Jane C. Martin
Extension Agent, Horticulture
Many gardeners grow herbaceous plants to add color and texture to the spring, summer, and fall landscape. "Herbaceous" means non-woody, and includes many common annuals and perennials grown for bloom or foliage effect. Success is nearly assured if a proper planting site is selected and good cultural practices are followed. Knowing the particulars of a plant's culture is beyond the scope of this fact sheet; do further research on your own for plants that interest you.
The term "annual" refers to a plant that completes its life cycle in one growing season. It begins from seed, flowers, produces seed, then dies. A "biennial" is a plant requiring two growing seasons. The first season, it is simply vegetative. It overwinters, and in the second season produces flowers and seeds and then dies. A "perennial" is a plant that is winter hardy and reappears year after year from its crown and root system. It can bloom and produce seed every season. Sometimes a perennial will not bloom in its first season. This may be due to establishment problems, improper photoperiod, or lack of a cold period that induces bloom. Plants started from seed in winter seedings (in greenhouses) often fail to bloom the first season.
When choosing the proper site for a plant, consider hardiness zone, light exposures, and soil characteristics. Hardiness zone refers to minimum winter temperatures. In general, the northern half of Ohio is in zone 5 with -10 degrees F to -20 degrees F; southern Ohio is in zone 6, with 0 degrees F to -10 degrees F minimum temperatures. Check a USDA zone map for more detailed areas. When selecting biennials or perennials, make sure they are hardy to your zone. Borderline hardy plants may survive in a colder zone if growing in well-drained soil, additional mulch is provided, and winters are not severe.
Light exposure is critical for plant growth and bloom. "Full sun" is considered to be six hours or more of direct sun. "Partial shade" is a half day of sun (morning sun and afternoon shade) or a filtered shade through high branched trees. "Full shade" is no direct sun exposure. Some plants are particular as to light required while others are more adaptable. Pay attention to lighting on your property before siting flower beds. Trees, fences, and buildings can all affect exposure. Sun-loving plants growing in shade will be lanky and flower poorly; shade lovers in sun will probably turn a light green, wilt easily, and exhibit leaf scorch.
Good soil preparation is essential for success. Be aware of soil type on your property and be willing to amend and work it to provide good aeration and drainage. Sandy soils may need additional organic matter incorporated (compost, dried manure, or peat moss) to help retain moisture and nutrients. Clay soils will typically need amendments, such as peat moss or compost, to improve aeration and drainage. Soils that drain poorly, especially in winter, cause the death of many perennials due to crown rot.
Start with a soil test that will give a pH reading and nutrient levels. Most perennials grow well in the 6.2 to 6.7 pH range and most annuals do fine in the 6.0 to 7.0 range. A few perennials for acid soil include: Iris ensata, Iris cristata, Chelone, Asclepias, Phlox, Baptisia, and Astilbe. Perennials preferring alkaline soil include Centranthus, Centaurea, Dianthus, Geranium, Dictamnus, and Gypsophila. The pH can be changed through the addition of lime or sulfur (or sulfur-containing) products. Phosphorous and potassium may need to be added. Follow soil test recommendations for rates. These items can be added and incorporated as the bed is being prepared.
Bed preparation is ideally done the summer or fall before planting the following year. Soil is typically drier than in spring and can be worked more easily. Perennial weeds can be completely killed, sod can be removed, and the action of winter freezes and thaws can break up clay clods. Perennial beds should be worked deeply, to an 18 to 24 inch depth if possible. Add one-third by volume of organic material and incorporate throughout the depth; sphagnum peat moss, dried manure, or compost work well. Gypsum, often sold as a "soil softener," does not work as an amendment in Ohio. Annual beds should be worked 8 to 10 inches deep and organic matter added (one-third by volume) and incorporated as well. Also, add and incorporate two pounds of a 5-10-10 fertilizer or the equivalent per 100 square feet of bed area, unless soil test recommendations state otherwise.
In wide beds, make some provision for paths or stepping stones to reduce soil compaction during maintenance. Some gardeners work from a board laid in the bed to distribute their weight over the soil.
Perennials and annuals can be obtained from many sources in spring and early summer. Hardy perennials can be planted as early as mid-April. Most annuals are tender and can safely be planted about mid-May in central Ohio. A few "hardy annuals" can be seeded directly outside before mid-May. These include Alyssum, Calendula, Clarkia, Eschscholzia, Godetia, Nasturtium, Nigella, Phacelia, Sanvitalia, and Lathyrus. Dates will be a week or two earlier for southern Ohio and a week or two later for northern Ohio. This allows for late frost and for soil warming.
Some perennials are available bare-root in spring and fall. These divisions should be soaked in a bucket of water for half an hour to thoroughly wet the root system before planting.
Planting depth should be the same as container depth or to soil line on bare root plants; don't bury the crowns. If planted too deeply, crowns and roots may rot; if too shallow, they may dry out. Plants in containers may become rootbound. If so, disturb the root system by gently pulling it apart before planting (figure 1 and 2). Most annual plant tags will give spacing tips. Use the following table for spacing perennials:
|Mature plant height||Spacing between plants|
|3 feet or above||2 to 3 feet|
|2 to 3 feet||1 1/2 to 2 feet|
|below 2 feet||1 foot|
Figure 1. Potbound circling roots.
Figure 2. Disturb roots slightly before planting.
After planting, water well with a "starter" solution of fertilizer that is high in phosphorous and aids root establishment.
