With enough space to put a white backdrop behind the plants, it's jut a case of lighting the backdrop brighter than the plant so that the white backdrop blows out before the plant does. The more space you can include between the plant and the backdrop the easier this will be.
Use a flash inside a large modifier placed slightly above the camera to light the plant. If needed, match the color of the flash to the ambient light using color correction gels on the flash inside the modifier. I would start with exposure settings that allow the ambient light to be about one stop or so underexposed, then set the flash for the power needed to give proper exposure at those settings.
- Remember to keep shutter speed at or below your camera's flash sync speed.
- Set your aperture to give good depth of field, probably around f/8 or f/5.6.
- Then set your ISO to give one stop or a bit more underexposure.
If you've got a good, stable tripod with a wired or wireless remote release and there's no wind blowing your plant around use base ISO (probably ISO 100 for most cameras) and set the exposure time long enough to get that -1 stop exposure before you add the flash.
If there's so much ambient light that even at f/8 and ISO 100 it won't get your exposure time at or below the camera's sync speed, then use neutral density filters on the lens to reduce the amount of light entering the lens.
Use a flash or two placed behind the plant to light the backdrop. If a single flash from below will not be seen, that often works. It's true the bottom will be brighter than the top, but if you use enough flash power to blow out the top, it will appear the same as the bottom in the photo: pure white. You don't need to use two strip boxes crossing from either side unless you want to keep the backdrop to uniformly just below full saturation and/or you're in very confined space and are concerned about light from the backdrop flash bouncing around inside a mostly dark studio. Also, since you're blowing out the background (fully saturating all three channels) you don't need to worry about matching the color of the flash lighting the backdrop to the ambient light.