I’d like to tell you how it’s possible that broccoli, cabbage, kale, and collard greens can all be the same species of plant and why you shouldn’t throw out your beet greens.
If you were a botanist and were studying the diversity of plant life that is grown on a farm your first approach would be to categorize all the plants by their phylogenic family, genus, and species. This is the typical way that all biologists organize life on earth. We have all been exposed to some of this organizing and re-organizing of the living world. Of course our own species, Homo sapiens, is included in the pantheon of life, along with the blue jay, Cyanocitta cristata, and broccoli, Brassica oleracea.
The organizing of life starts very broadly and becomes narrow as you hone to the category in which an individual organism belongs. The final two categories (usually) in the binomial are the genus (meaning “general”, the general category) and the species (the “specific” category). If two individuals share a genus then they are more similar to each other genetically then they are to a individual in another genus. Think of a beach rose and a pasture rose. Both types of roses are in the genus Rosa but they belong to different species categories. They are different from one another in that they probably will not interbreed and they display very different characteristics. However they are more similar to each other than they are to another genus in the Rose family (for instance, Spirea or Potentilla).
On the farm we can identify many different families of plants. Let’s look at a few examples. Squash and Cucumbers belong to the Cucurbit family (Cucurbitaceae). Lettuce, Sunflowers, and Burdock belong to the Aster family (Asteraceae). Beets and Spinach are found in the Goose-Foot Family (Chenopodiaceae) and Broccoli and Cabbage are found in the Brassica family (Brassicaceae). Within each of these family groups are many genera (general groups) and many species (specific groups). Many of the wild plants (aka “weeds”) that grow on the farm also belong to these families and cause implications for crop management because of their genetic similarities.
Some of the crops we grow are very genetically similar but look very different. On the other hand some of the crops we grow are genetically different but look very similar. Let’s look at squash for a second. Summer squash and Winter squash are distinctions made between different varieties of squash but they are somewhat arbitrary. Summer squash and Zucchini for example both belong to the group Cucurbita pepo and their growth habits and fruit structure are very similar. However both acorn squash and delicata squash are both Cucurbita pepo as well and these are known as Winter Squash because they grow on longer vines and are harvested as a hard skin fruit later in the year. Genetically the acorn squash and the zucchini are more similar then they are to a butternut or a pumpkin, but we see them as different because of how we decide to grow and harvest them. Even butternut squash (Cucurbita moschata) and pumpkins (Cucurbita maxima) are different species.
The Brassica family is an example of the other extreme. Many of the famous brassicas that are grown are extremely closely related. Broccoli, cabbage, collard greens, Brussel sprouts, kohlrabi and some types of kales are in fact all the same species of plant (Brassica oleracea). How can that be possible considering how different they are? This is a very good example of artificially selecting different traits of a plant and breeding these traits “out” to suit different culinary interests. Each one of the varieties within this species has been bred with a specific trait in mind. Collard greens and kale were bred for the thick, delicious leaves. Brussel sprouts were bred for large, fat lateral buds. Cabbage was bred for a enormous central, leafy head. Kohlrabi was bred for a fat, juicy stem. Broccoli was bred to have an exaggerated, tight knit flower cluster. Each of these varieties provides a different culinary interest to people. Many of these varieties developed independent of one another in different parts of the world, but they all originated from the same species (not the same individual).
Another example of this selective breeding can be seen among beets and swiss chard. Grown side by side you would notice that these two plants look very similar except for some striking differences. The chard would have taller, wider, often more colorful leaves and no visible tap root. The beet on the other hand would have relatively stouter leaves often of darker color and a visibly large taproot. Chard and beets are the same species (Beta vulgaris). They have become different varieties of the same species based on an interest in having a culinary option for a milder leaf (the chard) and the culinary option for a large, nutritious root (the beet).
Now to wrap up. When people breed plants they inadvertently make them less nutritious. We breed vegetables often to maximize the pleasant, sweeter flavors and to wash out the wild, bitter flavors. A perfect example of this is Swiss chard. The more nutritious, stronger tasting beet greens have been “watered down” to grow large and mild. By continuing to select for a milder, larger leaf people have bred some of the nutrition out of the beet and given us Swiss chard. This has been so effective that many people don’t even consider eating their beet greens and toss them out. But beet greens are in fact one of the most nutritious vegetables you can grow (up there with turnip greens and dandelion greens) so perhaps try cooking them or sprinkling them on a salad next time you get some beets.