Selfhosted

This sounds more like a job for QGIS considering you are working trees. Partnered with QField, the app, you can create any number of "surveys" that will capture what you do on each pass (depending on how and what you set up), which you can then poll that data with the myriad of tools that QGIS has available. QField can also poll that data and display it to you in a limited form (QField is a mobile QGIS).

If you are fully pumped on the selfhost part, QFieldCloud can be selfhosted which each QField device/QGIS Desktop can then sync to and from each project you set up on that "cloud". A syncing desktop user can see realtime what a QField user is adding to the map if they have a data connection available.

You can also go full data offline (if infield has spotty connection) if you create maptiles and share a basemap to each device via cable/network, or if you are selfhosting (or paid hosting) can also share those basemaps with each user. Borrowing a drone and mapping your property will get you extremely high quality basemap so you can zoom into even juvenile trees with a basic GPS but there are a lot of free imagery services about. There may be contour maps available for your area (our local government has 0.25m contours available), which you can stamp onto your basemap so people can instantly tell the lay of the land with a visual cue.

QField ships with 2 sample projects so you can get an idea (https://docs.qfield.org/get-started/sample-projects/). You can even import Excel sheets into QGIS if that interests you. Spreadsheet is good, but GIS adds a visual aspect that reinforces the overall project and helps laypeople work out exactly what's where. It basically is a spreadsheet/database under the hood with any number of visuals on top. However you write your survey/s will dictate what is captured in the database. And this can include timestamped photos over the years if visual tree assessments are included.

Edit: https://qfield.org/success-stories/ for some examples. It can be as complicated or as simple as you need.

This looks like it ticks all your boxes

I honestly believe a well made spreadsheet would be best.

• You can have it in "the cloud" for multiuser.
• You can put some automation around it through an API if you use google sheets or onlyoffice or one of the many other spreadsheet providers that have an API. (Like for example phone notification when you need to water or fertilize a plant)
• It's easy to get started.
• And it's infinitely flexible to suit your exact needs.
• You can probably even abuse a page and make a graphical layout map of your garden with clickable links to specific plant pages/sections.

Have you looked on here https://github.com/awesome-selfhosted/awesome-selfhosted

Maybe this?

Open Jardin: a software for managing garden crops

Download page - sourceforge

some more information here, OpenJardin.eu

This sounds like it wouldn't be far away from just a basic spreadsheet. Dates, treatments, produce, etc., all easily added in there without much fuss.

You can share a single Joplin profile with shared data across multiple users, but a shared spreadsheet might make more sense.