Sim, you need to have a ventilation system in place.
The modern grow tents for marijuana are made with the understanding that your plants need a certain duration of both light and darkness. This implies that they must seal sufficiently to prevent light from leaking through when it shouldn’t.
If you have the tent zipped up all the time, the air inside will be stagnant. So you still need to have a ventilation system in place.
Mold may grow within your grow tent if there is no ventilation. It goes without saying that you don’t want mold to develop on your marijuana. In addition to your plants, it could affect you and your family as well if the mold spreads throughout the air.
