Different people have different methods for this. Here’s what I do:
1. use more than one source of information (and that means please don’t use this blog as your sole method of IDing a plant). I usually refer to two or three different books, and the internet if that’s handy.
2. buy some good weed ID books. You can often pick them up in secondhand books stores and sometimes on trade me. Libraries often have useful books too.
3. ID plants by botanical name. Common names are often used for several different (and often unrelated) plants eg milk thistle can be Silybum marianum or Sonchus spp (aka puha). This is especially important if you are using the internet or books from other countries, as common names vary even more between continents.
4. cultivate relationships with people who know plants: gardeners, botanists, and farmers are all good sources of knowledge and often love to share it. They speak different languages, so become multi-lingual.
5. learn plant ‘keys’. These are structures of plants that help limit what the plant can be eg whether the leaves grow opposite or alternately along a stem. Leaf shape, texture (eg hairy, leathery, smooth), flower structure, colour etc etc are just some of the things that can help ID a plant. Good ID books will specify keys for individual plants as well as give a general guide to plant keys.
6. use different parts of the plant to help ID eg leaf, stalk, flower, root, seed. The more parts you have the easier it is. Take some of the plant home with you if that’s where your books are.
7. don’t eat or make medicine from any plant until you are certain what it is.
To give you an idea about the value of having different sources of information, here’s chickweed in three different books.
Common Weeds of NZ has a black and white photo of a straggly, sparsely growing, older chickweed plant. There are two close ups, one of the flower, and the other of the hairs that grow on one side of the stem (a key).
Weeds of Crops and Gardens in NZ has a black and white photo of lush chickweed showing leaf, flower and seed. There is also a good sized paragraph on the structure of chickweed and how to specifically ID. The second paragraph is about where and how it grows – these paragraphs are aimed at people wanting to control weeds, and there is usually a bit about herbicide use but you can ignore that ;-)
Wild Herbs of Australia and NZ has a simple line drawing of chickweed in seed or bud that shows the distinct characteristics of the plant, alongside a short description. There is also a colour photo relatively close up.
Chickweed is relatively easy to ID, and you could probably get away with one good reference. But it can be confused with mouse-eared chickweed, and the weed books also show that, so you can tell the difference.
Learning new edible and medicinal plants in the wild is fun and satisfying. Enjoy!