The best conditions for cannabis seedlings are a bright, warm (21-23ºC) and humid environment with moving air.
Propagators are simple to obtain and provide an ideal start for young cannabis plants. A propagator is a plastic tray with a transparent top which holds in humidity and warm air while allowing seedlings to receive light. This an easy way to maintain a good environment for very young seedlings . However, since the cover prevents normal air movement (which is very important as it encourages young plants to grow strong stems), cannabis seedlings should not be kept in a covered propagator for too long.
Once the young plants produce their second or third pair of serrated leaves, or if they grow taller than about 7 cm with only the first pair, the cover of the propagator should be removed. A small oscillating fan on the lowest setting is a good way to give indoor plants the air movement they need, while outdoor plants can start to receive the natural outdoor air movement when they have reached this size.
The simplest form of propagator would be a waterproof box with clear plastic sheeting over the top. The advantage of shop-bought propagators is that they often have built-in heating and/or vents in the cover to allow temperature and humidity regulation. They also usually allow more light to reach the seedlings.
If the seedlings are intended for outdoor growing, the covered propagator should be placed next to a window to give the seedlings light. By the time outdoor seedlings are big enough to have the propagator’s cover removed, the process of acclimatizing them to outdoor conditions can begin (weather allowing).