The 99th Chelsea Flower Show opens in London on Tuesday. It's the horticultural event of the year, and its roots go even deeper than those 99 years — British horticultural shows have been part of the gardening calendar since the 1870s. Trendspotters at this year's show can expect lots of water-logged foliage after the wettest April in the U.K. record books, and a wary acceptance of a new judging system set to take effect after this year's show.