Fall is the best time for crops from the Brassicaceae family. There are more than 3,200 species in this family worldwide including many of our common crop plants, including broccoli, arugula and radishes. Some of the oldest crop plants are brassicas, and over centuries lots of variation has occurred resulting in a diversity of tastes, textures and growing habits. Despite this, there are patterns that all Brassica's have to help identify them.
First, all Brassicas have four petals arranged in a "X" or "H" shape. Above are two flowering salad greens - tokyo bekana and arugula. The color and petal shape are very different, but they both have four petals in a cross shape. 'Cruciferae' an older name for the plant family means 'cross-bearing' and described the crucifix shape of the leaves. ('Cruci' also means torture, which some people might feel when they are forced to eat broccoli or brussel sprouts.) The modern name "Brassica" comes from the latin word 'brassic' which simply means cabbage.
A second way to recognize brassicas is by their heart-shaped cotyledons. Cotyledons are the embryonic leaves that first appear when dicots sprout. All brassica crops plants have two that are heart-shaped. I assume wild brassicas do to, but this isn't used to ID brassicas in the wild because it's rare to see wild plants when they are so small.
What are all these crop plants I've hinted at?
Even if you don't like eating them, I'm sure you're familiar with many of them. I'm only going to discuss the varieties that are typically grown in our region.
First, and most common for us North American, temperate folks:
B. oleracea or the cole crops
Broccoli, brussel sprouts and kale are all part of this species. Cole crops developed in Northern Europe prior to the Middle Ages - thus the reason why they like moderate, temperate climates. They have developed in all sorts of ways so the edible part of the plant might be enlarged flower heads (broccoli, cauliflower), dense headed leaves (cabbage), or a swollen stem (kohlrabi). Leaves develop a waxy coating as they mature, which is why some varieties can be eaten as salad greens when young, but have to be cooked when they are older. Arugula, Eruca sativa, is not a cole crop, but did develop in the Mediterranean region.
Brassica rapa contains many familiar salad and cooking greens. Many of the greens I trialed as part of my master's research were varieties of this species. Typically these greens are mild flavored, high in nutrient content and can have thick, crunchy petioles (stems).
A second big group of brassica's used for salad and cooking greens are the mustards - Brassica juncea. Varieties in this group are often hot! and spicy! Many varieties have appealing shapes and textures so they look nice in salad, and give it interesting taste. (Yet another name for the Brassicaceae family of plants is the 'Mustard family.' This common name seems to encompass these spicy greens, as well as many weedy plants that plague farmers fields and the side of the highway in spring.)
I've mapped a lot of crops that I didn't discuss. Some of these are important oil crops, or forage crops that aren't often grown in rocky, cool New England. The Wikipedia page on Brassicas is a good place to start for more information on the family. Other sources I've used include:
Botany in a Day, the pattern method of plant identification by Thomas J Elpel
Asian greens offer tasty, easy-to-grow source of nutrition by Orin Martin
Evolution of crop plants 2nd edition, edited by J Smartt and N W Simmonds