Marijuana Seedlings Moisture and warmth from the soil will cause your seeds to awaken. Within a few days, if the soil is warm enough, the seed casing will crack open and a fragile root and stem will form. When the seedling emerges it is at its most vulnerable and your weed clearing and good soil preparation will have paid off.Marijuana Vegetative Growth Once established your seedlings will soon enter a vigorous vegetative growth stage. From such fragile beginnings plants quickly become full and robust. Unlike growing marijuana indoors you have no control over the duration of the vegetative growth stage and over the course of a summer plants can become huge. It is a good idea during this phase to feed your plants well with some quality, nitrogen rich, fertilizer.