Growing Annual Flowers

Horticulture Information Leaflets

Introduction

Annual flowers offer the gardener a chance to experiment with color, height, texture, and form. Besides providing a massive display of color, annuals are useful for filling spaces where perennial flowers have died, to cover areas where spring-flowering bulbs have died back, and to fill planters, window boxes, and hanging baskets.

Annual flowers bloom more quickly and for a longer period than any other group of plants. They are easy to grow, sturdy, and relatively inexpensive. Plant breeders have produced many new and improved cultivars, such as plants that grow more compact, produce more flowers, and tolerate more sun or shade.

Annuals are plants that complete their life cycle in one year. Most annuals are planted in the spring, flower through the summer and fall, then die with the first killing frost. Winter flowering annuals, such as pansies, violas, and ornamental cabbage, are planted in the fall, flower in the winter and early spring, and die during the summer. Some flowers, such as begonias and snapdragons, that might otherwise be classified as perennial are classified with annuals because they do not consistently survive the winter in a specific location.

Annuals are transplanted or direct seeded each year. Some cultivars naturally reseed themselves. Examples include alyssum, ageratum, cleome, impatiens, melampodium, nasturtium, petunia, and vinca. The ability to reseed varies with cultivars and severity of the winter. Reseeding is not always desirable. Volunteer plants are often scattered or located in clumps instead of growing in their designated area. Also, volunteer seedlings of hybrids may not have the same flower quality as their parents.

Types of Annuals

Annual flowers differ in their tolerance to cold weather and frost. Hardy annuals are the most cold tolerant. They will survive light frost and moderate freezing weather without being killed. In most cases, hardy annuals can be planted in the fall or in the spring before the last frost date. Hardy annuals include calendula, cornflower, foxglove, larkspur, pansy, sweet alyssum, stocks, viola, and many dianthus cultivars. Most hardy annuals are not heat tolerant. They usually decline and die with the onset of hot summer temperatures.

Half-hardy annuals will tolerate periods of cold, damp weather but are damaged by frost. Most half-hardy annuals can be seeded outdoors in early spring since they do not require high soil temperatures to germinate. They are normally planted (seeds or plants) after the last spring frost. Examples of half-hardy annuals include baby's breath, bells of Ireland, blue sage, candytuft, cleome, forget-me-nots, love-in-a-mist, snow-on-the-mountain, strawflower, and torenia. Many half-hardy annuals decline in the midsummer heat but may rebloom in late summer or fall.

Most tender annuals are native to tropical regions of the world and are sensitive to cold soil temperatures and are easily damaged by frost. Wait two to three weeks after the last spring frost to plant (seeds or transplant) outdoors. Most seeds will not germinate when soil temperatures are below 60°F. Tender annuals include ageratum, balsam, begonia, celosia, coleus, globe amaranth, impatiens, marigold, morning glory, nasturtium, nicotiana, petunia, scarlet sage, verbena, vinca, and zinnia.

Annuals are sometimes categorized as cool- or warm-season. Cool-season annuals, such as geranium, petunia, and snapdragon, grow best when the temperatures are in the 70s and 80s during the day. Best flower production is in the spring and fall; flower production declines in the middle of a hot summer. Warm-season annuals, such as blue daze, four-o'clocks, vinca, and pentas, perform well when the daytime temperatures are in the 80s and 90s and the night time temperatures are in the 60s and 70s.

Starting Plants From Seeds

When starting plants from seeds indoors, the seeds are usually sown eight to ten weeks before the last spring frost. Transplants should be harden off by exposing them to outside conditions before planting in their intended site.

