One of the most common weaknesses of an experiment involves what

One of the most common weaknesses in scientific experiments involves a lack of appropriate controls. A control is needed to know what exactly causes the change in the experiment.

For example, if you want to measure the influence of water on the plants, then you should have plants which are watered and some plants that aren't - if you only water all your plants, you can't actually be sure if they grew because of water or if they'd have grown anyway without it.