Backyard Gardener: Using fertilizers wisely in landscapes
Fertilizers are compounds that contain one or more essential plant nutrient(s) and are formulated to be applied to plants either through the soil or by foliar spray. They may: be organic or inorganic; be complete or incomplete; contain micronutrients or not; be immediately available or time-released; and the list goes on.
The range of fertilizer products is virtually limitless. Fertilizers are useful in many situations. However, in my experience, nitrogen-containing fertilizers are often unnecessarily applied to woody plants in ornamental landscapes.
Nitrogen-containing fertilizers can be important for maintaining healthy lawns, productive irrigated pastures, flowering plants such as roses, productive fruit trees and vegetable gardens, for annual flower beds, and indoor plants.
However, when fertilizers are applied indiscriminately to woody ornamental plants, the result can be negative: causing excessive growth; weak wood; increased susceptibility to disease and insect pests; decreased drought tolerance; and potential contamination of nearby rivers, streams and groundwater resources.
In my experience, woody native and drought-tolerant plants rarely need fertilizers.
Overuse of nitrogen-containing fertilizers leads to rapid plant growth which, in turn, requires more irrigation and may necessitate increased pruning.
Fast growing wood is also more susceptible to breakage. While broken limbs should be removed by proper pruning, pruning wounds require additional energy to properly heal. If the wounds don’t heal properly, then disease can move in.
Insects and browsing wildlife are attracted to nitrogen-laden plants because they provide more succulent foliage and higher levels of nutrition. Locally-adapted trees and shrubs will often benefit from supplemental irrigation during times of drought, but will find the nutrients they need when properly planted and allowed to develop well-distributed root systems.
Many “experts” tell you that fertilizers should be regularly applied to all of your landscape plants multiple times per year. These recommendations are outdated, misguided and/or involve financial gain to the seller. My recommendation is to only apply fertilizers to woody ornamentals when you have observed a specific nutrient deficiency.
Fertilizers containing nitrogen are especially tempting because they encourage lush green foliage and rapid growth. Before applying nitrogen fertilizers, you should evaluate the plant’s growth rate and general health. You can do this by observing the amount of each season’s growth (looking at bud scars), the leaf color, and how well the leaves are distributed on the plant.
The initial symptom of nitrogen deficiency is when the oldest leaves are a uniform light green or yellow coloration. As the deficiency progresses, the growing tips of the plant maintain some green color, but the lower leaves are either yellow or have turned brown and dropped off the plant.
Sometimes growth will cease. This is because nitrogen remains highly mobile within the plant and as new growth develops, the nitrogen is sequestered from the older foliage to support the new growth.
Where alkaline soils are found in some areas of northern Arizona, some exotic landscape plants display iron deficiency symptoms in the spring. This is often noticeable in red-tip Photinia (Photinia fraseri). Iron deficiency symptoms are often recognized when newly emerging leaves have interveinal chlorosis (yellow or whitish leaves with somewhat greener veins).
Iron may be present in the soil but is not highly available due to alkalinity and cool soil temperatures. Iron deficiency can be treated with a foliar application of chelated iron to improve the leaf color, but in most cases, the chlorotic leaves “green up” on their own once the soil warms.
Iron deficiency symptoms can be made worse when nitrogen fertilizer is applied. Here the plant is trying to grow even more leaves while iron availability is limited.
Acid-loving ornamental plants such as gardenia, azalea, rhododendron, white birch, and pin oak regularly display iron deficiency symptoms in areas with alkaline soils.
These plants should be avoided where soils with a pH of 8 or greater are present.
Native and drought-tolerant plants have adaptations that allow them to grow and thrive in alkaline soil conditions. Among these adaptations are slow growth rates, root associations with mycorrhizal fungi, and the ability to conserve moisture during long periods without precipitation.
WHAT CAN I DO?
If you think you are observing a nutrient deficiency symptom, be sure to analyze the situation for evidence of insects and/or disease. Inspect the irrigation system to ensure proper function. Established woody landscape plants do best with deep infrequent irrigation applied in a fashion that encourages a widespread root system.
If you think you may have a nutrient deficiency, don’t hesitate to consult your local Cooperative Extension office. Also, visit the online edition (see URL below) to see photos of nutrient deficiencies and additional resources.
If you have gardening questions, call the Master Gardener help line at 928-445-6590, ext. 222, or email us at [email protected] and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.
Jeff Schalau is the agent in Agriculture & Natural Resources for the University of Arizona Cooperative Extension, Yavapai County.
Additional Resources: Fertilizing Woody Ornamentals, Texas A & M Agrilife Extension
Fertilizing Trees and Shrubs, University of Maryland Extension
A Gardener’s Guide to Fertilizing Trees and Shrubs, North Carolina State University Extension