While experimentation, why do you think it is difficult to assign any effect seen to any single hormone?

Most of the hormones are synergistic to each other in their mode of functions. Thus, during experimentation, we cannot judge whether, a particular effect is produced by a single hormone or is an additive effect of many hormones. For example,
(i) Auxins help to initiate rooting in stem cuttings (an application widely used for plant propagation). Cytokinins also show the similar function of root formation. Auxins promote flowering, e.g. in pineapples. They also induce parthenocarpy, e.g. in tomatoes.
(ii) Both gibberellins and ethylene are synergistic to auxin in initiating flowering and for synchronising fruit set in pineapples.
(iii) Cytokinins and gibberellins help overcome the apical dominance and delay the process of leaf senescence.
(iv) On the other hand, ethylene promotes senescence and abscission of plant organs especially of leaves and flowers. This shows that all hormones are synergistic to each other in their mode of action in plants.