To all plant enthusiasts, botanists, and biologists out there, how did you master identifying species to the point where you can tell, in less than a second, the difference between plant A and plant B?
Do you master one family at a time? Do you go out and identify as much as you can?
I asked this because I’m having trouble distinguishing one species of the genus Premna; I got frustrated that I couldn’t tell the difference between the species because they all look the same.
I can’t speak for the genus Premna, but for me it was a combination of continuously going out and literally just iNatting anything and everything, regularly communicating with other naturalists regarding identifications, using dichotomous keys, and recognizing what species prefer which growing conditions(dry/wet/sand/loam/clay/rock,etc).
By far the best way to learn is simply through practice and continued exposure to the species in question.
Here is a paper with a dichotomous key for Premna, it may help. https://www.researchgate.net/publication/236270791_The_genus_Premna_L_Lamiaceae_in_the_Flora_Malesiana_area
Going to organized walks (I am a member of a local branch of the botanical society), trying to identify with the key myself, reading articles about different species when they exist. Not usually the whole family at a time. Rather various taxons that appear in the area relevant to me. And then some interesting ones that I never saw myself from the literature.
However, some genuses are simply difficult and require a specialist. One way s to try ID yourself, but collect the specimen and submit to the herbarium where you know the specialist comes from time to time or even works there. Or submit to iNaturalist if a specialist is active here, but that is not true for some taxons I am interested in.
Reading keys, using different keys and repetition, repetition, repetition, repe…. ;)
With some (a lot) of frustrating moments “why still don’t you know this species, you had seen it so often”.
Start in your region, with species you can see very often and which are easy to recognize. Here in Europe it’s Bellis perennis, Trifolium pratense and repens, Abies alba, Fagus sylvatica, etc. Don’t start with complicated things, where experts still sometimes struggle with (Roses, Salix sp., Poaceae, …).
And try to document every feature of the plant. Leaves (top and bottom), hairs, flowers, fruits, the whole plant, …
And not every species is easy to recognize the whole year. It’s much easier in flowering periode than without flowers.
Just start and don’t hesitate to much - still experts make mistakes. 
And with iNat you have a good feedback, if you are right or wrong, when you can provide pictures in good quality.
Constant practice is the only way. The more you key and learn, the easier it gets. You may not figure out what an identification key is talking about until you’ve seen multiple taxa in multiple populations. Even then, some keys are not very good or contain errors. And some plant taxa are almost impossible to write a good key for as some taxa have many overlapping characters and only the combination of many characters sets them apart in the keys, though they may look very different in reality. When you get really good, you start seeing where the keys are wrong and what might be possibly new species that need further research. And, some plant groups are just really messy with hybrids and/or cryptic taxa that keep even the experts confused. Despite the tricky ones, with lots of practice, you can learn to identify most plants in an area with just a glance. Just always be aware that there are plant taxa in some areas that look very similar to different plant taxa in other areas, so it is always good to key when you are in a new area or have any doubts. A good botanist will question all identifications including their own. Overconfidence lead to misidentifications.
In one way, everything others have already said. But also, I will never be a master at identifying, my skills and knowledge will increase overtime but the more I learn the more I forget, it is simply human nature.
The “in less than a second” part is just about practice and familiarity. No one can scrutinize features and go over keys that quickly, but once you really know a given plant, you can recognize it instantly without having to. I sometimes compare this to recognizing people’s faces. We can pick a family member out of a crowd without going over a checklist of facial features, even if there’s nothing objectively all that distinctive about them. Nobody goes around with a tape measure checking nose length compared to eye width et cetera, but if there were such a thing as keys to human faces, they would be full of such minor differences that one might think, “How would anyone ever memorize that, and be able to tell these people apart in less than a second?”
Some people just have an innate eye for the plant pattern recognition and others don’t, I do believe it is trainable however.
another way around that is to annotate for phenology. When you work thru hundreds of obs, slowly slowly something starts to stick. And if you have learnt A vs B, then go thru the obs stuck at Genus and see if you can move some more. Or do a taxon sweep thru A - and catch one in error.
For me it has been partly about poring over my field guides and the iNat species accounts and other online sources, over and over and over again. Even for the taxa that I feel most comfortable with, I often refer to them.
A big help for me has been filtering for disagreements in a particular genus in identify mode, and then studying observation after observation, reading all of the comments by identifiers, and following their links to other resources. I find that identifiers are more likely to explain how to identify something when they are disagreeing with another identifier; there is a lot of valuable information there.
Occasionally I will ask an identifier that I know is reliable how to tell two species apart; if you politely say “I’m trying to learn this genus, thanks!” most will reply.
This approach is very slow going, but very rewarding, for those taxa that you’ve decided you really want to make an effort to learn.
Good luck!
Sometimes it’s as simple as looking at the taxon photo on Inat and comparing to the field. It’s much easier to identify plants like you would identify a face or landmark you know very well. Practice makes perfect as well.
Start small then snowball - I’ve started floristic mostly identifying plants very nearby me (my game was to findback all species in my village), because you need to known well a “base” of plants - then when you’ll seen another close species, your brain will send you the message “oh this one is like species B, but habit looks odd and it blooms 2 weeks later”.
iNat also helps me a LOT being able to pratice during winter and taking a look to the closely-related taxon in areas i would have never been gone. Once you start understand a genus/group with field experience combined to the internet ressources, related features will become a lot clearer (bc in most flora you actually need to know the plant to really understand key features).
Sorry, TMI.
I have to say that this is the most comforting message I’ve seen this day: to know the limits of being a human.
Thank you!
I didn’t, even though I can give a wrong ID quicker than the CV.
I started with grasses and mosses a couple of years ago. I am still only identifying grasses and mosses for other people.
Thank you for doing those difficult taxa that are under-represented among identifiers … Too tricky for me!
All the above, but also: it doesn’t all have to be done by photography. Don’t be afraid to pick a piece and examine it with a hand lens, within reason. If there is only one flower spike, don’t snap off the whole spike but removing one flower is legitimate in order to get an identification, so long as it isn’t part of a formal nature trail where you could be spoiling it for others.
No one told me they are difficult, I thought I am just a bit slow. Now it is too late.
Maybe we should tell how easy they are, so more people get started.
Tips from the past:
At some point you’ll find yourself detecting issues and errors in keying materials, herbaria, or other supposedly trustable resources. And later, identifying dandelions quite reliably just by driving past them. 