Explaining vitamin shots for trees

The shortest day of the year has passed. Days become lighter, little by little, and spring is just around the corner. Gardeners' thoughts turn to outdoor activities including fertilizing trees. As you plan your fertilizer regime, avoid the practice of injecting trees with fertilizer.

When tree leaves turn yellow, some landscape managers recommend a quick "shot" to turn leaves green again. Although fertilizer injection can be a short-term solution for improving the color of leaves, it can actually cause long-term problems, according to Dr. Linda Chalker-Scott, horticulture professor at Washington State University.

"This practice breaches the tree's bark barrier and leads to numerous health problems. Injection sites are portals for pathogens and pests; they can cause trunk splitting, decay, cankers and structural defects; and they are especially dangerous to trees already in poor condition," said Chalker-Scott.

Injecting trees with fertilizer increases the nitrogen content of leaves, and makes trees more susceptible to pests, particularly sucking insects such as aphids.

Chalker-Scott reports that researchers have been disproving the value of tree injection for decades. People think injections should work because vaccines work for humans and animals. Plants are not like ailing humans, requiring doctors to give them shots to get well.

The benefit of soil injection of fertilizers is another myth and does not improve significantly how a tree accesses nutrients from the soil. Research indicates that surface applications work just as well and can cost less.

The sustainable solution to improving the color of trees is to find out what is causing the yellowing leaves in the first place. Yellow leaves do not necessarily indicate a nutrient deficiency. There could be too much or too little water. The soil may be compacted, waterlogged or deficient in organic matter. There could be insect or disease problems. The tree may be competing for nutrients with weeds, lawn or other plants. Numerous stresses, including air pollution, mineral toxicity or poor root health, adversely affect trees. Fertilizer will not correct such things. In fact, fertilizing a stressed tree can magnify the extent of the existing problem.

"Trunk injection is harmful to the long-term health of the tree and should not be used for delivering fertilizers," said Chalker-Scott.

Good tree health depends on proper watering and good soil health. More information at theinformedgardener.com

JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at 887-2252 or skellyj@unce.unr.edu