I've installed the "Flora Incgnita" plant ID app on my phone, and have tested out a little bit in the last two days in between rain showers when I've had a break at work.
Although our campus is now starting to look very autumnal, there are still a few flowers about for me to test the app on or try and ID, and when it works Flora Incognita seems to be a pretty good way of getting a reasonable ID.
It is a very simple app to use, and unlike others seems to be 100% free with a database of several thousand species to ID against. You simply activate the app, take a photo of the top of the flower and if this isn't enough to get a positive result, it asks you to take a picture of the top of the leaf, then a sideview of the whole plant.
It was very quick to confirm pennyroyal, st john's wort, bristly ox-tongue (a new flower to me, and wow you can see how its leaves give it its name) and gave an ID on autumn hawkbit after three photos.
The one it seems confused about is clustertop vervain, AKA verbena. The wild garden seems to have two distinct varieties of vervain growing, one with just long stems leading to the flower head, which I think is your actual cluster top vervain, and another shorter variety with spiny leaves, and a sort of feathery body beneath the flower cluster.
It thinks this is clustertop vervain as well, but I'm not so sure.
It may be that the photo quality wasn't good enough, my Moto G 7 plus struggles with focus on flowers when there is a cluttered background.
I will keep trying it, and report back further.
All text and images copyright CreamCrackeredNature 27.08.20