I have crossvine growing up an iron handrail on each side of my steps leading to the front door of my house. This vines was planted about 3 years ago and have performed beautifully. The railing and steps are about 10 ft in length. After three summers, the vine has gotten very thin at the point of planting and the first third of the vine. The other 2/3 portion of the vine (newest growth) is very thick. Can I prune this vine back severly to make if fuller at the bottom instead of being so top-heavy. When is the best time to prune?