If your camera does not have Flash Exposure Compensation and only expensive ones usually do, a useful tip for using flash in low light, to prevent the colors looking bleached, is to stick a piece of thin white paper in front of the flash as you shoot. This reduces and diffuses the light and you get quite a nice " Victorian water color" effect. Also putting a piece of plain black paper behind the plant, as a sort of improvised screen, cuts out unwanted background. You may need someone's help to hold this in place.