One of the best ways to keep annual and perennial flowers blooming is to remove their faded blossoms. By removing withering flowers, in a process called "dead-heading," the energy of the plant will be directed toward continued flower formation rather than seed production.
There is a correct way and an incorrect way to dead-head flowers. The incorrect way is to try pulling or pinching off the faded flowers. This will remove the petals but likely leave the seed-forming flower ovary.
The correct way to dead-head faded flowers is to remove them by cutting off the entire flower structure (petals and the ovary flower base). Do this using sharp scissors or hand pruners, and cut the flower stem just below the base of the blossom.
In general, flowers that respond best to dead-heading are those with rather large blooms. These include roses, geraniums, marigolds, cosmos, dahlias and zinnias. Petunias respond best to a shearing back of the entire plant, flowers and stems, when the plant becomes too sprawling and leggy. When cutting roses, always trim back to a five-leaf leaflet to encourage the formation of new flowering shoots.
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Many perennials also can be stimulated to bloom for longer periods by dead-heading. These include lavender, guara, California daisy, Angelita daisy, desert marigold, chocolate flower, coreopsis and purple aster. When in doubt, go ahead and remove the spent flowers of any variety. Chances are that it will encourage more blooming to some degree.
In addition to cutting off faded blooms, supplying sufficient amounts of phosphorous also will stimulate continued, abundant blooms. Phosphorous is the middle of the three numbers on the fertilizer label, the first being nitrogen and the last potassium. Flowering-plant foods generally contain a higher percentage of phosphorous. So a typical formulation for flowers might be something like 5-10-5 or 6-15-8. Some superbloom products have much higher levels of phosphorous, sometimes up to 50 percent of the formulation.
It really doesn't matter how high the phosphorous level in a fertilizer is, as long as it contains some amount and the label directions are followed. Recommendations will call for making larger applications of lower-percentage phosphorous fertilizers and smaller applications of ones with greater levels of phosphorous.
To be effective, phosphorous fertilizers must be mixed into the soil. Unlike nitrogen, phosphorous cannot be watered into the soil. It must be physically mixed into the root zone. In the case of seasonal flowers, phosphorous can be mixed into the soil before planting. With perennial flowers, the best that can be done is to scratch the phosphorous into the top inch or two of soil. There, the shallower feeder roots can absorb the nutrient.
In containers, the potting soil is light and loose, with lots of pore spaces for phosphorous to physically move through. So phosphorous can be applied to the surface of the soil in pots, and it will move down into the soil with watering. Water-soluble plant foods with phosphorous are a good choice for container fertilization, as are beaded, time-release fertilizer products for flowering plants.
● John P. Begeman is the urban-horticulture agent for the University of Arizona-Pima County Extension. If you have questions, call 626-5161 to reach a master gardener.