When can seedlings be put under lights or in the sun?

“When can seedlings be put under lights or in the sun?” is a common and sensible question that is often asked by novice cannabis enthusiasts. For those who are growing indoors, cannabis seedlings may be put under lights as soon as they emerge from the soil or growing medium.

• When using HID lighting (usually a metal halide lamp), young seedlings should be kept at least 50cm from the bulb.
• With compact fluorescent lamps (usually 100w or more), a distance of around 15cm should be maintained between the top of the young seedlings and the bulb.
• If using normal fluorescent tubes (18-36w), seedlings can be kept within a few centimetres of the light source.

Cannabis seedlings growing under lights should always be given a gentle breeze from an oscillating fan (a small household fan on the lowest setting is fine), as constant gentle movement will strengthen their stems significantly. If seedlings grow tall and then fall over, this is almost always a result of growing in an environment with static air.

Seedlings intended for outdoor growing should be kept by a sunny window for the first week or two after emerging from the soil. When they have grown their second or third set of serrated leaves (after the the round cotyledons that initially emerge from the seed) seedlings are usually hardy enough to flourish in direct sunlight.

If outdoor temperatures are suitable, seedlings may be acclimatised to direct sunlight  by giving them progressively longer daily exposure to outdoor conditions. Starting with about three hours outside, at the sunniest time of day, seedlings can be given an extra hour of outside exposure each day, so that within about  two weeks they can be left outdoors permanently.