Fertilizer, Insecticide, and Fungicide Treatments
Fertilizer
Before talking about fertilizing trees, it may be useful to cover what fertilizer is (and what it isn’t). Fertilizer isn’t food for plants, nor is it medicine. Instead, you can think of it as more like a vitamin to support the tree’s growth and development.
The only way to know for sure what’s in your soil is to have it tested. Soil tests generally tell you the pH of the soil plus what nutrients are available in the soil to be used by plants. If some minerals are insufficient in the soil, fertilizer can be applied. However, it is important not to add in nutrients that are already abundant in your particular soil composition. Too much of any of the nutrients can cause fertilizer burn in your trees.
The three major components in fertilizers are nitrogen, phosphorus, and potassium, designated with the letters NPK on labels. Secondary nutrients such as calcium, magnesium, and/or sulfur may also be included in commercial fertilizers.
If you fertilize your lawn, you’ll also be fertilizing nearby trees, so be careful not to overdo it. Use your soil analysis to understand what ratio is best. For example, if your soil is low in nitrogen, you might choose a 3:1:1 NPK fertilizer as it has three times as much nitrogen as the other two nutrients.
Insecticides
Insecticides (also called pesticides) are used to kill insects that feed on trees. A healthy, mature tree can endure a certain amount of insect pests, so only use an insecticide if it’s truly necessary. The most common use for an insecticide is on fruit trees, but other kinds of insect infestations can damage or even kill trees.
Insecticides can be applied in the form of a foliar spray or a systemic treatment. Foliar spray is sprayed on the leaves and branches of a tree, while a systemic insecticide would be applied through absorption from the soil or, in some cases, by injecting the tree. Systemic insecticides are generally preferable, since spraying trees is difficult and messy. Foliar sprays can also affect areas other than your tree, and may inadvertently kill beneficial pollinators like bees. Systemic insecticides only affect those insects that are actively feeding on the tree.
Be sure to read all instructions before using an insecticide. Make sure it is safe for the tree and is the right one to use for the pest you are trying to treat. Also confirm the correct time of year and ideal environmental conditions before applying it.
Fungicides
As with insects, there are many types of fungi that may appear on your trees, and not all of them are harmful. However, if your tree has a fungal disease, your best course of action is generally to apply a fungicide. As with pesticides, both foliar and systemic fungicides are readily available.
The most important thing before treating your tree for fungal disease is to correctly identify the issue. Not all fungicides treat every type of fungus, and many types of fungal and bacterial diseases can look similar.
Since fungus travels around via spores that float in the wind, it can move quite easily from one tree to another. Even if you don’t see evidence of the fungus on more than one tree, you may want to treat all the trees in the area with fungicide as a preventative measure. Always read the label on your fungicide carefully and follow the instructions for proper application.