I am frequently asked what kind of fertilizers should I use for hollies? … yews? … roses? … azaleas and rhododendrons? Fertilizer manufacturers have brainwashed the public into believing each species of plant requires a special fertilizer.
The Bay Gardener has spent many years conducting research on the nutrition of ornamental plants. As a result of some of my early research, I developed Osmocote 18-6-12, a fertilizer that releases nitrogen, phosphorus and potassium slowly for one growing season. Those elements are present in differing proportions because, in my studies, I observed repeatedly that plants do not absorb much phosphorus (6) but do absorb more nitrogen (18) and potassium (12). Both nitrogen and potassium are more easily leached from the growing medium but are also required in greater amounts by plants.
Osmocote 18-6-12, which became available in 1972, was the first controlled-release fertilizer to release all three nutrients simultaneously. Similar brands have since been developed.
It was designed specifically for growing ornamental plants in containers filled with soil-less growing media. Osmocote was never intended to be used for growing plants in the ground.
My research in composting and in compost utilization demonstrated strong differences between the nutrient needs of plants growing in soil and plants growing in soil-less media. Soils are forgiving, because of their buffering capacity, which moderates nutrient availability. This is due primarily to the presence of clay and silt particles in most soils.
When it comes to satisfying the nutritional needs of established shrubs growing in your landscapes, yearly applications of fertilizers are not necessary. Matter of fact, yearly applications of fertilizers are more likely to create more work and more waste to dispose. The more you fertilize, the more you have to prune.
All shrubs shed their leaves yearly, even evergreens. If the leaves are allowed to decompose, the nutrients are recycled like grass when you cut it tall and let it fall. If you carefully mulch, only as needed, the mulch decomposes and releases nutrients. Take a walk through a forest and see how Mother Nature fertilizes her plants. By allowing natural recycling to occur, you not only save money but also reduce your workload.
In the 27 years we have lived in our home in Deale, I have never fertilized the foundation plantings, except for the azaleas. I fertilized the azaleas in mid-October with ammonium sulfate until my children gave me a leaf blower. Azaleas and related species can absorb nitrogen only in the ammonium form, which is why ammonium sulfate is recommended. Now, by blowing as many oak leaves under my azaleas as possible, I have eliminated having to apply ammonium sulfate. The decomposing oak leaves satisfy all of the nutrient needs of the azaleas.
Hollies, azaleas, camellia, acuba and viburnums only need a light pruning once each year. I allow the small clippings to fall to the ground while the larger branches that are removed are added to the brush pile behind the barn.
If you are fertilizing the shrubs surrounding your home every year, you are simply creating more work and you are forcing those plants to outgrow their usefulness.