(Editor's Note: This article was originally published on June 26, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
The most important thing is to be familiar with plants at 0 mph. Look at books and websites and learn the names. Get out and look at plants when you can stand still and really study them. Look for unique characteristics of plants. I find that color is a very helpful clue. Many flowers have unique colors and some plants have unique foliage or bark colors. For example, in my area, no other wildflower is the same color as prickly phlox, and no other bush has the same yellow-green foliage as redshanks. With just a glimpse of those colors, I know what plants I have seen. Plant size, shape, and textures can also be clues. Nothing else around here is anything like the chalk liveforever in color and form. A tall, straight tree with mottled bark is a eucalyptus. A tall, curvaceous tree with mottled bark is a sycamore. Observe your garden plants in all stages of their lives. You might know what a mature plant looks like, but could you recognize a field of seedlings? Learn to recognize deciduous trees by their bark or branch structure. Study your plants up close and from across the yard. If you are going to be in an area where you are not familiar with the plants, consider getting a field guide to that area and study the pictures ahead of time.
Next, consider the time of year. This is especially helpful for flowers. Many have a limited season. You aren't going to find asters in April or trilliums in August. It is true that occasionally some rogue plant will bloom out of season, but you won't encounter this often.
Also, know where you are. All plants are specialists to some degree. There is nothing that grows absolutely everywhere. Some plants like shade, some like sun, some like it wet, some like it dry, and so on. For example, I think that bigcone douglas-fir and sugar pine trees look alike. I have never seen either tree enough to learn how to tell them apart at a distance. However, if I am at 4000 feet elevation in the San Gabriel Mountains, I know that the tree in question can only be a bigcone douglas-fir. Sugar pines do not grow at that low of an elevation at this latitude.
You might sometimes be able to get a clue from things besides the plants themselves. When we were driving in North Carolina, we saw fields of some mystery crop. It definitely wasn't tobacco and it didn't look much like the cotton we had seen in Arizona at 75 mph. Then we went through a town that had a sign advertising a peanut festival. The mystery plants were peanuts. Some locales have plant names but be aware that these could be deceptive. The farmers of Apple Valley may have switched to a more profitable fruit. Pine Mountain might actually be covered in fir trees.
If you still are stumped as to what your plant might be, try to remember everything you can about the plant - overall size, colors, size or shape of foliage, growing location, and anything else that seems noteworthy. You may later find the plant identified at a museum or visitor center, encounter someone familiar with plants in the region, have the chance to look through a book, or ask about the plant in the Dave's Garden Plant Identification forum. Without a picture of the plant, you will need every descriptive characteristic that you can gather. Of course, if you are the driver, your first priority is to drive safely.
Lastly, learn to accept an incomplete identification. It takes time to learn the plants and their habits well enough to do high speed identifications. It is impossible to identify everything. Those blue-green transplants might be broccoli, cauliflower, or brussels sprouts, so just be happy with knowing that they are something in the cabbage family. There are dozens if not hundreds of species of lupines in the west. The plants with the spires of purple flowers and palmate leaves will have to be known only as lupine. Don't be discouraged if you can't figure out everything you see. Even a keen observer of nature like Robert Frost can get stumped.
A Passing Glimpse
I often see flowers from a passing car
That are gone before I can tell what they are.
I want to get out of the train and go back
To see what they were beside the track.
I name all the flowers I am sure they weren't;
Not fireweed loving where woods have burnt--
Not bluebells gracing a tunnel mouth--
Not lupine living on sand and drouth.
Was something brushed across my mind
That no one on earth will ever find?
Heaven gives its glimpses only to those
Not in position to look too close.