A dichotomous key is a classification tool that allows you to identify organisms and minerals based on a series of dichotomies, or contradictions that occur between two objects. According to the University of Nebraska-Lincoln website, when setting up a dichotomous key, you must establish one less step -- or dichotomy -- than the total number of organisms you are identifying; otherwise you won't be able to divide the organisms evenly. So for a dichotomous key for four common garden plants, you would need three dichotomies.
The first dichotomy in a dichotomous key for four common garden plants could be between herbaceous plants and woody plants. While herbaceous plants have flexible stems and branch structures, woody plants have more rigid, bark-covered structures. As West Virginia University notes, marigolds -- which produce rounded or flat-topped red, yellow, orange and maroon flowers -- are examples of common, herbaceous garden plants. People commonly use marigolds in flower beds and flower pots, as well as for walkway and patio edgings.
Boxwood is a slow-growing evergreen shrub. In contrast to marigolds, boxwoods are woody plants, which would put them in a different category under the first dichotomy of a dichotomous garden plant key. The trees typically feature dark green, glossy leaves and are available in over 160 cultivars. While some cultivars offer leaves with different sizes, colours, shapes and textures, other cultivars have higher resistances to cold climates and infestations. As North Carolina State University notes, people incorporate boxwoods into contemporary as well as traditional landscape designs.
Tomatoes, like marigolds, fall into the herbaceous plant category under the first dichotomy of the key. To distinguish tomatoes and marigolds from one another, you can establish a second dichotomy, such as between fruiting plants and non-fruiting plants. While tomato plants produce large, juicy fruits annually, landing them in the fruiting plant category, marigolds do not produce any fruits, which puts them in the non-fruiting category. According to Purdue University, tomatoes are the most popular vegetables for growing in the home garden. While many tomato cultivars produce red fruits, others can produce pink, orange or yellow fruits.
Lilac trees are woody plants, which puts them in the same category as boxwood. To distinguish lilac from boxwood, you can create a third dichotomy in your dichotomous key for common garden plants. One option is to use a dichotomy between plants with showy flowers and plants with inconspicuous flowers. While boxwood plants have inconspicuous flowers, meaning the flowers are hard-to-see because they are small or because the foliage is hiding them, lilac trees produce large, branching clusters of flowers known as panicles. These flowers are highly fragrant and typically white, pink or violet in colour.