Herbaceous Ornamentals

Missouri Master Gardener Core Manual

Mary Kroening
Division of Plant Sciences

Herbaceous ornamental plants comprise a large group of annual and perennial plants. They offer a variety of color, form, foliage, texture and fragrance that add distinctive touches of beauty and interest to personalize your garden and landscape.

Most herbaceous ornamentals are easy to grow and care for. They bring pleasure to the gardener and to those who observe their blossoms and foliage. They also provide a supply of flowers to cut for bouquets for one's own enjoyment or to share or sell.

Get ready to grow

With hundreds of different types of decorative herbaceous plants, the possibilities for their uses are nearly endless. Plants' varied color, texture, form, height, bloom season and environmental needs and adaptations make designing a flower garden an exciting labor. Yet this variation can also make it a daunting prospect to plan a flower garden or a landscape that will include multiple flower species.

Keep in mind that annual and perennial plants are flexible — more flexible than woody trees and shrubs. If you plant a flower bed, and you don't like the effect, or the flowers do not perform well in that location, most herbaceous ornamentals can readily be moved or replaced. Flower gardens are constantly changing. Annuals must be planted each year, and many perennials need to be divided and transplanted every few years.

Start with a good working knowledge of the plants you plan to use. This will ensure more satisfying results and prevent unnecessary reworking of flower gardens.

Most landscapes have several different microclimates, or combinations of light exposure, soil types, moisture, wind exposure and temperatures. Choose the right plant for each location to avoid maintenance and pest problems later.

Light

The amount of sunlight available is a primary consideration for placing plants in the right location. As you select plants for your landscape, look for the following terms that describe plants' light preferences:

  • Full sun is customarily defined as at least 6 hours of direct sunlight per day.
  • Partial or light shade are terms used when a plant receives direct sun much of the day, but is shielded from direct sunlight for several hours. These terms also apply when the plant receives only a little direct sun early or late in the day, under trees pruned with high clearance or where the main light is reflected from buildings.
  • Shade or dense shade refers to areas with low light levels most or all the time, such as under large trees with heavy foliage. Plants growing in such conditions not only must adapt to low light levels, but also must withstand competition from tree roots.

As a general rule, the more sunlight, the more profusely plants will bloom. A plant that prefers full sun might bloom in partial shade, but blossoms will be fewer. Similarly, one that blooms well in light shade might have only sporadic blooms in dense shade. In sunny regions, several hours of shade during the hottest part of the day help protect plants from the peak of summer heat. (See MU Extension publication G6911, Gardening in the Shade.)

Soil

Berms and raised beds

A berm is a raised bed area where the gardener elevates the bed by bringing in a higher quality topsoil and building the planting bed up from ground level by at least 10 inches. Soil in a raised bed warms and dries out more quickly in the spring, which can give plants a good start. Berms can be as small as one truckload of topsoil, or as large as multiple truckloads. They can be shaped to create a nice garden oasis, or shaped in a straight line to create a visual barrier. A berm should be proportional to the overall landscape and blend in as naturally as possible with the overall garden design. Berms also divert water or block the natural flow of water, so use caution in locating them to avoid creating drainage problems elsewhere.

Soil type, texture and condition are as important as light availability. A rich garden soil works best for most flowers. This type of soil holds moisture well and provides a good growing medium. Clay soils tend to crust over or form large cracks as they lose moisture. Rocky soils are too shallow to permit good root growth, and sandy soils dry out too quickly. In areas where clay, rocks or sand predominate, soil amendments or replacement will improve plant performance.

To correct any of these situations, either plan to bring in additional, high-quality topsoil to build raised beds on top of the existing soil or plan to improve the existing soil deeply and thoroughly by incorporating organic matter before planting. Normally, 2 to 3 inches of compost, peat moss or well-rotted manure worked into the top 6 to 8 inches of existing soil will be sufficient. After thorough soil preparation, roots will be able to penetrate more deeply to reach nutrients and moisture. Plants will be more vigorous and less susceptible to pest problems.
 

Water and drainage

The moisture-holding capacity of the soil and its internal drainage are closely linked to soil type. The ideal soil consists of about 45 percent mineral matter, 5 percent organic matter and 50 percent pore space, which should be split evenly between water-filled and air-filled pores. When pore space is too small, the soil lacks oxygen and is said to be poorly drained.

Excellent internal soil drainage is important. This is especially true for perennials. Without good drainage, plants are subject to crown rots, root diseases and heaving during winter's alternating freeze-thaw cycles. Soils can be improved, or amended, by adding organic matter. This will improve internal drainage.

External drainage of the site matters, too. If the spot is low and moisture that drains through the soil has nowhere to go, additional drainage may be needed. Creating a raised bed, or berm, is a possible solution (see box). If little or no drainage improvement is feasible at a certain location, limit your plant selection to those that can tolerate poor soil aeration and wet conditions.

Air movement

Air circulation is another important environmental consideration for a flower bed site. Poor circulation may be caused by windbreaks, fences, buildings, trees, shrubs or even other flowers planted closely together. Where stagnant air pockets occur, foliage will remain damp for extended periods from dew, rain or irrigation, which encourages diseases to develop.

Air movement may be increased by providing adequate spacing between plants, thinning out branches from trees and shrubs, or by pruning trees high to allow air flow beneath their canopies. Air movement can also be increased by funneling breezes through constructed landscape features such as arbors or narrow openings between buildings.

Where a site is exposed, there can be too much air movement, especially for plants that are not considered hardy. You may need to provide wind protection for such sites. Good air flow is desirable, but high winds can easily damage seedlings and tender blooms and knock down tall plants.

Diseases and pests

Although most herbaceous ornamentals have few serious disease or insect problems, the incidence of pests varies in type and severity from year to year in different areas. You can control most of them effectively if you follow these general recommendations:

  • Buy plants that are free of diseases and insects.
  • Purchase disease-resistant species and varieties when available.
  • Remove seriously diseased or insect-infested plants from your garden as soon as you notice them.
  • Apply fungicides and insecticides only as needed, and follow label directions carefully.

Temperature and climate

Herbaceous ornamentals may be classified as either cool-season or warm-season plants, according to their preferred growing temperature range. Many plants have both an upper and a lower temperature threshold, beyond which they do not thrive. Whether a plant is considered hardy or nonhardy depends on its ability to withstand cold temperatures. Most plants will sustain damage if the temperature drops below their winter hardiness threshold. For some plants, this might be zero degrees Fahrenheit. For others, it may be as low as minus 20 degrees or as high as 40 degrees.

Hardy plants that tolerate cold temperatures often have a correspondingly low upper temperature threshold: If the temperature rises above this point for several days, these plants are likely to sustain heat-related damage. These plants are referred to as cool-season plants, and they generally do not thrive in areas with higher summer temperatures. In contrast, warm-season plants more easily tolerate heat and will sustain cold injury at warmer temperatures.

Plant hardiness zone map of MissouriFigure 1
Information about an area's typical weather conditions such as the plant hardiness zone map of Missouri can help gardeners select species and varieties well-suited to local growing conditions.

Source
U.S. Department of Agriculture
 

Several maps have been compiled to provide gardeners with useful information about temperature and weather patterns in different locations in the United States (see Figure 1). The following maps portray climatic information in different ways:

At a smaller scale, microclimatic temperature differences that occur over a landscape or garden can also have a dramatic impact on the success or failure of herbaceous ornamentals. For example, southwest slopes exposed to full sun are much warmer than north-facing slopes. Match the plant's temperature preference to your microclimate for best performance.