I think this thread has three underlying issues being discussed all intermixed. The original core issue is one of edge cases in urban environments, cases in which whether the plant was planted are in doubt.
That is,
In edge cases this is clearly a judgement call based on the question “Was this planted by a person?” Age of the plant and whether the plant is “interesting” are not being weighed in this judgement. And, as with any judgement call, others may reach a different conclusion. That said, the only question being judged is, “Did someone plant this?” More nuanced categories might provide other information, but the core question is ultimately binary.
The second topic centers on the premium status in iNaturalist that being eligible for research grade brings. That status is reserved to wild organisms and that was the founding design intent of the application. The About blurb “iNaturalist helps you identify the plants and animals around you” should perhaps say “iNaturalist helps you identify the wild plants and animals around you.” so as not to mislead users into thinking they can identify their garden flowers with the site. iNaturalist was founded to connect people to wild nature, not planted plants. That is a mission level matter for the organization.
The third topic is the matter of having observations identified including casual observations. Underneath this is whether those who engage in identification can keep up with observations.
The discussion You know you’re seriously into iNat when… has 90 posts all of which are about observing and none are about identifying. There are no posts, at least not yet, that say “when you start identifying after breakfast in the morning and when you next look up from your computer the sun is setting and you missed two meals.”
A colleague of mine spends at most a month collecting specimens and then the other eleven months identifying and working on publishing results before returning to the field. I know I spend far more time trying to sort out a single identification than I spend taking images, even if perhaps not always an eleven to one ratio!
Identifying is usually more time consuming than taking a photo. I have no actual idea of what portion of observations are getting identified, but my own poking around regularly turns up organisms unidentified back as far as 2011. In my mind the problem is not the cultivated/captive versus research grade status so much as it is observations overwhelming identifiers. If there were more identifiers then perhaps the captive/cultivated, the planted plants, would also eventually get identified. I wonder how more observers might be encouraged to become identifiers, starting with their own plants. But that discussion is off-topic here.