04/25/2012 04:17 EDT | Updated 06/25/2012 05:12 EDT

Spring Gardening: Gorgeous Containers To Pot Your Plants In (PHOTOS)

TORONTO - Think "thriller, filler and spiller" while shopping at the garden centre and you'll end up with gorgeous planters that will be the envy of all your neighbours.

"Containers used to be very monochromatic. People used to buy one big pot of impatiens, begonias or geraniums and plop them in and that was it," says gardening guru Denis Flanagan.

"Now containers have got so much more exciting and varied and (with) different types of plant material."

And the way to make sure your planters are lush, bold and beautiful is to keep in mind the mantra "you've got to have a thriller, a filler and a spiller," says Flanagan, director of public relations for Landscape Ontario, the province's trade association with more than 2,000 members.

Check out some of the other interesting things you can plant to grow a great garden. Story continues below slideshow.

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"It's a little like laying out a traditional perennial garden. You're basically going to have three different levels, three different heights — a low ground cover, a medium height which is two to three feet (60 centimetres to one metre) and then a higher height, three feet plus, and by grouping those planters together and choosing the plant material to go in them you really get this tiered, levelled effect which is what most traditional gardens are built on."

The "thriller" should be a bold centrepiece, with height and dominance. Choices can include ornamental grasses, canna lilies or phormium, a stunning, upright ornamental plant that originated in New Zealand.

Tropical standards (they resemble miniature trees) like hibiscus, lantanas and shade fuchsias have come a long way, says Flanagan.

"You have that trunk and then that colourful head on the top. That allows you to plant underneath that with something that's a little bulkier, things like osteospermum or even geraniums, begonias, things that have a fairly big flower to them (the 'filler'), and then get into the 'spiller,' the trailer, which could be any number or ivies or trailing verbena, to give you that impact," explains Flanagan, a former host of the HGTV shows "Indoor Gardener" and "One Garden Two Looks."

Dwarf trees or evergreens can be used, but most containers are not insulated well enough to overwinter them, and Flanagan advises finding a permanent home for them in the garden before cold weather hits.

Some designers will opt for simplicity as a statement or element in landscaping, with one sculptural plant placed in a container with graphic lines.

Containers have come a long way from terra cotta and plastic. Metal planters are hot this season, especially bold pinks, though their vibrancy can add an extra challenge when it comes to choosing complementary plant material and placement in the garden.

Some designers will use coloured planters sparingly for a touch of whimsy and then choose more traditional black and terra cotta colours as the mainstay.

Consider the backdrop — house, fence, existing shrubs —when determining urn colour. "If those are a little bit complicated it's better to stick with probably solid colours in the container. That tends to pull things together, he said during an interview from Milton, west of Toronto. "Whereas if the background is a little more bland, that's when you can be more adventurous with the mixed colours in the containers."

People often try to create a balanced look by placing the same arrangement on either side of a door.

"It really works if you've got a centre-hall plan house and everything else is balanced and you've got pillars either side and then that very definite formal look is wonderful. But it doesn't always work," he says.

"Probably the majority of houses are a bit more asymmetrical and sometimes having a larger planter on one side and a smaller planter on the other plays up on that look."

Containers can be used on fences, arbours, trellises, in trees or on the sides of houses to create vertical interest, especially in smaller spaces.

Wrought iron-and-moss baskets are much more interesting visually than the bottom of white hanging plastic pots.

"The beauty with the moss-lined baskets is you can plant the sides of the baskets, not just the top ... you can plant trailing verbena, ivies and have this wonderful full trailing look which you're looking up towards, much more fun, much more exciting."

Garden enthusiasts should be planting containers now to get roots established. Just be aware of local conditions and the last frost dates. If your containers are movable, take them inside a garage or shed on cool nights.

Garden centres make it easy with plenty of preplanted containers that customers can slip into containers for an instant garden. But for do-it-yourselfers, Flanagan advises planning before shopping.

"You can be going in and spending several hundred dollars on plant material and it's really not a bad idea to sit down at the kitchen table, sketch out the containers you've got and start playing with some plant combinations and some numbers on paper before you go to the garden centre."

As for how many plants to put in a container, "you're undoubtedly going to use more than what you think. You really want a full effect." The root balls of annuals can touch one another, Flanagan says.

The advantage of a jam-packed container is that "you're going to get June, July and August out of it and then you're going to start pulling it apart a little bit to put some early fall-time colours in."

For what will do best in sun or shade conditions, do some research online and check signage in garden centres. Keep records of your successes and failures as a gauge for what to buy next season.

Salad crops and herbs can be grown in containers placed by the kitchen or side door or in a window box for those with a balcony.

Flanagan recommends adding 15 to 20 per cent compost into the soil mix, which helps hold moisture, along with slow-release fertilizer. Fertilizing with water-soluble fertilizer every 10 to 14 days through the growing season will keep your containers looking lovely.