Prescription Fertilization of Trees and Shrubs
Does your yard need some perking up? Fertilizer can be a great way to get the most beauty out of a yard by promoting flowering, growth and brighter colors in flowers and foliage. Dramatic differences can be made in one season: the worse the need for fertilizer, the bigger the impact deep root feeding will have!
We have the experience and equipment needed to do the job right and get amazing results.
Prescription fertilization is the practice of identifying what's missing from the landscape, and supplying only that which is needed. Not only is this cheaper, less wasteful, and better for the environment but can be better for your plants too. A soil or foliage test can be used to figure out exactly what's needed. Often, I am able to determine if there is a fertilizer defiency simply by examining the plants, particularly the foliage. Nitrogen and Iron deficiencies are the most common for our area, and fortunately are easy to visually identify.
Sometimes we may want to fertilize even if a deficiency isn't evident. Raking up leaves in the fall, for example, removes approximately 2 pounds of Nitrogen per 1000 square feet per year, and if not replaced can lead to a Nitrogen deficiency over time. When we grow landscape plants in rock beds, this can affect the normal nutrient cycling process as well. Manicured landscapes are different from a natural forest and have different needs. We often plant many trees and shrubs closely together, which puts them in competition with each other for nutrients. This could result in reduced growth, poor vigor and cessation of flowering without supplemental fertilization. For these reasons a low-rate maintenance type fertilizer can be done annually to keep everything healthy.
Fertilizers are plant nutrients. They are elements that are required for the growth, maintenance and reproduction of plants. Fertilizer is not plant food, but is needed for plants to be able to make their food.
Nitrogen: Perhaps the most important fertilizer because it is the most likely to be deficient, and is needed by most plants in fairly large amounts. Nitrogen defiency shows up as reduced growth, undersized leaves, and pale leaves, especially on older growth.
Phosphorous: Occasionally deficient in Front Range soils, Phosphorous deficency can manifest as poor root development and poor flowering. Care should be taken to not oversupply Phosphorous as it is an environmental pollutant when it leaches or washes into waterways.
Potassium: Rarely deficient in Front Range soils. Potassium deficiency can be harder to spot, but can result in overall reduced plant vigor. Leaves that easily tear may be a sign of Potassium deficiency (my own observation!).
The three above nutrients are called "macro nutrients" because they are needed in realtively large amounts. Every commercial fertilizer product will display the ratio of these three nutrients in that order. For example, a 30-7-10 fertilizer will be 30% Nitrogen, 7% Phosphorous and 10% Potassium by weight.
Sulfur, Magnesium and Calcium are other nutrients needed in fairly large amounts. Magnesium Sulfate is a great source of the first two for our soils and I frequently include it in my fertilizer formula, but Calcium is rarely found to be deficient here.
Then there are the trace minerals, needed only in small amounts. Iron is the most common one to be lacking here in Colorado- read more about that on my Iron fertilization page. Manganese is another that can be deficient. Boron, Zinc, Copper, Molybdenum and almost a dozen more minerals make up the rest of the nutrients trees need in some amount but they are almost never lacking in our soil. Only once have I seen Sodium come up deficient in a test, and once Boron was deficient.
Trees and woody shrubs need the same basic nutrients that lawn turf and annuals do, but their fertilizer differs in how it is formulated and delivered. We usually want to use a slow release fertilizer for delivering Nitrogen; one that breaks down and releases its Nitrogen in a controlled manner over the course of months or even a year. This allows the fertilizer to be present during all periods when the tree needs it and makes timing less critical. I would only use a quick release fertilizer on a tree if it has a severe deficiency, and then multiple applications per year would be needed.
Tree fertilizer should also have a low Salt Index. Slow release fertilizers tend to have a lower Salt Index by their very nature. Salt Index is essentially the ability of the fertilizer to burn roots. Have you ever seen "dog spots" in a yard, where dog urine kills the grass? That is fertilizer burn from the Urea in urine, which has a fairly high Salt Index. You'll also notice a ring of dark green, tall grass around the dead spot. That is where the Urea was not concentrated enough to kill the grass, but strong enough to act as a fertilizer. Today we have fertilizer formulations available to us that have a negligible Salt Index and are much safer for our plants especially during drought conditions.
How fertilizer is delivered to trees and shrubs is important. Liquid fertilizer applied over a lawn will virtually all be absorbed directly by the grass blades and little if any will reach the tree roots. Liquid or granular fertilizer applied over mulch will accelerate the breakdown of the mulch, and much of the Nitrogen can become tied up or evaporate into the atmosphere. Granulars can also wash away easier, running downhill in heavy rain where they can pool and be over-concentrated in low areas. For these reasons I prefer to use the "deep root" method, wherein I mix the fertilizer with water and inject it into the soil using a hydraulic pump. This method can more evenly distribute the fertilizer into the root zone of the tree than any other method, and because it comes with its own water, is safer to use in dry conditions. Granular fertilizers can be a good option if subsurface injection is not feasible. Foliar sprays of fertilizer is also sometimes an option, but has its limitations.
Fertilizer timing is important, but less so when we're using slow-release formulas that break down over the course of a year. I do most of my fertilization in spring or in October. During the summer months, extra care must be taken to not cause fertilizer burn. Although not everyone is in agreement on this, there is some thought that late summer fertilization of trees (August) could make them more susceptible to freeze injury come winter.