As you can see, last year was not good for the sunflowers as something ate them before they could get started. Therefore, this year I am going to start the seeds indoors hoping that it will allow them to get big enough so that they can fend off things that would like to eat them. It can also help your remember where you have an extra sunny or cold spot that needs to be taken into account when laying out your garden next year. Other benefits include keeping track of your germination rates of the seeds that you are storing.
They say that a picture is worth a thousand words, so another way to keep a log is to take pictures with the date on the picture. Digital pictures are also easy to find on the computer, as the date and sometimes location are stored with the picture. You can even add comments with the digital picture. The drawback is that without adding comments, down the road some of these pictures are going to be hard for your future self to interpret. For instance, how are you going to know what the plant is before its true leaves have appeared?
A final important aspect of a garden log is tracking your weather conditions throughout the year. The helps you see patterns in your local climate. Are you colder or hotter than other recorded local temperatures? Does one area of your garden get two hours more sun than another? The best tool for that is a weather station that records the information to a computer. Sites like Weather Underground will help you find someone who records the weather close to where you live, but temperatures can very widely on neighboring lots. For example, the weather station used to make prediction's for our city is on a different hill top several miles away and not on our hill slanting west like our property – this can alter the local weather patterns enough that our weather is slightly different than the evening news. So the best is to set up your own weather station. I like the Oregon Scientific WMR200 as you can have multiple temperature sensors, which helps you keep track of those different areas (shade, sun, etc.).
So happy gardening and good luck keeping track of how your garden grows.