Oregon Medical Marijuana Grower Laws. How much medical marijuana can I grow? The laws in regards to growing are very straightforward; you cannot grow more that 6 mature plants and 18 seedlings at any one time. A seedling is a plant less that one foot tall or wide and a mature plant is anything over 1 foot. There are no specific laws saying how big a mature plant can get either. Once the plants are harvested you will want to make sure you have only 24 ounces of usable marijuana. A grower: