Gardening recommendations from DU’s horticulturalists

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.   

Ice plant

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. 

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