The simplest and most helpful visual aid for this experiment would be if the student sets rulers of some sort or maybe pieces of wood that are properly marked with certain measuring unite. If they are set right next to each plant, the student will be able to monitor constantly how are the plants growing, and the smaller the measuring unites the better, as the student will be able to monitor the situation in higher detail.
Over time, the plants will most surely develop differently, so the student will have it easier to notice the changes. Considering that it is an experiment in question, the student should use proper measurements and to be able to have detailed numbers for it.