Are there any online data analysis websites for arduino IoT projects?
To send data to cloud you will need an internet shield and given that the individual plants are far apart, you will need a wifi shield for each plant that you want to monitor most likely the yun shield. 
Now for online services, there are a few like Temboo, Pushingbox, Initial State, Cayenne, Xively.
Each of them works differently and you will need to choose which best suit you.