As long as you put down the cardboard with a wide berth around the hydrangeas – say, a good 2′ or so – it won’t hurt them. However, in a warm, humid climate like yours, the cardboard is unlikely to last long enough to accomplish the weed control you need – especially if the weed is indeed Ornithogalum umbellatum, also known as star of Bethlehem, as we suggested. It’s very difficult to control, and you’d need to keep a thick layer of cardboard over the area for several months of hot weather for it to have any effect – especially when the weeds go dormant and the bulbs nestle into the much cooler soil to avoid the stifling effect intended by using the cardboard.