Peter, you are missing one very crucial aspect of the scientific method: peer review. In no particular order, science is observation, hypothesis and peer review. Peer review is especially important to the former in the way that a verifiable hypothesis must also be falsifiable: if no one reviews your hypothesis and record of observed data, how can anyone verify your results? Consider that knowledge is empirical verification of what is (else how do you know what is?) and that for your record of observations and hypothesis to be verified as useful for making predictions or simple as stating the case, other people must review and scrutinize the hypothesis and experiments with their own observations and critique.
For example, as your peer and while reviewing your suggested experiment of cutting off leaves I am motivated to question several aspects of what is otherwise an example of taking a hunch, making observations, suggesting a hypothesis and proposing a test to confirm or refute that hypothesis. Does this test the change of color from aging, or just from decay? How would you conduct an experiment to establish the cause of color changes during the life cycle of the plant and its leaves? How will you distinguish cause from correlation? What evidence might show that your hypothesis is false, or are you simply confirming what you want to be the case? Is it enough to make a general observation that certain species of trees have significant changes in the color of their leaves during the Autumn? Is your theory that tree leaves change colors related to decay anything more than a mundane statement of the obvious? Can you think of an abstract way of formulating this process of color changes during life cycle or from decay in such a way that it is applicable to other trees beyond just the particular varietals you have studied?
Not to discourage a sense of investigating the world around you, however, it is also worth noting the investigations, conclusions and confirmed hypotheses of those who have gone before you (these are also your peers and their work is subject to review by you!), such as: https://www.na.fs.fed.us/fhp/pubs/leaves/leaves.shtm or http://harvardforest.fas.harvard.edu/leaves/science Shall we take these authors at their word, or shall we put their claims to the test?