Annuals seeded in the garden sometimes fail to germinate properly because the soil surface crusts and prevents entry of water. One way to overcome this is to make a furrow in the soil about 1⁄2 inch deep and fill with vermiculite (if the soil is dry, water the furrow before filling with vermiculite). Then make a shallow furrow in the vermiculite and sow the seed at the rate recommended on the package. Cover the seeds with vermiculite and use a nozzle adjusted to a fine mist to water the seeded area thoroughly. Keep the seed bed well-watered or cover with a mulch, such as newspaper, to prevent excessive evaporation and soil drying. Remove the mulch promptly after germination begins so young seedlings will receive adequate sunlight.

When most outdoor-seeded annuals develop their first pair of true leaves they should be thinned to the recommended spacing. Often excess seedlings can be transplanted to another spot.

Starting With Transplants

Transplants will produce flowers several weeks earlier than direct-seeded flowers. This is especially true for annuals, such as scarlet sage and verbena, which germinate slowly or need several months to bloom from seeds.

Annual flowers can be purchased at a variety of retail businesses in the spring. Buy only healthy plants, free of insects and diseases. Retailers often purchase flowers from a wholesale grower instead of growing the plants themselves. While the quality of plants is often excellent when they first arrive, some retailers are not plant experts or equipped to properly care for plants. Avoid purchasing plants that have been neglected: not watered properly, or have been stored under stressful conditions (hot, paved surfaces) for extended periods. Ask when the plants arrived or if a new shipment will be arriving soon. Freshly stocked plants are preferred to plants that have been held for several weeks. Choose plants with compact foliage, side branches, and good color. It may be tempting to only select the plants in bloom, but younger non-flowering plants are often the best choice since they establish more quickly in the landscape.

Delay purchasing plants until the proper planting time. Plant tender annuals after the danger of frost has past. Plant hardy-annuals, such as pansies after the soil has cooled in the fall. Planting too early can result in cold damage in the spring or heat damage and disease in the fall. Place the plants outdoors in a partially shaded location until they can be planted. Since the soil volume is limited in the small containers, they dry out quickly. Check the plants daily and water as needed. While plants appear to fully recover after wilting it can stunt their potential growth.

Site Selection

The key to successfully growing garden flowers is to match the planting site with the needs of each specific flower. Before selecting plants to grow, accurately analyze the site. Critical factors include the amount of sunlight, microclimate temperature, competition from tree roots, and soil drainage and aeration.

Light - Plants vary in the amount of light they required for optimum growth. Too little light can lead to reduced flowering and leggy plants. Too much sunlight can burn or fade the foliage of shade-loving plants. Light, temperature, and water are closely interrelated. Plants listed as preferring partial shade may tolerate more sun if temperatures are moderate and adequate water and mulch are provided.

When evaluating light exposure, note the duration and intensity of sunlight the site receives. There are many types of shade. Four hours of full sun during the morning is very different from four hours of strong and more intense afternoon sun. The amount of light in shaded locations varies with the type, number, and size of trees in the area. If the site receives more than three hours of unfiltered mid-day sun, treated this area as a "full sun" site. "Partial shade" can be defined as receiving unfiltered morning sun, but shade during the afternoon hours, or moderate shading throughout the entire day. A "heavily shaded" site would receive very little direct mid-day light and less than 60% of the sun's intensity during the remainder of the day. Few flowering plants do well in deep shade.

Temperature - Very few plants look attractive and flower profusely from early spring through late fall. Cool-season flowers such as dianthus, pansies, and snapdragons grow best when the temperatures are mild; they slow or stop flowering when exposed to high summer temperatures. It is possible to extend the flowering season of cool-season annuals by placing them in a protected location, shaded from direct sunlight from about 12:00 to 4:00pm. Placing them next to a paved surface or brick wall will increase temperatures and shorten their flowering period. Heat-loving flowers such as gaillardia, portulaca, verbena, and vinca do not begin to flower until early summer; they should be planted in full sun. Planting them on the north side of the house in light shade will delay and reduce their flower production.

