University of Illinois Extension
http://www.urbanext.uiuc.edu/tropicalpunch/

Tropical-Looking Perennials

Many perennials can give the garden a tropical feel. Choose from Arunucus (goatsbeard), Flipendula (meadowsweet), large ornamental grasses such as Erianthus (hardy pampas), Rheum (ornamental rhubarb), Crambe, and many hardy, large-leaved ferns. Hardy hibiscus (Hibiscus moscheutos) with their large flowers (8-10 inches across) can bring the feel of the islands to a midwestern garden. Some of the large-leaved hosta cultivars such as ‘Sum and Substance’, ‘Blue Mammoth’ and ‘Sun Power’ can add a tropical look. Other winners in the big foliage category include Butterbur (Petasites giganteus) with huge 2-3 foot leaves and ornamental rhubarb (Rheum sp.) that grows to an imposing specimen with two-three foot wide leaves.

One of the most basic, diverse and dramatic garden plants is the canna. Cannas are easy to grow, offer a vast array of foliage and flower colors and range in height from 18 inches to over six feet. They are easy to overwinter and can be used just about anywhere a tropical accent might be needed. They prefer full sun for best performance and can tolerate moist to wet areas such as around water features.

Many seed grown annuals can easily be used to portray a tropical look. Castor beans grow into huge imposing plants, 6-8 feet tall with large colorful leaves and interesting flowers and seedpods. Nicotiana sylvestris (candleabra tobacco) has large green leaves reminiscent of large tobacco plants along with white fragrant flowers that give the look and feel of the tropics.

Tropicals fit almost every situation in the garden ranging from full sun to shade. Whether tropicals are grown in the ground or in containers, attention will need to be given to moisture and fertility. Most tropicals prefer soils that are uniformly moist so water thoroughly. Allowing them to dry out affects foliage quality. They are heavy feeders meaning they should be fertilized once every week or two with a liquid fertilizer. Fertilizers high in nitrogen will help keep the plant actively growing and producing large, healthy leaves.