In the early spring, the High Line gardeners cut back the previous growing season's perennials and grasses to make room for new growth. The entire process is called High Line Green-Up, and it takes place every year as soon as the temperature starts to warm up.
Links:
[1] http://www.thehighline.org/content/high-line-green-up-preparing-the-planting-beds-for-spring
[2] http://www.thehighline.org/yt-category/nonprofit
[3] http://www.thehighline.org/yt-tags/high-line
[4] http://www.thehighline.org/yt-tags/friends-of-the-high-line
[5] http://www.thehighline.org/yt-tags/park
[6] http://www.thehighline.org/yt-tags/nyc-park
[7] http://www.thehighline.org/yt-tags/gardening
[8] http://www.thehighline.org/yt-tags/piet-oudolf
[9] http://www.thehighline.org/yt-tags/spring
[10] http://www.thehighline.org/yt-tags/nyc
[11] http://www.thehighline.org/yt-tags/horticulture
[12] http://www.thehighline.org/yt-tags/james-corner-field-operations
[13] http://www.thehighline.org/yt-tags/diller-scofidio-renfro
[14] http://www.youtube.com/watch?v=GtrWOchhJZI