Soil moisture - Examine water drainage, moisture retention, and soil aeration of the site. All three factors are closely interrelated. Frequent, heavy rains in combination with poorly drained soils will cause excessive soil moisture and limited air space in the soil, thus reducing plant growth and increasing the chances of root rot problems. One method to check for adequate drainage is to dig a hole 10 inches deep and fill it with water. After it drains, refill it with water. If the water drains in 8 to 10 hours the site is adequately drained for most flowers. Subsoil compaction or the presence of a hard pan beneath the bed will affect water drainage and soil aeration. It may be necessary to deep till beds to break up the subsoil and increase drainage.

The amount of air in the soil depends on soil type, soil compaction, and how quickly water drains from the soil. Clay soils normally have poor drainage and aeration but excellent water retention. Water does not always enter clay soil easily. It often puddles on the soil surface rather than soaking in. Sandy soils have good drainage and aeration but retain little water. The addition of organic matter, such as pine bark or composted yard waste, will enhance soil aeration, water movement into the soil, and drainage of clay soils. It will also improve water retention in sandy soils.

Preparing the Soil

The best amendments for clay soils are pine bark (less than 1⁄2 inch in diameter), composted leaf mold, or small pea gravel (less than 3⁄8 inch in diameter). Be careful when selecting leaf mold, making certain that the material is fully composted and not merely "aged." Partially decomposed materials will compete with plants for nutrients, especially nitrogen and sulfur, resulting in nutrient deficiencies and poor plant growth. Peat moss, sand, hardwood bark, wood chips, and pine straw are not recommended because they will not adequately improve the physical properties of a clay soil. Organic matter content must be increased to 25%, by volume, to be effective as a soil conditioner. For example, to achieve approximately 8 inches of amended soil, a minimum of 2 inches of material should be incorporated into the top 6 inches of soil. This also raises the bed which improves drainage. Incorporating more than 50 percent organic matter may have a negative effect on plant growth. Incorporating less than 25 percent by volume is a waste of time and material.

Amendments such as pine bark, composted leaf mold, or peat moss will improve water retention in sandy soils. Similar to clay soils, these amendments need to be added at a minimum of 25% and a maximum of 50% by volume.

Soil testing and fertilizing - A soil test is the only way to determine if phosphorus, potassium, calcium, or magnesium must be added or if a pH adjustment is needed. Over application or application of unneeded fertilizer could result in salt injury to plants, cause nutrient imbalances, and is environmentally unsound. If soil test results indicate nutrient or pH adjustments are needed, the materials should be mixed into the soil uniformly since bedding plants have a very root system.

The soil pH for bedding plants should be between 5.5 and 6.5. Lime should be thoroughly tilled into the soil prior to planting. If the pH must be lowered, elemental sulfur can be incorporated into the soil. If only a small decrease in pH is required, acid-forming fertilizer such as ammonium nitrate can be used as a nitrogen source and to slightly lower soil pH.

If a soil test is not made, incorporate 2 to 3 pounds of 10-10-10 fertilizer per 100 square feet of bed area. An easy conversion to remember is that 1 pound of fertilizer is equal to about 2 cups or 96 teaspoons. Therefore, use 2 pounds (4 cups) of dry fertilizer per 100 square feet or about 2 teaspoon per square foot. An alternative to 10-10-10 is a slow-release fertilizer, such as 16-4-8 or 12-4-8.

It is impossible to determine the amount of calcium and phosphorus required without a soil test. However, because most North Carolina soils are low in phosphorus it is usually safe to add 2 to 4 pounds of triple superphosphate (0-46-0) per 1,000 square feet. Phosphorus and calcium move very slowly through the soil profile so they should be incorporated into the top 6 to 10 inches.

Avoid guessing at the amount of calcium to apply without a soil test. First, it is possible to get too much calcium in the soil which can lead to potassium and magnesium deficiency. Second, many sources of calcium such as calcitic and dolomitic limestone raise soil pH. A high pH can cause some nutrients to become unavailable to plants. If calcium is low, but a pH change is undesirable, gypsum can be incorporated into the soil prior to planting.

