The two primary reasons for fertilizing ornamental shrubs and trees are to encourage growth, and to create a healthy, vigorous, attractive plant. Most home gardeners are typically more concerned with the long-term maintenance of the plant. There is often a temptation to over-fertilize in the hopes of producing an even healthier, larger or more beautiful plant. Be careful! If you force a plant beyond its natural growth rate by over-fertilizing, you can cause it to grow too quickly. This can result in structural problems, predispose the plant to insect or disease infestation, and reduce tolerance to drought or temperature extremes.
It's a Good Idea to Evaluate Soil Conditions First
Before fertilizing your shrubs or trees, the best starting point is a soil test. Soil tests are inexpensive, typically about $10, but they can provide a lot of valuable information. Your Local Extension Service might provide soil testing services. Construction, and many other factors, often result in soils that differ from one home landscape to the next, or even from the front yard to the backyard. A good general rule of thumb is, if the soil looks significantly different in color or texture, it is probably also very different in fertility as well.
Determining what fertilizer to use can be an overwhelming task. Which formulation do you need for your particular situation? Should you get the bag of 10-10-10 or 16-4-8, go with a specially formulated Shrub & Tree fertilzer, or should I look for something organic?
What do those numbers mean anyway?
It may seem a little confusing, but you can figure it out. When selecting a fertilizer, the first question to answer is, "What analysis do I need?" The analysis is the three large numbers you see on every fertilizer label, such as 12-6-6 or 10-10-10 or 6-2-0. These numbers represent the percentage (by weight) of the three major nutrients required for healthy plant growth. These three major elements are always in the same order: Nitrogen-Phosphorus-Potassium (N-P-K). Each of these nutrients affects plant growth differently.
The first number represents nitrogen.Nitrogen provides plants with the ability to produce more chlorophyll, which in turn sponsors more rapid growth to the part of a plant that is above the ground: the foliage. With each additional nitrogen application, plants should grow taller and greener. Typically, fertilizers that are high in nitrogen content are not recommended for use on most ornamental plants (shrubs and trees). A well-balanced shrub and tree fertilizer might contain 10 to 15 perecent nitrogen, such as 12-6-6, which contains 12 percent nitrogen.
The second number represents phosphorus. Phosphorus aids in root development and therefore can help to increase bloom size and bloom production. A fertilizer high in phosphorus content can be used when planting new trees and shrubs, to help them "root in" more quickly. Once a plant has become more established, a well-balanced shrub and tree fertilizer such as 12-6-6, or something similar should be sufficient for most shrubs. If you have blooming plants that havent been blooming so well, you might consider using Triple Super Phosphate (0-45-0) to boost root growth and increase bloom production.
The third number represents potassium. Potassium has many functions: It guards the plant against diseases and aids in drought protection and cold tolerance. It also serves a role in improving root development, and helps in the process of photosynthesis.
You might have noticed that all the numbers in the fertilizer analysis dont add up to 100 percent. Thats because there are other nutrients and fillers often added to the mix.