Tropical plants and tender perennials can give an exotic flare to your garden, spicing up containers and perennial borders. Many of them are easy to care for, others need a little pampering, but as long as their light, watering and fertilizing requirements are met, they do well.
Some tender perennials can be used as decorative fillers for containers or as effective edging for a border (when planted in groups of five or more). Scented geranium (Pelargonium), plectranthus (Plectranthus), lantana (Lantana) and licorice plant (Helichrysum) are fairly drought tolerant once established, require very little care and fill out into beautiful masses of flower and/or foliage. They make great, no-fuss additions to the garden.
Scented and ivy-leaf geraniums (Pelargonium) come in a beautiful array of textures and patterns. 'Vancouver Centennial' is a popular red and chartreuse-leaved variety; 'Madame Salleron' has cream-colored leaf edges; and 'Lady Plymouth' provides wonderful textural contrast with delicate finger-like foliage. The licorice plant (Helichrysum) and plectranthus (Plectranthus) come in silver, green, chartreuse and variegated forms.
If it's a purple foliage that you desire, try Persian shield (Strobilanthes), purple spidewort (Tradescantia), blood leaf (Iresine) or Joseph's coat (Alternanthera). All of these make vibrant additions to container plantings and offset annual and perennial displays at the front of your border.
Dahlias are wonderful complements to any perennial border. They pair well with the bold foliage of cannas or the wild architectural structure of honey bush (Melianthus). Dahlias are demanding and insist on nutrient-rich, garden soil that is high in organic matter, a sunny spot and generous feeding and watering throughout the season. Several of the cultivars, with dark, bronze foliage such as the classic 'Bishop of Llandaff', 'David Howard' and 'Fascination', stay a reasonable height (2 to 3 feet) and are easy to grow. The Gallery Series of Dahlias are fairly new on the market and ideal for the no fuss gardener. They have a nice bushy shape, fantastic flowers and only grow to 1½ feet tall so they require no staking.
For late-season color, the temperate and subtropical salvias are a wonderful addition to the garden. They thrive in full sun and well drained sites and are happiest in lean soil that should not be heavily fertilized. Many are tender perennials that will need to be dug up and taken indoors for the winter.
Salvia discolor has a grayish sheen with nodding black flowers, while the flowers of Salvia patens are one of the clearest blues you'll find in the garden. The silver salvia (Salvia argentea) makes a wonderful addition to container displays with its bold, fuzzy, gray, basal rosettes. If you find the Midwestern Salvia azurea 'Nekan' with sky blue flowers, buy it; it flowers from July until September and is hardy in this region. The Mexican bush sage (Salvia leucantha), with its fuzzy purple and white flowers, acts as a magnet for hummingbirds late in the season.
The anise-hyssop (Agastache) cultivars are a nice alternatives to salvias. They enjoy the same growing conditions as the half-hardy salvias, but will survive through the winter. The cultivar 'Blackadder' flowers from July to frost with deep purple blooms and fragrant foliage that is reminiscent of menthol. Agastache 'Tutti Frutti,' 'Heather Queen' and 'Sunset Hyssop' are also outstanding choices that have tall, graceful flower spires in pink and salmon.
Begonias make stunning partners for ubiquitous impatiens in the shady border. The 'Dragon Wing' begonia has small red or pink flowers through the entire season, but its full habit with shiny, lance-shape foliage steals the show. It can handle full sun to shade. The silvery, speckled-leaved Begonia coccinea 'Sinbad' is also superb for full sun to part shade situations. The fantastic, marbled foliage of rex begonias do best in part to full shade. They are good additions to container displays.
None of these plants are hardy enough to survive a winter outdoors in the northeast. If you would like to over-winter one these plants indoors, you will need to treat them carefully and allow them to complete their normal, annual cycle, be that dormancy or continued warmth and sun. A discussion of the over-wintering needs of various plants can be found by following the US National Arboretum "Preserving Your Container Plants" link in the right column of this page.
Encyclopedia of Exotic Plants for Temperate Climates by 
Hot Plants for Cool Climates by 
The New Book of Salvias by 
