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I see that on some other threads there's a lot of confusion about the meanings of these three words. This is an inevitable consequence of the human tendency to use a somewhat different vocabulary for "common usage" of scientific terms than the scientists use. Gardeners can benefit from coming to understand BOTH meanings of a lot of scientific terms.
Well, almost everybody understands that "fruit" means something different to a botanist than it does to the average person, so we don't get confused when somebody speaks of a tomato as a fruit, we just understand they're using the botanical meaning rather than the common usage.
But when it comes to "annual, perennial, biennial" there's still a lot of confusion. For example, over on another thread, somebody wrote "My favorite annuals are tomato, cabbage, potato, and a few other veggies."
Botanically speaking, not one of these is an annual. Tomatoes and potatoes are perennials, and cabbage is a biennial. So let's talk about what these terms mean botanically:
-- Annual: The plant is genetically programmed, as it were, to die soon after it has produced X number of seeds. Includes many ornamental flowers and most grains, corn (maize, to you Brits) being a prime example. Peas are a true annual, as nearly as I can figure, but keeping them picked before they're fully ripened will help extend their lifetime, as will deadheading ornamental flower plants. Some legumes are perennials, though; there are many leguminous trees such as carob, and if the guy over on the other thread says he kept a lima bean going for seven years, I'll take him at his word and conclude that lima beans are in fact a tender perennial. It would be useful to know if that also applies to the common pole bean, Phaseolus vulgaris, but I don't happen to know about that one.
Most cultivated plants in the cucumber family are true annuals, I'm pretty sure, but there are a few exceptions; it's my understanding that chayote squash is a perennial.
-- Biennial: The plant typically grows foliage in the first year, may (or may not) die back to the roots in a cold winter, survives through winter as best it can, and produces X amount of seeds in the second year, after which it dies off. Includes most of the cabbage family, and several other families.
There is a bit of gray area between biennial and annual. Some varieties of beet will produce a flowering stalk in the same year they are planted, others (e.g. Lutz Winter Keeper) pretty much insist on waiting until the following spring. Even within the same variety there may be some variance; the only Tall Top Early Wonder beet I ever had produce a flower stalk for me, waited until spring to do it, but I have a sneaking suspicion that if it had been grown in a warmer-winter area, it would've produced its flower stalk late in the same year it was planted.
Most varieties of radish will produce a flower stalk in the year of planting, but the big daikons usually will not. I suspect that's why they're so difficult to grow from a spring planting, often dying in the middle of summer without even having grown much; something in the daikon plant's genetic makeup reads the lengthening and shortening of the days, and causes it to "know" it was planted at the "wrong time of year," so it just goes into a sulk and stops trying to live.
-- Perennial: The plant can live for more than two years, if nothing kills it. With our most familiar TENDER perennials, tomatoes and peppers, the something that kills it is usually going to be frost, in most parts of the USA.
Almost all cultivated members of the nightshade family are tender perennials, in fact. Now, I'm going to have to backtrack a bit on the potato. I suppose you could argue that the potato IS a true annual. You would point out that the potato eye, from which next year's generation will spring if the tuber is just left in the ground, is in effect an entirely new plant from this year's mother plant. The thing is, it's basically a CLONE of the mother plant, genetically identical. Is my clone, if I were cloned, just me in new clothes, or is he his own person? I'll leave it to the philosophers to figure that one out. More to the point: What exactly happens to the roots of this year's mother potato plant, after the foliage dies back? If the roots are kept watered and warmish through a mild winter, will new foliage come back from this year's root tissue, separately from the foliage from the potato eyes on its tubers? I suspect not, but I don't happen to know.
The overall point being, though, still the same: many plants that we CALL annuals in common usage are really tender perennials or biennials that we customarily grow as annuals. Understanding that distinction can be very much to the gardener's advantage, especially for seed savers.
If you were wondering, that guy was me. I kept the plant going by providing LFWW, (light, fertilizer, water, and warmth). A warm window, plenty of moisture, a dash of fertilizer occasionally, and some warmth gave the plant a period similar to 7 years, all without winter. To be honest, I did not expect it to live so long, and it grew only in a plastic Dixie cup! It stayed bonsai size, but the flower and bean output was outstanding!
Hmmm... Well, now, that's interesting. Lima beans don't need a whole lot of room underground to produce a full-sized bean plant, but they do need more than a little Dixie cup.
Any time a plant becomes potbound, it's basically being grown hyproponically from that point on, which is why most container-grown plants need to be watered each and every day.
Mother Nature is not totally unfamiliar with this condition; long before grass and weeds began poking up through the cracks in man-made sidewalks, she had taught plants to survive in a handful or two of dirt trapped in a cleft between two rocks. So it's not a totally unnatural condition, just rather unusual.
The way a lot of plants respond to this condition is, they start putting out fruit, and hence seeds, far earlier than they would otherwise. I once bought transplants of Early Girl and Juliet in medium-small pots that only had about half a gallon soil apiece. I never did get around to putting them outside, and they started fruiting less than a month after I brought them home and put them in the greenhouse. Seeing how well they were doing at cranking out maters, I just left them where they were until the summer heat and spider mites eventually did them in. (As happens sooner or later to almost everything I put in the greenhouse, during the hottest part of the summer.)
The same phenomenon accounts for why you often see nursery transplants of tomatoes and peppers in April or May that already have fruit on them. The plant doesn't know if it's in jail for "life without parole," or if it has any possibility of being paroled someday, but in the meantime, it has conjugal visiting rights (what with being self-pollinating and all,) so instead of devoting its meager resources to adding new stem and leaf growth, it decides to start a whole new generation to carry on the family name.
Now I'm wondering how long that same lima bean would've lived if it HADN'T been so severely potbound.
When we buy seeds here in UK the packet info always tells you the plant is either a biannual but can / should be grown as an annual IF planted early, or is a perennial (like Tomato) but is best grown as an annual, or is a biannual and is sown ???? it will be ready for harvesting the following ???, so if the proper sewing instructions are read, there is no confusion of annual, bi anneal or perennial, I learned that to get the best flavour / colour, or size depending on what you are growing, most veg are harvested the same year of planting or they become tough, woody or tasteless, however things like the cabbage family are best grown here early summer, harvested late winter and some types the following spring, BUT, they are sown, grown and eaten within 12 months /year.
I think the confusion comes with beginner gardeners who perhaps as you rightly said, think to sow and harvest in one season makes the plants an annual, thats because we grow them as such,
The latest thing here now is for Tomato's to be grown from rooted cuttings from the previous year, then grown on as from a seed grown plant, the Tomato's are said to be fruiting earlier and have more fruit, the plants are said to be less prone to the normal diseases too. To buy the plants ready rooted are more expensive but for me it's too early to know if it would be worth my while taking rooted cuttings from my plants and growing them on for the following year, will do an experiment one of those days.
Happy gardening, WeeNel.