Hardening Off
Definition: Hardening off is the gradual process of moving indoor-grown cannabis seedlings or clones outdoors over about 7 to 14 days, increasing their exposure to sun, wind and temperature swings a little each day. It toughens leaves and stems so plants avoid sunscald, wilting and transplant shock when they go into the garden for good.
How Hardening Off works
Even strong grow lights deliver less intensity and far less ultraviolet than full summer sun. Indoor leaves also grow with a thinner waxy cuticle because the air is still and humid. Put those plants straight into midday sun and wind, and leaves can bleach white or crisp within hours.
A simple schedule
- Days 1 to 2: 1 to 2 hours outside in bright shade, sheltered from strong wind.
- Days 3 to 4: 3 to 4 hours, including some gentle morning sun.
- Days 5 to 7: half a day in direct sun; bring plants in at night if temperatures drop below 50°F (10°C) to avoid cold stress.
- Days 8 to 10: full days and nights outside, then transplant.
Tips for success
Water before plants go out, since sun and wind dry small pots fast. Watch for leaf curling or pale patches and step back a day if you see them. Transplant on a cloudy day or in late afternoon to soften the shock.
Timing matters for photoperiod plants. Moved outside in early spring while nights are still long, they may start flowering, then revert to vegetative growth as days lengthen. Waiting until after your last frost, often mid-May to early June across much of the US, avoids both cold damage and this premature flowering.