Q: What exactly is a pregnant onion?
A: There are lots of different species of onion. Some are edible and some are not but two different plants are called “pregnant onion.”
In the first place, there are three kinds of edible onion. The bulb onion, Allium cepa, that you eat on a hamburger grows from seed and makes a single large bulb.
The Egyptian onion, Allium cepa var. prolificum, forms small bulbs at the top of its green stem, which can fall to the ground and form roots —giving rise to the name “walking onion.”
Another onion, Allium cepa var. aggregatum, forms small bulblets alongside the mother bulb. For this reason, it is called “multiplier onion” or, sometimes, “pregnant onion.”
Meanwhile, in a whole different family of plants, we have the houseplant Ornithogalum caudatum, also sometimes called “pregnant onion.” You might recognize the scientific name —each spring the tiny white flowers of star-of-Bethlehem, Ornithogalum umbellatum, blanket metro lawns.
The houseplant onion forms a large bulb that bulges outward at the soil line as it produces bulblets, giving it the appearance of “pregnancy.” This is the one that is most commonly called a pregnant onion.