Every child knows what a flower bulb is. It's a round thing you plant underground when it's dormant, and next spring it makes flowers. Like a Tulip or a Daffodil. These bulbs fit the botanical definition of a bulb = a modified stem and shoot surrounded modified (usually) fleshy leaves that contain stored food.
But sometimes the definition's not so clear. Some of the plants that most people consider bulbs don't ever go dormant (crinums, for example, are definitely bulbs, but most species never lose their leaves). Some plants go dormant underground every year, just like a bulb does, but don't actually grow from roots that look like a bulb.
Some people look at the leaves -- if a plant's leaves look like bulb leaves, the plant is a bulb. But then what do you do about yuccas and agaves? They look a lot like bulb plants, but most don't have any sort of underground bulb.
The more closely you look at the question, the harder it is to define exactly what is and is not a bulb. You either end up leaving out some plants you would have liked to include, or you include some plants that you strongly suspect don't really belong.
It would be nice just to leave the definition a bit vague, but for the purposes of this wiki we have to decide what "counts" as a bulb (and so gets listed), and what is left out. That sometimes leads to passionate discussions with the advocates of a particular plant, so we want to be as open as we can about the process we use to decide.
We ask four questions:
1. Does the plant go dormant completely underground at any point during the year? (For example, a tulip disappears completely underground in mid-summer.) If the answer is yes, we generally include it. If no, go to question 2.
2. Is the plant generally recognized to grow from some sort of thickened storage organ (a bulb, tuber, corm, thickened rhizome, thickened root, or fleshy pseudobulb)? In other words, would scientists call it a geophyte? If the answer is yes, we include it. If no, go to question 3.
3. Does the genus/plant belong to a family/genus in which the majority of species are considered geophytes? If the answer is yes, we include it. If no, go to question 4.
4. Do our members collect and grow it as a bulb, even though it doesn't exactly fit the questions above? If the answer is yes, we include it. If no, we don't.
We include some species that technically aren't bulbs but grow like them. Some species Delphinium (Larkspurs) go dormant and more or less disappear underground in summer. They are often grown by bulb collectors, so we included them under rule 4.
We include some orchids. Most people would not think of orchids as bulbs, but some Orchid species, known as terrestrial orchids, grow from below-ground tubers. Some even go dormant in the summer like other bulbs. Their flowers often have the same extravagant beauty as other orchids, and unfortunately they range from difficult to impossible in cultivation. We classify terrestrial orchids as bulbs.
We exclude succulents. A number of our members grow Lewisia species. They live alongside bulbs in many parts of California, and respond to the exact same growing conditions. We decided to include only the couple of Lewisia species that go dormant underground, but we excluded the majority because they keep their leaves all year. We decided those really count as succulents (and there are other organizations dedicated just to them).
When botanical definitions fail, we use our best judgment.
Left to right: Salvia patens, Xanthorrhoea australis, Nivenia stokoei, Lewisia rediviva, Delphinium nudicaule, Disa graminifolia (a terrestrial orchid).