Editor’s note: The University of Denver is known for its beautiful campus, punctuated by colorful gardens that dazzle the eye, even in the dog days of summer. To help you plan your own flower garden, we asked DU arborist Bradly Hanks to share some tips for selecting high-achieving annuals and perennials.
Despite Colorado’s challenging conditions and high altitudes, the right selection of perennial and annual plants can produce a colorful garden that lasts throughout the season. Both types of plants, however, come with advantages and disadvantages.
Annuals provide strong color throughout the growing season, but every year, they need to be replanted and their planting site tilled and weeded. Perennials, meanwhile, can last for several years and grow out roots that benefit soil. When weed barrier is properly applied, weeds can be greatly reduced or even eliminated. The drawback to perennials is their shorter blooming period; some bloom only for a week.
Over the years, DU’s horticulturalists have observed that certain perennial varieties retain their blooms for many weeks, and when the early bloomers and late bloomers are properly sequenced, the garden can have color from spring to fall.
Mixing interesting plants and colors
The task of choosing a variety of plants for a colorful display can be daunting, given all the options. Three factors can help in the process of choosing: color, texture and shape.
When selecting for color, remember that the contrast of warm to cool colors pleases the eye. Red, orange and yellow—considered warm colors—work well with their cool counterparts: green, blue, purple and violet. Choosing colors on the opposite side of the color wheel is a good way to go. The warms and cools are shown on opposite sides, so orange is across from blue, yellow is across from purple, and red is across from green. A good example of contrasting colors that complement one another is a leafy lavender kale with a peachy dahlia.
Different textures, shapes and sizes help to create drama in a garden. Texture usually refers to the leaf size, as well as the leaf’s roughness or smoothness. Grasses are considered to have narrow, fine-textured leaves, while hostas are considered to have rough textured leaves. Experimenting with different texture is as easy as going to a local nursery and putting different plants together. Try visualizing those plants assembled in a garden. It is important to note that coarse texture dominates fine texture, so it should be used sparingly.
Plant height adds vertical structure, which draws the eyes to interesting elements. Differing plant shapes add substance and depth.
For those new to gardening, small plantings near the house are a good place to start. Design the beds so they can be enlarged in subsequent years and the different elements slowly introduced.
And now, here is our list of favorite perennials and annuals that consistently grow well and provide a color blast that lasts. Blooming times for perennials are included, so they can be sequenced to provide color throughout the season.
Perennials for plots with full sun and dappled shade
Filigree daisy provides long-stemmed blooms from May through July. They have good drought tolerance and work well as a dry border. Even after the blooms fade, the well-branched, grayish–white foliage provides an interesting texture throughout the growing season. Filigree daisies grow 4-10 inches tall.
Phlox is considered a staple of any garden. Phlox paniculata grows upright and can reach 2-4 feet with full round heads of fragrant white, lavender, pink, rose, red and bicolor blossoms. It blooms from July to September.
Phlox subulata grows short, to about 13 centimeters, and works well as a colorful border planting and groundcover, blooming from May to June.
Stella de Oro daylily, a compact plant that grows 9–12 inches tall, is known for its long blooming period from May through July. It requires little care. Colors range from yellow to gold.
Becky shasta daisy really holds up in hot weather and does best in full sun. Its blooming period is from July through September. The flowers, white with yellow centers, can reach 2–3 feet tall.
Rudbeckia are cheerful flowers in the sunflower family with 3-inch yellow petals and a black raised disc in the center. They grow 2–3 feet tall and bloom from mid-summer to fall. Rudbeckia do well with at least six hours of full sun.
Perennial salvias generally produce blooms all season long if deadheaded. The variety that produces the longest blooms is Salvia x sylvestris. The blue to violet flowers bloom on spikes that grow 18–36 inches tall, depending on variety. They are drought-resistant and do best in full sun.
Firefly amethyst yarrow has the showiest flowers of all the yarrows. Its flowers are a bright lavender pink, and it grows 18–22 inches tall. Other yarrow varieties include white, yellow, red, pink and rust brown. Yarrows are the easiest perennials to grow. All they need are full sun and well-drained soil to bloom from early to late summer.
Ice plant is another low-lying perennial, growing 3–6 inches tall. Drought hardy and fond of full sun, it has red to purple flowers and an interesting texture. The blooms last from June through September.
Russian sage is a tall, attractive and wispy sun-loving plant with silvery foliage and lavender/blue blooms. It can grow up to 5 feet tall and blooms from July through October.
Perennials for shade
Hostas are known for their beautiful leaves, but they also put out a stalk of lovely white flowers. Several different varieties are available with different leaf textures and sizes. Some varieties will grow to a couple of feet in diameter. Blooming times are from late May or June to September.
Bleeding heart has inch-long, heart-shaped flowers that hang from arching stems. Flowers can be pink to white, and they bloom from mid- to late spring until early summer.