Magnesium may be deficient especially in low pH soils. If magnesium levels and soil pH are low, dolomitic limestone can be used to raise pH and to supply needed magnesium. To add magnesium without affecting pH, epsom salts can be applied. The general rate for epsom salts is 1 pound per 100 square feet.

Transplanting

Plants in cell packs or containers should be watered thoroughly and allowed to drain before removing them from their container. A damp root ball is less likely to fall apart. Do not pull plants from their container. Remove plants from individual containers by tipping the container and tapping the bottom. To remove plants from cell packs, turn the container upside down and squeeze the bottom of the container to force the root ball out of the pack. If the plants are in fiber pots, remove the paper from the outside of the root mass. When transplanting plants grown in peat pots remove the upper edges of the pot. If the lip of the peat pot is exposed above the soil level, it may produce a wick effect, pulling water away from the plant and into the air.

Dig a hole for each plant large enough to accept its root system comfortably. Set the plants at the same depth or just slightly deeper than they were growing in the container. When filling the hole, firm the soil lightly and water thoroughly. A starter solution made from 1 tablespoon of a high grade phosphate fertilizer in 1 gallon of water can be used at this first watering.

Space plants so they will fill in but not be crowded -- crowding increases the likelihood of disease development. Tall upright plants such as snapdragons should be spaced about one-fourth as far apart as their mature height. Tall bushy plants should be spaced about one-half as far apart as their mature height. Rounded, bushy annuals should be spaced about as far apart as their mature height. A good rule of thumb is to space plants 6 to 8 inches apart. For more uniform beds, use a staggered spacing instead of setting plants in straight rows.

Transplanting in the cool part of the day or on an overcast day will minimize stress. Check newly planted transplants for moisture stress frequently until new roots grow into the surrounding soil. Remember that the root mass is initially only as large as the original container and irrigation water should be directed towards the base of the plant until it becomes established.

Mulch after planting - Mulches help keep the soil surface from crusting, reduce soil temperature, conserve moisture, and prevent weed seed germination. Organic mulches can add humus to the soil. Use a 2- to 3-inch layer of material such as pine bark nuggets, compost, or pine straw. Apply only 1⁄2 inch of mulch at the plant crown; excessive mulching of the crown can cause disease problems.

Weed Control

Options for weed control include mulching, cultivation, hand pulling, and using a herbicide. Mulching and spacing plants so they produce a solid mass are the best options to minimize weed problems. You will need to hand pull some weeds regardless of which methods you use. Weeds are easier to pull when small and after a rain or irrigation.

Cultivating to control weeds should be limited to early season. As annual flowers grow, feeder roots spread between the plants; cultivation is likely to injure these roots and may uncover weed seeds that can then germinate.

If a herbicide is used to control weeds, read the label carefully. There is no one herbicide that can be safely used on all annual flowers. Herbicides that are labeled for some but not all flowers include: Betasan, Surflan, and Treflan. Time and rate of application will vary with the herbicide selected. A preemergence herbicide can be used to prevent many weed seeds from germinating. Some preemergence herbicides are applied before planting, others are applied after planting but before weeds emerge. The flower bed should be weed-free when the herbicide is applied. Only a few postemergence herbicides can be used to control grassy weeds after flowers have become established.

Fertilizing

Nitrogen is the nutrient that most frequently limits plant growth. Unfortunately, nitrogen is the most difficult nutrient to manage. Soil tests for nitrogen are not dependable and nitrogen is easily leached from the soil. The challenge is to maintain adequate nitrogen levels to meet the plant requirements without damaging the plants or the environment.

Growth rate and foliage color should be the primary guide in determining the need for additional fertilizer applications during the growing season. However, some general guidelines can be used. Nitrogen can be applied in a quick-release, water soluble form using a liquid or granular fertilizer or in a slow-release granular form. If a quick-release fertilizer (such as 10-10-10) is used, apply 1 pound per 100 square feet every four to eight weeks throughout the season. Liquid, water-soluble fertilizers should be applied about every two weeks.

