Understanding Fertilizer Numbers

• Fertilizers sold in the United States are require to list three numbers called the NPK ratio. The numbers, such as 10-10-10 or 20-0-0, represent the percentage by weight of nitrogen (N), phosphate (P) and potassium (K) in the fertilizer. For example, a 10-pound bag of 10-20-10 fertilizer would have 1 pound of nitrogen, 2 pounds of phosphate and 1 pound of potassium in the mixture. All plants needs these nutrients, but in differing amounts. Nitrogen promotes vegetative or leaf growth, phosphate promotes rooting and flower and fruit production, and potassium is important for cellular strength, fruiting and blooming, and cold hardiness. Fertilizers with all three nutrients are called "complete," and fertilizers with the nutrients in equal proportions, such as 10-10-10, are called "balanced." Fertilizers with a larger middle number, such as 5-10-5 or 10-40-10, are called super-phosphates or bloom-boosters, and are formulated to encourage extra bloom or fruit production.
  
  

Hibiscus Fertilizer Needs

• With the exception of the shrubby rose of Sharon, hibiscus plants grow best with frequent, light feedings of a balanced fertilizer such as 10-10-10. While in active growth, fertilize hibiscus every two or three weeks with half the amount recommended on the fertilizer package. They need less frequent feeding when grown indoors and in winter. Some tropical hibiscus exhibitors prefer to use a low-phosphate fertilizer such as 7-2-7, and avoid high-phosphate fertilizers such as 5-10-5, feeling they weaken the plant. Rose of Sharon should be fertilized only once in spring with a balanced fertilizer, when the shrub begins to break dormancy.
  
  

Signs of Improper Fertilization

• Applying either too much or too little fertilizer to hibiscus plants causes bud drop. Additionally, over-fertilization causes leaf burn, and with rose of Sharon, increases the shrub's susceptibility to aphid infestations. Discolored leaves, such as dark green veins but pale leaf tissue, are usually a sign of a nutrient deficiency caused by incorrect soil pH, so check the soil acidity with a home meter or by sending a sample to your local extension service. Hibiscus grow best in slightly acid soil, around 6.0.
  
  

The Bottom Line

• Bloom-booster fertilizers are best used on annual flowering plants and vegetables, because you're growing these plants specifically for the flowers and fruits they'll produce in a short period of time. For perennials and shrubs, however, long-term use of these fertilizers weakens the plant by encouraging bloom production at the expense of leaf and root development. The bottom line: Use a balanced fertilizer like 10-10-10 on your hibiscus plants, not a super-phosphate like 5-10-5.