...more features: 5) "Heat map" view of the "property map". Based on previous readings, show me, on average, how the locations' sun level, temperature, etc. change over time. 6) FWIW, the previously mentioned property map could just be a picture I take with the phone of a room or my front yard.

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.

I think a mobile version of the site would be just as good. (I have an Android phone :)). See something like http://m.wegmans.com/. There isn't much an app can do that the website couldn't, as the functionality of the device is all via the website. It just needs a more finger-friendly interface.