking process softens the seed coat, allowing moisture to penetrate more easily.
The choice of starting medium is important, as rosemary roots are highly susceptible to rot from excess moisture. Use a sterile, light, and well-draining seed-starting mix, avoiding heavy garden soil or standard potting soil. A mix containing perlite, vermiculite, or coarse sand blended with peat moss provides the ideal structure for aeration and quick drainage. Select small containers or seed-starting trays that have excellent drainage holes to prevent water from pooling.
Sowing and Initial Setup
Rosemary seeds are tiny and require light to stimulate germination, so they must not be buried deeply. After moistening the seed-starting mix, gently press the seeds onto the surface of the medium. If you cover them at all, use only a very light dusting of fine vermiculite or sand, ensuring the seeds still receive light.
Since the seeds are small and easily displaced, initial watering must be done carefully to avoid washing them away. Use a spray bottle or mister to gently dampen the surface until it is consistently moist but never soggy. Immediately after sowing, cover the tray with a clear plastic humidity dome or plastic wrap to trap moisture. This setup maintains the high humidity necessary for germination.
Maintaining Optimal Germination Conditions
Achieving a high, consistent soil temperature is the most important factor for encouraging rosemary seeds to sprout. The ideal temperature range for successful germination is between 70 and 85°F, which is often difficult to maintain indoors. Placing the seed tray on a specialized seedling heat mat will provide the necessary warmth directly to the root zone, increasing germination speed and reliability.
Supplemental lighting is necessary, as a sunny windowsill alone will not provide enough intensity or duration of light for robust growth. Position a full-spectrum LED grow light directly above the covered seed tray. Rosemary requires a long photoperiod, so the lights should be kept on for 14 to 16 hours each day.
The humidity dome helps maintain the consistent moisture needed for germination, which typically takes two to four weeks. While high humidity is necessary, a lack of air circulation can encourage fungal growth, leading to damping off. To prevent this, lift the humidity dome briefly once or twice a day to allow for air exchange and check the soil moisture. If the surface appears to be drying out, mist it lightly, ensuring the soil remains damp but not saturated.
Caring for Seedlings and Long-Term Growth
Once the seedlings emerge and develop their first true leaves, remove the humidity dome completely to allow for better air circulation. Continue to keep the grow light positioned close to the seedlings to prevent them from stretching (etiolation). If multiple seeds sprouted in one spot, gently thin them by snipping the weaker ones at the soil line, leaving the single strongest seedling.
Transition your watering method to bottom-watering by placing the container in a shallow tray of water for 15 to 30 minutes, allowing the soil to wick moisture up from the drainage holes. This technique encourages deep root growth and keeps the top layer of soil drier, reducing the risk of fungal disease. Since the seedlings remain in the starter mix for several weeks, they will eventually deplete the limited nutrients.
Begin feeding the seedlings with a liquid fertilizer at one-quarter strength, only after the true leaves have fully formed. When the young plants reach three to six inches and have developed a sturdy root system, they are ready for their permanent home. Transplant the rosemary carefully into a larger, well-draining terracotta pot, which helps wick away excess moisture. Use a gritty, sandy potting mix that closely resembles the well-drained soil of its native habitat.