I avoid touching the plant and often shoot in areas where I can't disturb the surrounding plants or ground. Therefore, I rarely use a tripod.
The best days for shadows are those with bright sunshine and little to no wind. Early morning or late afternoon is preferred because of the longer shadows. I look for shadows that make a nice composition, which usually means photographing several different plants until I find the right angle of sunlight. It's great when I can photograph the plants in bloom, but some plants have more visual interest right before they bloom or as they begin to decay. Sometimes it's the seed pods or the remaining heads that make the best shadow. Although I've discovered that not all plants create beautiful shadows. Some just look like blobs. This summer I'll be visiting some of the blobs again to see if I can get a better angle.
The other challenge is trying to get as much of the shadow in focus as possible, as the part of the plant closest to the white board is obviously the sharpest. However, sometimes having part of the image out-of-focus adds to the composition and feel of the image. So it's just trial and error, following my gut......and sometimes just working with what I've got. :)