Trees and shrubs are living investments that grow in value with each passing year. When properly selected, sited, and planted, they can be expected to thrive with the right care, which may include pruning, watering, and fertilizing. Just as certain established drought-tolerant plants may not require supplemental watering during dry spells, mature trees and shrubs growing in favorable soil conditions may require little or no fertilizer.
In nature, shrubs and trees survive without fertilizers because they’re growing in undisturbed soil and receive nutrients through the recycling of organic materials, such as fallen leaves, that decompose and release their nutrients to the plants.
In urban landscapes, nutrients may be unavailable or deficient due to disturbed soils that have an inadequate nutrient-holding capacity, soils that are mixed with building debris, or where there is no recycling of fallen leaves to provide organic matter to the soil. Fruiting shrubs and trees, such as blueberries and pawpaws, may require fertilizer to replenish the nutrients used to produce the harvested fruits. In other cases, fertilizer may be applied with the intended purpose of encouraging the growth of leaves and flowers.
Although fertilizing has often been called “feeding”, plants actually produce their own food in the form of sugars through photosynthesis. Fertilizers contain elements or nutrients supplied to plants when they are lacking or absent in the soil. Inadequate levels of essential nutrients affect plant growth and development. Fertilizer is not a cure-all or an elixir for ailing plants. Fertilizer will not remedy unadapted plants growing in adverse site conditions, shrubs and trees that have been improperly planted and cared for, or plants affected by drought or injury from pests.
To determine if your shrubs and trees will benefit from additions of fertilizer, visually inspect them and test your soil. Look for signs of poor growth, such as poorly colored leaves (pale green to yellow); leaf size smaller than normal; earlier than normal fall coloring and leaf drop; stunted growth; or twig or branch dieback. Keep in mind that these symptoms of poor growth are not always related to low levels of nutrients in the soil, nor should you assume that fertilizers will cure these problems. Heavily compacted soil stresses induced by insects, diseases, and weeds; or adverse weather conditions can cause these symptoms.
To determine the presence or absence of essential nutrients, get your soil tested. Refer to HGIC 1652, Soil Testing. In addition to providing information about the level of nutrients available in the soil, a soil test also provides information about the soil pH, which affects the availability of nutrients and the activity of beneficial microorganisms in the soil. The combination of visual cues and soil test results will help you determine if fertilizing is necessary.
Homeowners should avoid the indiscriminate application of fertilizer. Excessive fertilizer use can cause your shrubs and trees to be more susceptible to certain pests or even harm or kill the plants. Furthermore, excess fertilizer not absorbed by roots can leach down through the soil and impact groundwater or move across the soil surface as runoff and contaminate surface water.
Fertilizers should not be applied routinely or used without a defined need and purpose. Consider fertilizer as a tool to address nutrient deficiencies and to improve productivity in fruit-bearing shrubs and trees.
Strive to follow nature’s lead by recycling organic materials in your landscape. The simplest thing you can do is mulch your shrubs and trees with a two- to three-inch layer of organic mulch. Keep the mulch a hand’s width away from the trunk and spread to the outermost branches. As the mulch decomposes and breaks down, it releases nutrients that support the below-ground ecosystem of earthworms, fungi, and bacteria. It also improves soil fertility and tilth–its physical condition or structure.
Regularly evaluate and appreciate your landscape shrubs and trees, and work with nature to maintain their health and appearance. It can save you time and money.
For more information on fertilizing trees and shrubs, see HGIC 1000, Fertilizing Trees and Shrubs.