Astilbe has plumes of flowers in pink, lavender, red, white and salmon, as well as fern-like foliage. One of the most common perennials for shade, it works well as a border or along paths. Astilbe blooms from early to late summer.
Hydrangea is one of the most popular shade-loving perennials, offering big round clusters of flowers in pink, blue and white. A woody flowering shrub, hydrangeas flower from mid-spring through summer.
Annuals for full sun
Wave petunias grow aggressively, and when planted 12 inches apart, they mound up 16–22 inches high and resemble waves. We get more compliments on the wave petunias than on any other of our flowers. They beautifully flow over when planted in pots. They are available in a variety of colors—red, blue, pink, salmon and white.
Marigold is a cheerful flower that is easy to plant, and it retains its brightness all summer long. It comes in gold, copper and brass. Marigolds can grow to between 6 inches and 2 feet tall.
Dianthuses are a striking carnation-like flower that emit a spicy cinnamon and clove scent. The colors are red, pink, rose, lavender, white and yellow. They grow between 6–18 inches.
Zenia have a daisy-like flower on a single stem. They grow quickly and bloom heavily to create a burst of color. Colors include pink, red, purple, orange, yellow, lavender, white and even green.
Coreopsis is a drought-tolerant and low-maintenance plant that is great for borders and fillers. Colors include red, pink, white and yellow. Many sport dark brown and maroon centers that offer an interesting contrast to the petals.
Dahlias come in a rainbow of colors and several sizes. Dahlia lovers will tell you that once you grow one, you want to grow more. The roots of dahlias are tubers, which can be dug up, stored and replanted the following season to produce yet more dahlias.
Celosia provides shapes that resemble plumed candle flames, coral or something depicted in science fiction. They come in a variety of colors and provide great contrast to other flowers. Most varieties are 6–12 inches in height, but some can reach a height of 3 feet.
Annuals for shade and part sun
Wax begonias are available with green, variegated or bronze-colored leaves and white, rose, pink or red blossoms. They grow to a height of 6–12 inches and are great as a mass planting.
Torenia offers the garden purple, pink and white snapdragon-like flowers. This annual looks especially good draping over planter boxes.
Coleuses have sensational colored foliage that comes in a variety of sizes and shapes. They provide great color for dark, drab corners.
Impatiens provide great colorful blooms to a shade garden. Few annuals that grow in shade provide the intensity of color that impatiens offer. They grow 10–16 inches tall and about 10–15 inches wide.
Oxalis, a low-growing annual that produces lush mounds of shamrock-shaped leaves, beautify the garden with delicately small flowers. They are great for borders, rock gardens and containers.
Browallia are also known as sapphire flowers because of their vibrant blue star-shaped blooms, which contrast dramatically with their bright green foliage. They grow into a tidy mound 12–16 inches high.
Fucshia delights with pendulous flowers that come in a variety of colors, from soft pink and white to magenta and purple. The flowers consist of either single or double blooms.
Sure-fire plants that thrive at high altitude
Gardeners tilling the soil at higher elevation will encounter different challenges from their counterparts in Denver. The high country has various microclimates created by wind and different levels of moisture. Winters are colder, and the local wildlife often finds landscape plantings delectable. Here is a list of perennial plants that do well above 8,000 feet. Some are native, while others come from cold climates in North America.
Serviceberry is a deciduous woody plant that is available as a shrub or a small multi-stemmed or single-stemmed tree. In early spring, white flowers bloom in short, erect clusters.
Potentilla is covered in white or yellow flowers from early June until fall. This shrub will reach 1–3 feet in height and responds well to rejuvenation pruning, which is a hard prune done in late winter or early spring.
Snowball viburnum blooms in mid-spring, displaying white flowers arranged in spherical clusters. Blooms fade in early summer and are followed by clusters of red berries. The leaves resemble maple leaves and are a dark emerald green.
Rock spirea is a native shrub that grows upright with slightly arching branches. Small, serrated leaves turn yellow, orange, gold, red or purple in the fall. Its tiny white flowers bloom in profusion from midsummer into August. Once the blooms fade, a tan fruit remains and lasts through the winter.
Lupine is known for its striking flowers that grow on spikes. The wildflower strains come in hues of blue and white, while domestic varieties offer yellow, pink, blue and purple flowers. The spikes can tower as high as 3 feet and are a good selection for the back of a garden. The flowers contrast well with the texture of the lupine’s foliage.
Columbine blooms in a variety of colors from midspring to early summer. The flowers contrast well with the dark green foliage. Columbines like the sun but don’t like the heat, so the high country is a great place to plant them. After all, they grow naturally and flourish in mountain meadows.
Catmint is a hardy and showy plant with soft gray-green foliage and clusters of lavender-blue flowers. It also seems to be wildlife resistant and blooms for most of the season.