Annual Flowers: Maintenance

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 are the primary guides for 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. Make applications of a quick-release fertilizer (such as 10-10-10), every four to eight weeks throughout the season at the rate of 1 pound per 100 square feet; liquid, water-soluble fertilizers should be applied about every two weeks.

With slow-release fertilizers, make only two applications. 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. The total seasonal application of slow-release fertilizer should not exceed 4 to 6 pounds of nitrogen per 1,000 square feet.

Watering
Although some flowers will tolerate moderate periods of dry weather, others must have a continuous supply of water. Flowering of most annuals will slow or stop during extended hot, dry summer weather. To minimize the need for watering, select drought-tolerant annuals such as globe amaranths, blue blaze, Dahlberg daisy, gazania, gomphrena, portulaca, and creeping zinnia.

Supplemental irrigation will probably 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 need to be watered only once a week. Bedding plants grown in a sandy soil may have 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 exposed to high temperatures or windy conditions. 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 flowerbeds. 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 wet the flowers and foliage and make them more susceptible to diseases. The impact of water drops falling on the surface may change soil structure and cause it to puddle or crust, preventing free entry of water and air.

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 so they do not apply enough water and do not distribute it evenly over the bed.

Weed Control
Using mulch and spacing plants so they produce a solid canopy are the best ways to minimize weed problems. Weeds can also be controlled by cultivation or use of a herbicide, but some weeds will still need to be pulled by hand. Weeds are easier to pull after a rain or irrigation.

Weeds can only be controlled by cultivation early in the season. As annual flowers grow, the feeder roots that spread between plants are likely to be injured by cultivation. In addition, cultivation stirs the soil and uncovers weed seeds that can then germinate.

Any herbicide used in flower beds must be chosen carefully. Read the label: no one herbicide can be used safely on all annual flowers. Herbicides that are labeled for some but not all flowers include: Betasan, Enide, 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.

Staking
Many tall annuals such as cosmos and celome may need support to protect them from strong winds and rain. Begin staking when plants are about one-third their mature size. Many materials can be used for staking: wire cages, bamboo stakes, tomato stakes, twiggy brushwood, or wire rings. 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 them 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 to form a figure-eight. Never loop the tie around both the stake and plant. The plant will hang to one side and the stem may become girdled. Plants with delicate stems (like cosmos) can be supported by a framework of stakes and strings in crisscrossing patterns.

Figure 14-2: click to see larger version

Deadheading/Pruning
Deadheading is the removal of dead or faded flowers and seed pods. When annuals expend energy to produce seeds after the flower fades, flower production often decreases. To maintain vigorous growth and assure neatness, remove spent flowers and seed pods. Although this step is not necessary for all flowers, it is a good practice with ageratum, calendula, celosia, coleus, cosmos, geraniums, marigolds, scabiosa, salvia, rudbeckia, and zinnias. Check plants weekly. Many modern cultivars are self-cleaning—their spent flowers disappear quickly. Some cultivars are sterile and do not produce seeds.

Some bedding plants such as polka dot plant 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 cultivars of petunias. Cut back plants as needed leaving approximately one-half of the shoot.

Prepared by: Erv Evans, Consumer Horticulturist