there's allot we would need to know. It could be you are trying to grow a sativa or sativa dominate plant which will grow tall and slender in the beginning , the problem is, in the northern hemisphere the sativa doesn't have a long enough season. 1 and a half feet though isn't anything out of the ordinary for even an indica dom strain out side in five weeks. make sure its getting at least 6 hours of DIRECT sunlight during the day. If you want bushy clip the very top off and it will stop growing up for a while and start growing its branches out. each branch can have the tops clipped to make it even bushier. but usually if you have a plant outside growing tall and spindly it isn't getting enough light. at least 6 hrs direct preferably during the most intense period of light between 10am and 4 pm