Heat – Marijuana seeds can germinate in many temperature conditions, but grow best between 75 and 80 degrees Fahrenheit (23-26 C).Cooler temperatures slow seed germination and promote fungal growth such as fusarium and pythium which can rot the seed and root. Temperatures above 90 degrees can also inhibit germination.ealthy, viable marijuana seeds should be slightly oblong and shaped like a teardrop with a point at one end. Size varies ,but marijuana seeds are usually about 1/8th of an inch wide and 3/16th of an inch long.