The vegetative phase of a cannabis plant is also known as the growth phase. It starts as soon as the plant stops being a seedling and its length will vary if we're growing indoors or outdoors. In the open air, the vegetative phase often coincides with the spring-summer months, right when the sunlight is more intense. During this stage, the plant will grow and develop a strong stem and vigorous branches for the forthcoming flowering phase that will start when the days shorten and the hours of sunlight lessen.

If you're growing indoors instead, you control the length of the vegetative phase. For example, you can keep your plant into a vegetative stage as much as you wish by using an 18/6 light cycle until she's big enough. Once you switch it to a 12/12 cycle, the plant will think the time to flower has finally come. Some people actually keep their plants into a vegetative state for years in order to get cuttings out of them. These are called mother plants. This is only possible with photoperiod marijuana plants. Autoflowering genetics, as the name suggests, don't depend on the light cycle but flower automatically once they're mature enough.