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Choosing an Ornamental Plant Fertilizer
By Ray Buckwalter
Why fertilize ornamental plants?
One of the main considerations when choosing a fertilizer is to determine the reason for fertilizing in the first place. What do you want to accomplish? The two primary reasons for fertilizing are to encourage growth, or to create a healthy, vigorous, attractive plant. These goals are not mutually exclusive. However, a high growth rate does necessarily equate to health. Nurseries fertilize to stimulate managed growth to produce a large, attractive plant for sale. Faster growth means reduced time to market and professional growers have mastered the science of managing growth by careful selection of varieties combined with controlled growing conditions and fertility programs.
Homeowners, on the other hand, 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.
Evaluate soil conditions
The best starting point is a soil test. Soil tests are inexpensive, typically about $10, but they can provide a lot of valuable information. In the landscape, construction often results in soils that differ from one house to the next or even from the front yard to the rear. 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. Even if you’re landscaping in the same general area don’t assume the soil conditions are similar without first conducting a test.
In the absence of a soil test, you can assume that most plants in a landscape setting require more nitrogen than they get from natural sources. One reason is that the leaves that have fallen from the plant that could have put nutrients back into the soil are often raked and removed. Ornamental plants often have showy foliage and exceptional blooms that are achieved by genetic selection. They are grown in nurseries under ideal conditions. These plants are less likely to be adapted to natural soil conditions and the more austere fertility levels that sustain their native cousins. Most ornamentals will benefit from 2 to 6 pounds of applied nitrogen per season. The exact amount, and ratio of other nutrients, depends on the type of plant and fertility of the existing soil.
Not all plant fertilizers are created equal
There are almost as many fertilizers on the market today as there are plants to fertilize. Trying to get beyond the marketing hype and choose the right one can be a daunting task but understanding some of the basics can make it easier.
Every fertilizer has a guaranteed analysis that represents the concentration of plant nutrients. Some products are more concentrated than others. For example, most natural organic fertilizers contain about 3 to 5% nitrogen. Synthetic organic or mineral fertilizers may contain 15 to 30% nitrogen. You need eight times more of a 3% product to supply the same amount of nitrogen as a 24% product.
…understanding some of the basics can make it easier
Another factor closely related to the guaranteed analysis is the cost. Let’s assume that in the example we just stated both products cost $18 per 50-pound bag. With the 3% product there would be a total of 1.5 pounds of nitrogen in the bag, so the cost per pound of nitrogen would be $18 / 1.5 or $12.00. With the 24% nitrogen product there would be 12 pounds of nitrogen in each bag so the cost is $18 / 12, or $1.50. The 3% material is actually 8 times more expensive. You need to use 8 times as much to get the same amount of nitrogen.
Coverage is a measure of how many plants can be fertilized with each product. Again we focus on nitrogen because that is the nutrient used in the largest amount by vascular plants. If we want to supply ½ pound of nitrogen per plant the 3% material will fertilize three plants and you will need 16.6 pounds of product for each one of them! That’s a pretty big pile of fertilizer so you probably need to make several applications just to get that amount applied. The 24% material will fertilize 24 plants and you would need just over 2 pounds for each. There are no rules governing usage recommendations on fertilizer labels so compare products based on the amount of nutrients they supply, not on the manufacturer’s coverage recommendations.
Quality is a function of particle size, consistency, lack of dust, and other physical characteristics. However, most of the time we equate quality to the amount and type of controlled release nitrogen. Of all nutrients, nitrogen is the most critical to control in order to provide an extended feeding. Occasionally, potassium may also be provided in a slow release form. A slow release formulation is the most desirable because it feeds plants for a longer period of time and eliminates time and effort required to make frequent repeated applications. There is less chance of problems arising from over fertilization since the nitrogen is released slowly and the plant is able to use more of the nitrogen so less is lost to leaching or volatilization. This is often referred to as nitrogen efficiency and can also apply to other nutrients in slow release form such as potassium.
Low salt index and slow-release nitrogen
When choosing an ornamental fertilizer, an important consideration is the salt index. Fertilizers with a low salt index, a measure of salinity that a fertilizer contributes to the soil, are preferred. Almost all fertilizers have some salinity, but too much salt can prevent the plant from pulling moisture into its roots. This type of injury is often referred to as fertilizer burn, but it is really an induced drought stress. Typically, a less expensive fertilizer will have a higher salt index. If the plant being fertilized does not have a well-developed root structure, as is the case with a new transplant, it is very susceptible to moisture problems that may be exacerbated by a high soil salt content.
IBDU® isobutylidene diurea is an excellent source of controlled slow release nitrogen and has a low salt index. IBDU releases by hydrolysis so it relies on moisture, but is less dependent on temperature. This makes IBDU a great source for plants that grow in cooler soils. By using IBDU, Greenview offers a fertilizer that works great under a wide range of conditions for ornamental plants, shrubs and trees .