Hardening off is the gradual acclimatisation of a plant to a tougher environment. Indoor-raised plants have thinner cuticles and leaves adapted to low light and still air; put straight into full sun and wind they can bleach, wilt or stall. Over one to two weeks they thicken their leaves and cuticles and adjust stomata.
In practice
For outdoor planting, start with an hour or two of morning sun or shade outdoors, increase daily, and bring plants in on cold nights until they spend full days and nights outside. Night temperatures should be reliably above about 10 °C (50 °F) before planting out. For clones coming out of a dome, the same idea applies to humidity: vents opened over several days.
Note that full sun can exceed 1,500-2,000 µmol/m²/s PPFD at midday, several times what most indoor plants have seen.
Don't confuse with
- Transplant shock: damage from root disturbance; hardening off addresses the above-ground environment.
- Stress training: deliberate bending or damage to shape plants, unrelated.
Microscopic pores, mostly on the underside of leaves, each opened and closed by two guard cells to control gas exchange and water loss.Cold nights are deliberately low night temperatures in late flower; they can bring out purple colour in cultivars that carry it, but don't raise terpenes or resin.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.Transplanting moves a plant and its root ball into a new container or the ground; done gently it causes little shock, but GenetixBay advises against it for autoflowers.A cloning dome is a clear lid over a tray that keeps humidity high around unrooted cuttings or seedlings, reducing water loss until roots form.Outdoor growing is cultivating cannabis in the ground or containers under natural sunlight, where the season's day length controls when photoperiod plants flower.Light stress is damage when leaves or buds absorb more light than they can use or dissipate, showing as bleached white or yellow tops nearest the fixture.
