Light is key to making a great photograph.
If not used correctly the color can be dull and make your garden look lifeless or washed out in the sun. Pick an object in your garden. It should be something that can be photographed over a period of time without changing a lot. You can make this an exercise last a week or even a year if you would like to really see how light changes. If you choose to do only a week a long blooming flower will work.