Every host has done it: While picking up some last-minute ingredients at the supermarket, you snag a cellophane-wrapped spray of flowers, bring it home, and plunk it into a vase ― where, well, it just kind of sits there.

But it doesn't have to be that way, says Carolyn Berner, a Seattle florist whose company, Flora Tropica, supplies local markets with flowers. In fact, buying supermarket flowers can actually be a brilliant idea.

First, there's the price: Mixed bouquets generally go for $10 to $20. "It's an affordable way to have cut flowers," Berner says, "and selections are becoming more varied all the time."

It's all a matter of making more of your bouquet than what you picked up in the store.

So the next time you're rushing around before a dinner party, go ahead and grab that bouquet. But follow Berner's simple advice for transforming it into a centerpiece that's truly worthy of being the center of attention.

FIVE TIPS TO TRY

Mix gerberas with homegrown greens The cheerful, daisylike blooms look great when paired with lush greens. Berner first fills a vase with leaves from the garden. Then she arranges four gerbera blooms evenly around the vase, saving one slightly taller gerbera for the center of the arrangement.

Ditch the clear glass vase To make familiar flowers look fresh, Berner puts them into teacups, woven baskets, vintage trophies, enamelware pitchers, or terra-cotta flowerpots (line nonwatertight containers with a hidden jar to hold the water).

Make 12 roses add up to more than a dozen Berner likes to divide a bunch of long-stemmed roses into several smaller arrangements. She tosses out the baby's breath and subs in sprays of flowers from the garden, such as Leptospermum (New Zealand tea tree), as a filler between the flowers.

Change your arrangement to fit the spot If the arrangement is for display against an entry or a hallway wall, place tallest flowers at the back of the bouquet; shorter ones at the front. If your design is for a centerpiece, place the tallest stems in the middle of the vase and shorter ones around the edges.

Reinvent the carnation Miniature carnations are absurdly cheap and can be found any time of year, though many hosts snub these humble blooms. But Berner says it's all about making them surprising: "I love to pair dark burgundy carnations with lime green Kermit poms. The colors create a lot of energy together."

FOR LASTING BLOOMS…

Carolyn Berner has a few must-dos for keeping a bouquet fresher longer.

Be a fussy shopper Always pull the flowers out of the store's bucket to make sure there's not slime along the bottom of the stem. Even when the top of a bunch looks great, you don't know what's under the water. Opt also for tight or partially closed flower heads, which will last longer when you bring them home.

Take the time to prep Use a sharp knife or garden pruners to trim at least an inch from the bottom of each stem. Cut at a 45° angle to allow the stem to soak in more water. Remove any foliage from the portion of stems that will be underwater in your vase. And before you start arranging, place the flowers in tepid water for 30 minutes to hydrate them before you add them to cooler water in the vase.

Don't forget to water Be sure to change the water every day or two. "It's more important to give stems fresh water than to use those little packets of floral food," Berner says.

Design: Flora Tropica (206/228-3568)