Euphorbia ‘StarDust White Sparkle’
* Common name: Euphorbia ‘StarDust White Sparkle’ 
* Botanical name: Euphorbia ‘StarDust White Sparkle’
* What it is: An annual flower that grows in a compact mound and is nearly covered by dainty, babys-breath-like flowers all summer. More compact and even heavier in bloom than the excellent ‘Diamond Frost.’
* Size: 10 inches tall, 12 inches wide.
* Where to use: Spot as a specimen annual throughout any sunny mixed garden or use in clusters toward the front of sunny gardens. Also does well in pots and flowers nicely in even half-day sun.
* Care: Plant after danger of frost in early to mid-May. Fertilize monthly with a high-phosphorus fertilizer to maximize blooms. In pots, use the same kind of fertilizer every 2 weeks. Will overwinter next to a sunny window inside for planting back out the following spring. Just cut back the leggy inside growth when planting out. Very bug-, disease-, drought- and animal-resistant. If not saving, yank when frost kills them in fall.
* Great partner: Pink shrub rose. Purple or bright-pink petunias make a good annual-flower partner.
— George Weigel
I’m having trouble understanding how this is classified as an annual unless nurseries want repeat sales. I received one as a gift and there wasn’t a tag so I don’t know the exact cultivar. This was in 2008 so maybe it predates Diamond Frost. It was a large hanging plant cascading several feet. This is its eighth winter that I have brought it into the bathroom. The base has become quite woody and branched. There was no secondary wood when I received it. Some years it trails 5 feet. Each year it falls back to about 18 inches and then fluffs out again when put outside. ( I am zone 7.) It never becomes a shrub or small tree, just an awesome hanging plant. I tell my local non-chain nurseries about it and they are either shocked or obviously don’t believe me.
So definitely not the botanical definition of “annual “.
Having done research in Petunia, I know that it is sold as an annual when it is really a tropical perennial. I had some approaching 2 decades old.
Ellora,
The designations “annual” and “perennial” aren’t commonly used as hard-and-fast botanical definitions but labels we give to plants to describe whether they’re going to make it through more than one season in a particular climate.
That can vary from place to place. In frosty winters like ours, petunias and that euphorbia of yours are classified as annuals. When you let them outside, they normally die when our first freeze comes along in fall.
They’re acting like “perennials” because you’ve protected them from the factor that makes them an annual here – i.e. sub-freezing cold.
Move those same plants to a frost-free environment, and they can live multiple years. In other words, the same euphorbia that’s an annual for us is a perennial in southern California.
Technically, there’s a difference between “true” annuals and ones that act like annuals in a particular climate. “True” annuals are ones that go through their entire life cycle in a single year no matter where they’re growing.
Examples of those are marigolds and zinnias. Flowers that are annuals for us but perennials in frost-free zones include geraniums, begonias, impatiens, snapdragons and coleus in addition to the petunias and euphorbias you mentioned.
By definition, a perennial is a plant that lives more than 2 years. A plant that produces vegetative growth the first year, seeds the second and then dies… that’s a biennial.