Basil, a culinary herb, is known for its aromatic leaves and vibrant flavor. Originating from tropical regions, this plant is sensitive to cooler conditions, making understanding its temperature tolerance important for gardeners.
Basil’s Temperature Needs
Basil grows best in warm conditions, with optimal daytime temperatures ranging from 70°F to 85°F (21°C to 29°C). Growth can decline above 95°F (35°C). Basil plants show signs of stress when temperatures consistently drop below 50°F (10°C), leading to stunted growth and leaf damage.
Temperatures below 45°F (7.2°C) can be lethal for basil, and frost at 32°F (0°C) or lower kills the plant. This is due to irreversible cellular damage as ice crystals form within plant tissues. Younger basil plants and tender varieties are particularly susceptible.
Signs of Cold Stress and Damage
When exposed to cold, basil plants display several visual indicators of stress. Leaves may wilt, appearing droopy. This wilting often progresses to a water-soaked appearance as cellular structures break down.
The foliage then darkens, turning black or brown. This discoloration indicates significant cellular damage, from which the plant cannot recover. Cold stress also results in stunted growth and a reduction in the plant’s characteristic aroma.
Protecting Outdoor Basil from Cold
For outdoor basil, temporary protection can shield plants during cold snaps or early autumn chills. Covering plants with breathable materials like frost cloths, old sheets, or blankets before temperatures drop helps trap warmth. Cloches or inverted buckets also provide a protective barrier for individual plants.
Ensure any covering does not directly touch the basil leaves, as this transfers cold and causes localized damage. Once daytime temperatures rise above 40°F (4°C), remove covers to allow air circulation and sunlight. A layer of mulch around the base of the plant offers insulation to the soil and roots.
Bringing Basil Indoors for Winter
To extend basil’s life beyond the outdoor growing season, bring plants indoors before nighttime temperatures consistently fall into the 50°F (10°C) range. When transplanting, carefully dig around the plant to preserve as much of the root ball as possible. Choose a pot with good drainage that is at least 8 to 10 inches deep for established plants.
Once indoors, basil requires ample light, ideally 6 to 8 hours of direct sunlight daily from a sunny, south-facing window. If natural light is insufficient, supplemental grow lights set for 10 to 14 hours a day, positioned a few inches above the plant, provide necessary intensity. Water when the top inch of soil feels dry, keeping it consistently moist but not soggy, and reduce watering frequency compared to summer.
Harvesting Basil Before Frost
As cold temperatures approach and frost is predicted, harvest basil generously to maximize yield. Instead of picking individual leaves, cut entire stems approximately a quarter-inch above a leaf node. This allows for efficient processing of the harvested herb.
Preserve collected basil through various methods. Freezing is effective for retaining flavor, either by blanching whole leaves or by pureeing them with olive oil and freezing in ice cube trays. Drying, either in an oven on a low setting or by air-drying, allows for long-term storage, as does preparing pesto.