Issue: February 9, 1998

Fertilizing topped shade tree

A tree company topped my shade trees. I hired them to trim my trees, then I went shopping. When I returned, they had ruined my beautiful trees. I wanted to cry; instead I got angry and they said that they would not charge for the job they had done. (Good thing!) They said for a fee they would feed my trees so that the branches would grow back and the tree would be as good as new. When should I have them feed the tree, and what should they use to feed the trees?

They probably offered to put pruning paint on the wounds they made when they topped the tree too, didn't they? When should you have them feed the trees? Never! They have shown that they do not know how to properly care for trees.

I am glad that you realze that topping is bad horticultural practice. I agree that it destroys the beauty of the trees. I also feel that the tree will never be "as good as new" again because the tree, while it can grow new branches, cannot replace those lost with branches as strongly attached to the tree. You didn't identify the type of tree, but the most common shade trees in New Mexico are fast growing and produce weakly attached branches, especially when topped. Some of our slower growing trees, some I consider more desirable, will produce branches which are more strongly attached to the tree and less subject to breakage by wind and ice. However, even these trees cannot produce the most strongly attached branches after topping. This is due to the biology, the anatomy, of the tree and the manner in which branches form following topping. My negative comment about the pruning paint, or pruning sealer, is due to the fact that it has been shown that this does not benefit the tree; it can actually interfere with the tree's attempt to close the wounds, so it can be harmful. However, some so-called tree care professionals who do not continue to learn how to care for trees do continue with outdated practices long after they have been shown to be damaging.

So to cease my diatribe against topping and companies that top trees, I would like to address your question. When should the tree be given nutrients, that is fertilized?

Trees need nutrients available to them when they are actively growing, but the need is not the same at all times in the growing season and for all ages of trees. Applying fertilizer at the wrong time can create problems for the trees. It is recommended to apply nitrogen containing nutrients after leaves are produced and reach full size. Other nutrients can be applied at other times with less concern because they do not have as great an effect on stimulating vegetative top growth. Nitrogen, if applied too early, results in the production of too many new leaves and stems at the expense of other aspects of tree growth. This can harm the tree, especially one which has been damaged by topping or by other injuries such as the breakage we saw in the Northeast U.S. following the ice storms this winter. Over-stimulating growth depletes the food, carbohydrates, stored in the tree. These carbohydrates are needed for growth but also for protection from disease and insect attack, as well as repair of injury. Reduce fertilizer application following pruning; reduce it much more following topping. Then apply it at the best time for the tree, that is after the leaves are fully expanded.

In New Mexico, many trees exhibit summer dormancy and cannot or should not use much nitrogen after early July or late June, so don't fertilize from July until late September. However, there may be reason to fertilize the trees in the autumn as the trees become dormant. That reason is to provide nitrogen for root growth. Many of our shade trees do most of their major root growth in the fall when the top of the tree is dormant or becoming dormant. A little nitrogen applied at this time may be useful in development of a better root system.

For your topped trees, I would fertilize lightly or none at all this year. Oh yes, a final thought. Find a tree care company which keeps its employees educated regarding changes in horticultural practices by sending them to school or to a reputable conference such as the annual Think Trees Conference in Albuquerque.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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