Roots and Shoots: Fertilizing Woody Plants and Trees

By Pamela Doan

The fertilizer aisle at the garden center presents tantalizing dreams of lush, healthy plants and making sense of what’s really needed can be daunting when there are so many choices. Recently, Roots and Shoots covered the basics of fertilizing perennials (April 18) and vegetables (May 2). Woody plants and trees are another landscape feature that raise a lot of questions for gardeners about proper care. Whether you have new plantings or established, mature plantings, because of their longevity and impact in the yard, trees and shrubs are a bigger investment that we want to thrive.

Established, mature plantings probably don't need extra nutrients. (Photo by P. Doan)

Established, mature plantings probably don’t need extra nutrients. (Photo by P. Doan)

The basics of care are water, pruning and fertilizing. Mature plantings may not need any extra nutrients if they’re planted in healthy soil. As has been mentioned previously, test the soil before bringing home that bag of fertilizer or succumbing to an offer from a landscaper to “just take care of it all.” Fertilizers should only be used when the minerals and nutrients the particular species needs are not available in the soil.

If you’ve noticed a visible symptom of the tree or woody plant that indicates it’s in poor health, like leaf discoloration, bare spots, or stunted growth, identify the cause before assuming that fertilizer will fix it. Depending on the type of tree, these issues could be the result of pest damage, improper pruning, disease or poor site conditions, like lack of sunlight or under- or over-watering. Fertilizer can’t compensate for these problems, but could exacerbate them.

Mulching around a tree or woody plant will help it retain water and keep other plants from growing under it. Grass and weeds will compete with the roots for water and nutrients from the soil, depriving both of needed food. Bark, aged wood chips, shredded leaves, and compost are all examples of organic mulch and are preferable to gravel or landscape fabrics because they contribute nutrients to the soil as they decompose. Inorganic mulches suppress weeds but don’t offer anything.

Trees and shrubs growing in lawns that are fertilized will absorb those nutrients and don’t need anything else. They may suffer from burning and herbicides if the lawn treatment is applied too closely to the roots, though, so proceed with caution when making choices about lawn care, too.

Once the results of the soil test are back and it is determined that the plantings could benefit from added fertilizers, there are a few guidelines to follow. To avoid stimulating growth at the wrong time, apply fertilizers in early spring and late fall, never in the summer or early fall.

Contrary to what may seem like common sense, don’t fertilize new plantings. In the early stage of growth, trees and shrubs concentrate on establishing roots and nitrogen — one of the main ingredients in fertilizer — will suppress root growth.

Consider the soil type when choosing a fast or slow release fertilizer. Since fertilizers are water-soluble, in quick draining sandy soils, the fast-release fertilizers may process through the soil before the roots absorb the nutrients.

Avoid situations where fertilizer will simply run off and not be used for its intended target. Fertilizer should be applied so that it stays where it is supposed to be. Resist the urge to use more than is recommended on the label of the fertilizer, too. More is not better and will have to find a place to go, usually into ground water or a nearby stream.

Organic fertilizers include compost, blood meal and processed sewage sludge. Mulching trees with shredded leaves releases nitrogen as the leaves break down and will feed the roots, too. A simple approach is to apply a nitrogen-based fertilizer once a year if needed. The rate is about 2 to 3 pounds per 1,000 square feet.

The Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses, a resource book by Michael Dirr, Bonnie Dirr, Margaret Stephan, Asta Sadauskas, and Nancy Snyder, is an invaluable reference for shrubs and trees. Other good sources that do not have a commercial interest can be found at the Cornell University website, accessed locally with the county Cooperative Extension office and website. Local educators can offer diagnostic and identification assistance, as well as specific advice for treating problems.

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