I have 2 young Maple saplings that I transplanted 4 years ago when they were very thin and about 2 feet tall. They were among many trees that spouted from the seeds of an older tree. I should be thankful that they are thriving and vigorous, however they seem to be growing too tall too fast. They are nearly 20 feet tall, but no more than 1 1/2 inch diameter. I have never fed them. They are in full sun more than half the day. No other trees/bushes around them. Should I be concerned about this vertical explosion of growth? How do I encourage horizontal growth? Should I prune the lower sprigs/limbs yet?