This web site matches my experience:
"Lombardy poplars are fast-growing trees, growing as much as 6 feet per year. This makes them a popular choice when people want "living wall" privacy screens or windbreaks in a hurry. To serve this function, they are planted in a row, and spaced about 8 feet apart. However, they should be considered only as a stop-gap measure for privacy screens and windbreaks, as they are short-lived, being susceptible to a number of pests and diseases, as mentioned above."
http://landscaping.about.com/od/landscapedesignprivacy/p/lombardy_poplar.htm