With slow-release fertilizers, only two applications should be made. The first application should be incorporated into the bed just before planting and the second should be broadcast over the bed midway through the growing season. If a slow-release fertilizer that contains phosphorus and potash is used, additional application of phosphorus and potash may not be needed. The total seasonal application of slow-release fertilizer should not exceed 4 to 6 pounds of nitrogen per 1,000 square feet.

Watering

Some flowers will tolerate moderate periods of dry weather; others must have a continuous supply of water. Most annuals will slow or stop flowering during extended hot, dry summer weather. For sites with minimal irrigation, select drought tolerant annuals such as globe amaranths, blue blaze, Dahlberg daisy, gazania, gomphrena, portulaca, and creeping zinnia.

Supplemental irrigation will be required at some point during the growing season. Soil type, as well as, growth stage and temperature influence watering frequency. Bedding plants grown in a clay soil that has been properly watered may not have to be watered more than once per week. Bedding plants grown in a sandy soil may need to be watered several times a week. This will vary with the time of year, amount of sunlight or shade, plant growth, and other environmental factors. Most plants need 1 inch of water per week, but may require more when flowering or when plants are exposed to high temperatures and frequent wind. Moisten the entire bed thoroughly, but do not water so heavily that the soil becomes soggy. After watering, allow the soil to dry moderately before watering again.

A soaker hose is excellent for watering beds. Water seeps directly into the soil without waste and without wetting leaves and flowers. The slow-moving water does not disturb the soil or reduce its capacity to absorb water. Sprinklers are not as desirable as soaker hoses since water from sprinklers wet the flowers and foliage, making them susceptible to diseases. Sprinklers also allows large amounts of water to be lost through evaporation.

The least effective method for watering is with a hand-held nozzle. Watering with a nozzle has all the disadvantages of watering with a sprinkler. In addition, gardeners seldom are patient enough to do a thorough job of watering with a nozzle; not enough water is applied, and the water that is applied is usually poorly distributed over the bed.

Staking

Many tall-growing annuals, such as cosmos and cleome, may need support to protect them from strong winds and rain. Begin staking when plants are about one-third their mature size. Many materials (wire cages, bamboo stakes, tomato stakes, twiggy brushwood, or wire rings) can be used for staking. Ideally the staking material should be 6 to 12 inches shorter than the height of the mature plant. Place stakes close to the plant, but take care not to damage the root system. Sink stakes into the ground far enough to be firm. Loosely tie plants to the stakes, using paper-covered wire, plastic, or other soft material. Tie the plant by making a double loop with one loop around the plant and the other around the stake forming a figure-eight.

Deadheading/Pruning

Deadheading is the removal of dead or faded flowers and seed pods. Annuals expend a considerable amount of energy in producing seeds after the flower fades -- the results often is a decrease in flower production. To maintain vigorous growth, flowering, and assure neatness, remove spent flowers and seed pods. This step is not necessary for all flowers but is recommended for calendula, celosia, coleus, cosmos, geraniums, marigolds, petunias, scabiosa, salvia, rudbeckia, or zinnias. Check plants weekly.

The removal of spent flowers is not necessary for all species to remain free-flowering. Many modern cultivars are self-cleaning --- spent flowers disappear quickly. Some cultivars are sterile and do not produce seeds.

Some bedding plants, such as polka dot plant, petunias, and impatiens, may benefit from pruning back for size control and rejuvenation. Others such as gomphrena can be pruned or sheared into shapes. Pruning can stimulate greater flowering of some petunia cultivars. Cut back plants as needed leaving approximately one-half of the shoot.

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Author

Ervin Evans

Extension Associate (Consumer Horticulture)
Horticultural Science