Rosemary (Salvia rosmarinus), a perennial herb native to the Mediterranean region, is a plant adapted to specific climatic conditions. Its ideal indoor temperature range is not a matter of human comfort but a direct reflection of its evolutionary biology. To thrive rather than merely survive, indoor rosemary requires an environment that mimics its native habitat's warm, sunny, and well-ventilated conditions.
From a physiological standpoint, the ideal temperature range for an indoor rosemary plant is between 65°F and 75°F (18°C and 24°C) during the day. Within this range, the plant's metabolic processes, including photosynthesis, respiration, and transpiration, function at their most efficient rate. The enzymes responsible for converting sunlight into energy operate optimally, supporting robust growth, the production of essential oils (which give rosemary its signature aroma and flavor), and overall plant vigor. This range provides the warmth the plant craves without pushing it into a state of heat stress.
Perhaps even more critical than daytime warmth is a significant drop in temperature at night. Rosemary plants require a nighttime temperature that is 10-15 degrees Fahrenheit (5-8 degrees Celsius) cooler than the daytime temperature. Ideal nighttime temperatures should be between 55°F and 65°F (13°C and 18°C). This diurnal temperature variation is a key environmental signal for the plant. It mimics the natural coolness of Mediterranean nights and helps to reduce respiratory loss of energy (sugars) accumulated during the day. Without this cooler period, the plant's growth can become weak and etiolated, and it becomes far more susceptible to diseases.
Straying outside the ideal range triggers significant stress responses in the plant. In conditions above 80°F (27°C), especially if coupled with poor air circulation or high humidity, the plant can enter heat stress. It will close its stomata to conserve water, which simultaneously halts the uptake of carbon dioxide needed for photosynthesis. Growth stalls, and the leaves may become brittle, dry, and begin to brown. Conversely, temperatures below 50°F (10°C) significantly slow the plant's metabolism, putting it into a state of dormancy. While mature plants can tolerate a very light frost, prolonged exposure to temperatures near or below freezing (32°F / 0°C) will cause cellular damage, blackening the foliage and ultimately killing the plant.
A discussion of temperature is incomplete without considering its interaction with light and humidity. The ideal warm temperature range is only beneficial if the plant receives a minimum of 6-8 hours of direct, bright sunlight daily. Without sufficient light, a warm environment simply accelerates the plant's decline as it depletes its energy reserves faster than it can create them. Furthermore, rosemary prefers low to moderate humidity levels (around 45-55%), as high humidity in combination with cool temperatures or poor air flow creates a prime environment for fungal pathogens like powdery mildew and root rot, which are among the most common killers of indoor rosemary.
