Spiky, mounding, trailing.
That's the plan to keep in mind when filling a container with plants: You want a tall one like salvia in the back or the middle, flanked by masses of something shorter like begonia, and the whole thing ringed with ivy that tumbles over the sides.
But what if you're having a Georgia O'Keeffe attack, craving big color and contrast, feeling like you want to stick some 4-foot curly willow in there with raging-rose ti plant, pumpkin celosia, and black sweet potato vine on the side?
"You can't go wrong," says Karl Gercens, Longwood Gardens' container expert and someone you might expect to be Mr. Traditional.
He can do traditional. But he prefers bright and bold, envisioning "container ops" everywhere. Longwood's elegant Conservatory Complex, for example, the glass-wrapped horticultural showplace in Kennett Square that routinely prompts visitor gasps, is just "one big container garden" to this guy.
Most container gardens are less grand, which helps explain their continuing popularity. A National Gardening Association survey done by Harris Interactive Inc. put container-gardening sales at $927 million last year, with roughly one in five American households filling pots with flowers, vegetables or herbs.
Nancy Terramin of Newtown Square has dozens of containers on her second-floor deck, which measures 55 feet by 12 feet and looks out into the woods. She fills them with annuals, perennials, vines, bulbs and herbs, even Japanese maple trees.
"I move them around a lot, to sunny parts or shade, or you can say 'This doesn't work' and move it, and you haven't ripped up the yard," Terramin says. "That's the wonderful thing."
Containers are also easy-care - and getting easier.
You can now buy self-watering containers and water-wise irrigation systems that attach to hose bibs and hook up to pots. The systems are so energy-efficient they've been exempted from water restrictions in the drought-dry South, according to Pamela Crawford of Georgia, author of the new Easy Container Gardens (Color Garden Publishing, $19.95).
"You're watering directly into the roots of the plants. It's watering slowly, which gives the roots time to absorb it, and it's not giving off any evaporation," says Crawford, who subscribes to the "spiky, mounding, trailing" formula with fervor.
You can also buy preplanted containers or small preplanted pots that fit inside bigger ones that you can change with the seasons. Customers under 30 especially like these, says Jim Feeney of Feeney's Nursery & Garden Center in Feasterville.
"The younger generation doesn't have time to come in, get a container, select the plants, and put them in there. They want instant gratification, and price isn't that important to them," says Feeney, who sells the planted-pots-with-inserts for about $65.
As for empty pots, we may daydream about Tuscan terra cotta and our new life snipping sage by the villa's front door. But hard-fired clay is a not-so-smart choice in this part of the world.
Even "frostproof" terra cotta can crack in winter. It's expensive, heavy, and prone to mold, too.
Untreated wood's good, but it can rot. Metal lasts, but it can cook your plants or rust. Ceramic's arresting but expensive, and it can't take frost. And cast stone is forever; just try moving it.
Hands down, the experts like plastic. It's lightweight, crack-resistant and durable. Though "faux," it can be quite attractive, says garden designer Carreen Wright of Berwyn.
If you're not wild about plastic pots, Wright says, "you can camouflage them with trailing plants or spray-paint them. There are paints that will adhere to plastic. That's a perfectly easy fix for a cheapo pot."
Cheapo or not, big is best. "With container gardens and fish ponds," Gercens says, "you're going to get it and immediately wish it was bigger."
No matter which type, containers need holes in the bottom, feet underneath (no saucers, please), and - very important - potting mix inside. This is an airy, easy-draining blend of sphagnum peat moss, vermiculite and perlite. (You can also use half potting mix, half compost.)
"If you squeeze the bag of potting mix and it's nice and soft, light and fluffy," Gercens says, "it's good for your plants."
And you can put lots of them in a container without worrying that they'll smother one another. Think of it as a living flower arrangement, one that, despite the organic trend, should be fertilized.
Many potting mixes already contain fertilizer. Terramin makes her own, Crawford markets her own, and Wright uses fish emulsion. What to do?
"There's a lot of confusion on this," Crawford acknowledges. "Every year, it seems there are 462 new fertilizers on the market."
Gercens suggests fertilizing "weekly and weakly," using half the manufacturer's recommended dose. But do fertilize.
"If you don't," Wright says, "you're not going to get the flower power out of your annuals that you could. You can make that choice, but you sacrifice a lot."
And containers, by nature, are not about sacrifice. They indulge taste, mood and season to an extraordinary degree. Best of all, much of the time, they do it using ordinary plants.
Virginia A. Smith blogs about gardening at http://go.philly.com/kisstheearth.
Contact gardening writer Virginia Smith at 215-854-5720 or email@example.com.