Annual: a plant that goes from seed to flower to seed in one growing season; aka one and done. Included are most of the flowers commonly used in planters and hanging baskets, like impatients and begonias, as well as those sown for ‘cutting’ gardens, like cosmos and zinnias.
Biennial: a plant that forms in year one, flowers in year two, sets seed and dies; aka two and through. Many of these plants, like Dame’s Rocket, appear to be perennial because they self-sow and the patch continues to flower.
Bulb: a nifty little self-contained system, often related to garlic or onion, that produces flowers. With the exception of tulips, which seem to peter out over time no matter what you do, bulbs will continue not only to flower year after year but continue to produce more of themselves by forming bulblets, connected to the parent bulb but capable of being split off and planted on their own.
Some bulbs, like my favorite Turk’s Cap lilies, also produce bulbils which form at the leaf junctures all along the stem. These bulbils drop like seed, sprout, and eventually form plants of their own. Clever things, having two forms of reproduction!
Perennial: plants which, with few exceptions, come back up year after year, often doubling in size in just a season or two. The exceptions are the so-called ‘tender‘ perennials, like delphinia, which never seem to acquire the strength (at least in Northern gardens) to survive and thrive. Their more rugged brethren ~ Siberian iris and day lily ~ are virtually indestructible even in the hands of the most derelict gardener.
Save for peonies, the Garbos of the garden, clump-forming perennials should be divided every three to five years to avoid the center of the clump becoming so congested it begins to die out and forces all new growth to the outer edge of the ring. Anyone who’s ever tried to separate an old, untouched clump of Siberian iris knows how impenetrable that center can be, impervious to all but the sharpest of spades. Dig and divide in three to five, for the good of both the plant and your back! If you’re faced with such a clump, use a straight spade (your local hardware store likely has someone who can put a sharp edge on it) and be ruthless. Ordinarily I’d say hand-separate to tease out each individual rhizome (or tuber, in the case of day lilies) but when the clump is that far gone your spade and a sturdy work boot are the best course.
There are a couple of other circumstances that cause this type of center die-back as well: crown rot, a fungus which affects hosta, among others, causing die-back at the crown; and iris borers, pesky little caterpillars that eat their way through the meaty flesh of rhizomes, their favorites being bearded iris. Both are pretty easy to spot and pretty easy to correct.
Herbaceous plants: die back at the end of every year and flower solely on new growth ~ think green herb rather than brown spice.
Woody plants: do not die back every year and flower on both new and old (aka woody) growth. Many of these plants, oddly enough, are herbs ~ the landscape workhorses of lavender, English thyme and Russian sage among them.
Architectural plants: usually refers to those plants with bold, clean shapes that make a ‘statement’ in the landscape. I prefer a broader definition, one that encompasses plants which directly reflect architectural shapes ~ a vase-shaped tree, for example, is the flip side of a sharply-angled roof, while a globe-shaped tree is its complement. This still leaves out all material too delicate or fussy to be considered a statement of anything, but keeps the design process focused on the interrelationship of all shapes on the site, whether architectural, botanical or topographical.