Design question for an agronomic trial

I have a trial, 5 treatments with 3 replicates. I want to survey these 15 plots a total of 7 times throughout the year. I had thought about using cases, but I dont want the data from each survey to be updated or changed, I need each survey to be a single data set, comparable to the others. I have it currently designed with 5 individual surveys, group by the 3 treatments. But i just wondered if there was a better way of designing it. I basically have 15 surveys, the be collected at 7 different intervals.
Thanks in advnace for any feedback/help!!

Assuming all surveys over time will contain similar questions I would set it up like this:
Registration form --> to create a case and store it’s characteristics (e.g. case name (1-15), treatment)
Follow up form --> for all subsequent surveys to track over time (e.g. growth, # of leaves etc).
For analysis you just have to link these two and you could use the case characteristics to group.

Thanks for this Jos! The surveys will be the same, so ill have a go at setting it up as you have described.