Ah, I hadn't considered offline access. This is actually possible to do on a website with the data storage APIs in HTML5 (which both iOS and Android support -- at least, they are supposed to).
An app may be nicer in general for more advanced features, anyway. I'd like to see these:
1) Searching of the plant library.
2) Browsing of plants I own/want to own, along with readings and suitable locations.
3) Lists of locations I have measured with access to the list of plants that will grow there.
4) "Property map" type overview. For example, let me provide a scan of my property map or a sketch and indicate where on that image I have taken readings and where my plants are.
4a) Let me specify watering/other maintenance schedules on the map (or at least show them there). That is, I'd like to indicate how often I expect to water plants and prune them and get reminders of some sort.
4b) "Planning mode"; Use the property map to indicate places I have measured in recommend mode. Let me select each place, show me plants that work there, then let me pick one. In the end, I should be able to save the selections in some form to take to the store or just play with until I come up with something I like.