Welcome binchix.
CloudDB is similar to FirebaseDB. From your Project description, you probably can also use a Google spreadsheet as a database. All of these require use of the Web. The TinyDB stores data on the device.
Alarm extension: You may not need this tool. If you open your app every day, your app can check for key dates when it is reactivated and provide your notifications at that time. Much simpler. The alarm extension might be useful if you use your app in conjunction with a mini computer like an Arduino to automate sprinklers etc. at specific days and times.
Several plant watering Projects have been discussed in the community. You might search for them to get ideas for your Project. Here is one File component issue (Using Bluetooth, TinyDB to control a Plant watering system) - #3 by Patryk_F that parts could apply to your Project.
Also, Projects in the MIT Gallery
https://gallery.appinventor.mit.edu/?galleryid=6182311175782400
https://gallery.appinventor.mit.edu/?galleryid=5144113010311168
https://gallery.appinventor.mit.edu/?galleryid=4685162689462272
or enter the MIT Gallery and search for 'water plant'
Good luck with your Project