Cold Stress
Definition: Cold stress is slowed growth or tissue damage in cannabis caused by temperatures below the plant's comfort range, generally under about 60°F (16°C). Mild cold slows nutrient uptake and metabolism, while frost near 32°F (0°C) can kill leaves and buds outright, which is why outdoor timing matters so much in northern states.
How Cold Stress works
Cannabis is a warm-season annual. It tolerates a chilly night or two, but sustained cold slows every process from root growth to resin production.
Temperature thresholds
- Below about 60°F (16°C): growth slows noticeably, and roots absorb nutrients less efficiently.
- Below about 50°F (10°C) for several nights: chilling damage, stalled plants, and possible leaf discoloration.
- 32°F (0°C) and below: frost can rupture cells, leaving leaves and buds limp, dark, and dead within a day.
Indoor signs
Basement and garage grows often run cold during lights-off in winter. Slow growth, droopy leaves, and purple stems or leaves are common. Purple color can be harmless genetics, but on a strain that normally stays green, cold nights reducing phosphorus uptake are a likely cause. Keep pots off cold concrete floors, and hold the night drop to 10°F or less.
Outdoor timing
Outdoor growers face cold at both ends of the season. Plant after your last spring frost, and choose genetics that finish before your first fall frost. Cold nights also bring dew and high humidity, so late-season chill increases the risk of bud rot. Your USDA hardiness zone gives a rough sense of season length, but local frost dates are more useful.
Protection
Indoors, a thermostatically controlled heater or a warmer room solves most problems. Outdoors, frost cloth, moving pots against a south-facing wall, and choosing autoflowers or fast-flowering strains for short seasons all help.