Plants evolved seeds relatively late in their history, but this single development shifted the course of the plant kingdom. Today, seed plants dominate the landscape, encompassing species as diverse as tall trees to the tiny grasses and flowers underfoot. When you want to identify an unknown plant in your yard or garden, it is usually a seed-bearing plant.
Before you begin, you will need to determine if the plant species you're trying to identify counts as a seed plant or not. Luckily, the evolution of plants makes this distinction fairly easy to make. Plants began in an aquatic environment, reproducing with spores, so the earliest land plants--the mosses--stayed small and close to a water source. Ferns grew larger but still needed water to aid in reproduction with spores. The earliest seed plants, the gymnosperms, developed exposed seeds on cones, and flowering plants followed. Therefore, unless the plant is a moss or a fern, it is a seed plant.
You can find a variety of guides and resources to help you identify seed plants. Field guides provide pictures of plants, followed by detailed descriptions and more information on range, habitat and behavior. According to naturalist Jim Conrad, field guides are generally organized according to physical descriptions of the plant--for example, all plants with yellow flowers are grouped together--or according to relationships between species, such as grouping all members of the sunflower family together. You can find field guides at your public library, and universities and nature groups maintain online versions, often aimed at a very specific geographical region.
Identification keys lead you to choose among defining characteristics. Unlike field guides, you rely on written descriptions to select the best match from among multiple options. Identification keys generally aid in identifying less common plants, but they are harder to find and more difficult for the amateur to use.
Using a plant identification guide primarily calls on you to use your observational skills to identify the characteristics that set the plant apart from related species. Most people pick up on the color and basic shape of the flowers and leaves right away, but you should also observe how the leaves and flowers are arranged with respect to each other. Note the number and arrangement of different parts, such as leaf veins, petals and stamens. If you can't work with the plant or a sample in front of you, make sketches, take photographs or write careful notes about what you see.
What if you've checked every book and website on hand and still can't find your plant? Botanist William A. Niering recommends trying next to identify the family to which the plant belongs. Plants belong to families based on shared physical characteristics, so identifying the family may lead you to species identification. At the least, you will learn more about your plant based on characteristics of the family.