You will need to think long term if you want to grow the plants described in this article. Some may take a few years to emerge from the spot where they were planted and others will be slow to develop their full potential. However in each case the final result will be well worth the wait. The seven slow growers are described in an article by Adams County Master Gardener Kay Hinkle which I found on The Master Gardeners website.

Do you plan your landscape on an annual basis with a vision while planting in springtime of what will be blooming within the next few months? Does your planning process span just one growing season? For those of you with “long-distance vision”, this article suggests a few plants that require patience because they don?t bloom every year, but when they do bloom a few years after planting, are well worth the wait!
I encourage you to check each of these late-bloomers on-line to see for yourself what they bring to the garden in the way of size or spectacular blooms. Admittedly, the patience they require is an investment in time and money for you, the gardener. On the other hand, these beauties are not your average petunia, either!
First is the Martagon lily (Lilium martagon). A mature clump of these plants sports at least 50 blooms on dozens of stems. The 2-inch blooms face downward on stems that resemble those of the Oriental Lilly. Colors vary and all sport a strong, sweet scent. The stems do not require staking in spite of the fact that they can reach heights of 6 feet. Be sure to mark the spot where the bulbs are planted as they may not emerge from the soil for a few years. Once planted, do not disturb. These slow movers require sun to part shade and are cold-hardy in zones 3-8.
Next is the Clivia (Clivia miniata). From strappy leaves (much like an amaryllis) come orange, red or yellow blooms in late winter to early spring several years after planting. These plants are cold hardy only in zones 9-11 but can be containerized here in Zone 6. Indoors, Clivia does well in a bright north window. A mature plant can send up as many as 15-20 fabulous blooms on a half dozen stalks. This plant likes to be crowded, so keep the plant in the same pot as long as possible for the best blooms. Water sparingly in winter months to promote bloom. Be sure to plant in a pot that can be moved indoors in for longevity in South Central Pennsylvania!

See more at The Master Gardeners