Established perennials can be transplanted in early spring just after growth starts, or in late summer (early September). Dig as large a rootball as possible and keep plants watered to aid establishment. If plants must be relocated during the growing season, cut off all flowers, cut back foliage, and attempt to transplant on a cloudy day. Dig a large root ball and follow good watering practices.
Additional water may be required during the growing season. In general, plants need one inch of water per week as rainfall and/or applied water. Watering should be done in the morning, and at soil level, rather than sprinkled over the top of plants, which may spread disease. Soaker hoses, bubblers, and water breakers all aid in watering. Plants that are recently planted or transplanted may need additional water during the establishment period.
Mulches are used to help prevent mud-splashed blooms, conserve moisture, moderate soil temperatures, and suppress weeds. Spring mulch can be applied about June 1 in central Ohio, once soil has warmed. A depth of two inches is sufficient and should be kept away from plant stems. Compost, chipped or shredded bark, pine needles, shredded leaves, etc., are all suitable to use. Perennials that are transplanted or newly divided in fall should have three to four inches of mulch applied over the crown after the ground freezes to prevent soil from freezing and thawing and potentially heaving plants out of the ground. Plants that are marginally hardy should also have three to four inches of mulch for winter protection. Pull these mulches back from the crowns once new growth begins to emerge in spring.
Perennial weeds are best killed prior to planting with glyphosate (see Soil and Bed Preparation section). Preemergent weed control products (trifluralin, dacthal) are labeled for use around many flowering plants. These are applied to a weed-free soil surface in spring, after the bed is planted, to disrupt germination of annual weed seed. However, these products do not control established perennial weeds. Hand pulling, cultivation, and mulching are also useful methods of weed management.
"Pinching" is done to promote bushiness and, consequently, more flowering. Many annuals are "soft pinched;" the terminal is pinched out to allow for branching. Gardeners may give annuals a pinch at planting, and then pinch again later in the season to rejuvenate and encourage more new growth. Do not pinch cockscomb, poppies, stock or balsam which tend not to branch. Perennials are pinched or pruned for the same reasons. Research specific plants to know when and how much to pinch.
Both perennials and annuals should be deadheaded (dead flower removed) throughout the season. This encourages rebloom, eliminates seed production and self-seeding, and helps maintain compactness. Don't deadhead, however, if you wish to dry flowers, save seed, or are wanting seed pod development for drying. Leave flowers on most ornamental grasses and on Sedum 'Autumn Joy' for some winter effect.
Some plants require staking since stems and flowers may flop or fall over in the garden. Plants with heavy flower heads and/or long thin stems tend to blow over or be beaten down in heavy rains. Overly fertile soils that cause succulent, soft stems, or inadequate sunlight can also lead to floppiness.
Staking should be done when perennials are about six inches in height; put stakes on annuals at planting time. This allows plants to grow up through and around the supports, usually hiding them by mid-season. Perennials often requiring support include: Delphinium, Digitalis, Achillea 'Gold Plate,' some peonies, tall lilies, mums, and asters. Tall annuals, such as Cosmos, may also require staking.
Division is a necessary chore in maintaining most perennials. A few plants never like to be disturbed, and should not be moved or divided; Aconitum, Baptisia, Dictamnus, Eryngium, Helleborus, Limonium, and Papaver are good examples. Most others will need division every three to four years or so. It's time to divide when a dead center forms in the crown area, with a ring of plants around it; blooms are fewer and smaller; or growth appears crowded.
A general rule is to divide the plant in the non-bloom season. Midsummer bloomers should be divided in spring. In spring (April/early May), divide when plant growth is two to three inches in height. Fall divisions are done in late August or early September; plants should be semi-dormant and temperatures cooling.
Use a spade to dig the clump and cut off divisions. If you don't want to divide an entire clump, divisions can be cut from the edge of a clump using a spade and trowel (Figures 3 and 4). Some plants have tough, thick root systems that are a challenge to divide; Hosta, Hemerocallis, and Astilbe may be hard to divide.
Figure 3. Heuchera - cut through the crown.
Figure 4. Iris siberica divisions.
Annuals and perennials have their share of pest problems, which are too extensive to discuss in this fact sheet. Research the plants that interest you and become knowledgeable of specific problems.
Rabbits, squirrels, and chipmunks can be a nuisance in flower gardens. Rabbits often feed on young, tender perennial growth as it emerges in spring, or on young transplants. But, they will also feed on foliage, flowers, and even stems of some plants later in the season. They often feed on Cinquefoil, Heuchera, Coreopsis roseum, Lilium foliage and stems, and Cosmos. Squirrels, and sometimes chipmunks, do most of their damage by digging and uprooting transplants or perennial divisions. Some management options for rabbits include 18-inch high fencing (usually impractical in flower gardens), live trapping and relocating, or use of repellent sprays. It is harder to deter squirrels; live trapping and relocating may work unless there are many in an area. Chipmunks can be live trapped or trapped in rat traps baited with peanut butter.
At season's end, after a hard killing frost, the garden can be cleaned out. Cut off annual plants at the soil surface, leaving their roots in the soil to add organic matter. Perennial tops can be cut off too, above the crown. Leaving a short piece of stem, especially if it's a bit woody, may aid in locating plants the following spring. These tops can be chipped or shredded for composting, or otherwise worked into a compost pile. Plants that have a tendency to emerge late in spring, such as Platycodon, should be marked with a wooden or plastic tag, in order to avoid destroying them when working the garden early the following spring.
Other fact sheets in this series that relate to growing herbaceous ornamentals include:
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868