iNaturalist
For both beginners and experts, I highly recommend using iNaturalist.
It is a website and an app. It is extremely useful to have the app on your phone, which allows you to photograph a plant in the field and often get an immediate identification based on its image analysis software.
You can also take photos with a camera and upload them on a computer.
While this should never be completely trusted, it is surprisingly accurate. Other users will often suggest IDs for plants you record. And it has lists of similar or suggested plant ID’s you can browse through and compare to your plant.
It is an excellent record of plants which you find. It keeps a map of all your and other users’ observations. So, if you want to return to a plant that you know has berries in its fruiting season, just refer to the map.
I use it to find rare species I know occur in my area, as other users have happened to record observations and I just go there.
Tips for ID with iNaturalist:
Try to have a clear photo of just one plant. If other plants are intermixed, separate them out.
A contrasting background is very helpful. I isolate the one plant and provide a contrasting background by putting my hand behind the plant I wish to photograph. That also helps my camera focus and gives a scale. A sheet of paper can also be used.
Getting photos of the flowers and leaves is most helpful, but additional photos of the whole plant and its other significant parts can also help.
Plant ID Groups
Plant groups such as Facebook pages dedicated to plant ID or plants in your area can be very helpful. For example, this Texas Flora Facebook page quickly gets accurate identifications for users posting photos. There are many such forums and groups online, and I recommend joining one based on your area.
Field Guides
Look for a field guide to plants in your area. These can be helpful for beginners when there is a good photo section. Other more complete listings of plants and their descriptions can be useful for experts trying to distinguish similar or rare species.
Never assume a field guide is a definitive method of identification, as they never include every species, and there may be a close look-alike not mentioned.
My favorite field guide (beginner) for Texas:
Tull, Delena and George Oxford Miller. 1999. Wildflowers, trees, and shrubs of Texas. Taylor Trade Publishing, Lanham MD.
My favorite plant guide (expert) for Texas:
Vines, Robert A. 1960. Trees, shrubs, and woody vines of the Southwest. University of Texas Press, Austin, TX.
My favorite plant guide (expert) for California:
Hickman, James C. (ed.). 1993. The Jepson manual: higher plants of California. University of California Press, Berkeley, CA.
Online Databases
General databases with range maps, photos, and more:
I use these every day for many reasons.
A Community for Naturalists · iNaturalist
USDA Plants Database (USA only)
2014 BONAP North American Plant Atlas
Taxonomic databases:
These are excellent when facing any confusion with scientific names, e.g. which is valid or what old ones are considered now.
Home (worldfloraonline.org) (definitive current accepted names)
International Plant Names Index (ipni.org) (find any scientific name)
Integrated Taxonomic Information System (itis.gov) (easy to navigate lists of names / taxonomic indices)
Other useful databases:
The National Gardening Association (planting guides, flower / fruit seasons)
FNA (floranorthamerica.org) (technical description, ID keys)
Flora of North America @ efloras.org (ID keys)
Texas specific:
Lady Bird Johnson Wildflower Center – The University of Texas at Austin (excellent resource for info)
Native Plant Society of Texas (npsot.org)
California specific:
Calflora – A non-profit database providing information on wild California plants (excellent resource for ID and info)
General Tips
Annual herbaceous plants tend to be more difficult to identify, especially without flowers.
A guide to wildflowers in your area can be helpful for this, but generally are limited to herbaceous plants with showy flowers.
In winter, many trees and shrubs can still be identified by their twigs.
Grasses are quite difficult to distinguish for beginners, and a dedicated field guide is recommended.
For Texas, this one is excellent:
Hatch, Stephan L., Umphres, Kelly C., and A. Jenét Ardoin. 2015. Field guide to common Texas grasses. Texas A& M University Press, College Station, TX.
Build experience by identifying every unfamiliar plant, and reminding yourself of plants you know each time you see them.
Pro tips
The odor of the crushed leaves can be a very useful ID characteristic. However, this characteristic is rarely described as an ID method. It must be developed by personal experience. One exception is the genus Allium. Everything in that genus smells like onions, and it’s extremely helpful since they are rather inconspicuous grass-like plants with deadly poisonous lookalikes. Many species in the Lamiaceae (mint family) can often be identified as belonging to that group by smell alone.
Leaf texture and color and bark texture and color are also very helpful to consider when developing a good gestalt sense of plant ID.