"Amateur astronomy"[1] doesn't have negative connotations. And I don't think this is an exception that proves the rule - as a native speaker of North American English, amateur can have negative connotations, but that isn't the default.
That said, in this particular case, something like "hobbyist" might fit better.
The word means non-professional. So you don't do it for a living. Which is directly factually a description of what's happening here.
In some contexts is could be negative, because the concept of a non professional doing the thing is scary. Like there's nothing positive about an amateur heart surgeon, or amateur parachute designer.
But in the context of gardening it's clear and descriptive. The needs of a professional farmer and someone with a backyard garden are very different. The term clears up which one this app is made for.
Amateur definition:
"sloppy, not professional looking." https://www.urbandictionary.com/define.php?term=amateur
"a person who is not skilled" https://www.oxfordlearnersdictionaries.com/definition/americ...
"one lacking in experience and competence in an art or science" https://www.merriam-webster.com/dictionary/amateur
It's neither positive nor negative most of the time.
Context: I've lived primarily in the Midwest and the East Coast, with a few years spent in New Zealand. I've also been either involved in or peripheral to a few hobbies that style themselves as "amateur X."
Same here. Native English speaker, lived on the East Coast all of my life. I don't find "amateur" to necessarily have negative connotations, but clearly it depends on the context.