If you are thinking about how to grow vegetables, and how to grow them well, then one of the key things to know about plant structure is what is meant by the “crown” of the plant. The crown is the spot where the above ground parts of the plant meet with the below ground parts.
If you are growing plants for food this is a an important distinction, because the plant part you are cultivating (root, shoot or fruit, for example) will differ in its needs, depending on where on the plant, and in the environment it is located. Tomatoes need some sun to ripen well, while their cousin, potatoes need to be covered in soil or they will develop mildly toxic green spots.
Figuring out where to put plants in your annual crop rotations — and how to treat them — is part of this. Crops like Kohlrabi and Celeriac (and yes, Asparagus) are all swollen stems, though they might not seem that similar. Tomatoes and Eggplants are fruits, while Peas and Beans are seeds. Fruits and seeds are easier to discern as “above ground” but what about onions (and garlic and leeks and shallots)? Most gardeners would consider them (after years of hearing so) root crops. But they are not. NOT.
The bulbs of onions are “compressed” leaves. They “bulb” in the same way that cabbages (and lettuces) “head” but for different — and less anthropocentric — reasons. In the arid climate of the Western Asian steppes, the formation of bulbs was a necessary evolutionary development. But that those bulbs are just leaves is readily apparent when you examine the bulb before planting and note that that [sic] bulb is ABOVE the crown, not below.
Thus the love of nitrogen early in the spring, and the danger of nitrogen later on (double centers, poor storage). As a unique kind of leaf crop, the whole onion family (Allium spp.) requires a particular (and very specific) treatment in the vegetable garden. Yet you wouldn’t know this from reading the garden press, which seems to think that an onion is a root crop.
There is a Garden Smarts entry for Onions, if you’re curious.
The takeaway from this page: LOOK at the plant (whether in garden or market) and find the crown (if there is one). Appreciate.