1 - Fall is a good time for planting trees and shrubs in New Mexico for several reasons.

Q.

I remember that you have said that autumn is a good time to plant trees. That seems counterintuitive. Why is autumn a good time for planting?

A.

Autumn is a good time for planting trees and shrubs (as well as spring flowering bulbs) for several reasons. Sometimes nurseries, both local and online, are having clearance sales and these plants may be available for good prices. However, this also may mean that what they are selling are the left-overs that no one else wanted. Choose carefully! Bare root trees and shrubs are available in the spring and are often less expensive, but they can also be obtained in the autumn from some sources. Container grown plants available in the fall may be for sale at clearance prices and are often easier to plant.

Another reason, especially relevant for New Mexico, is that in the fall we often experience less windy conditions than during the spring in New Mexico. Winds can quickly dry plants and cause stress. Winds can uproot newly planted trees. Plants are often damaged by improper staking to help them stand against the wind. Autumn planting helps avoid these problems.

Temperatures are declining in the autumn. This reduces evaporative water loss and stress on newly planted trees and shrubs. Lower temperatures in the autumn result in less need for irrigation to sustain the newly planted trees. While the plants still need water through the fall and winter, the need for irrigation is greatly reduced as temperatures fall. If a thick layer of organic mulch is placed around the tree, the need for irrigation will be reduced even more. Remember to leave a 2 to 4 inch separation between the mulch and the tree trunk.

Declining temperatures have a physiological effect on trees and shrubs that makes autumn planting beneficial. In the spring, air temperatures warm while the soil is cold. This results in greater metabolic activity in the top of the tree. This results in the initiation of growth in buds. Nutrients and water from the soil are directed to the upper portions of the tree to support new growth. This is at the expense of the development of the root system of the newly planted tree which has trouble supplying sufficient water for new leave development.

The newly forming leaves begin to photosynthesize, turning the water from the soil, carbon dioxide from the air, and energy from sunlight into carbohydrates. Carbohydrates are the food that plants (and animals) need to perform all other metabolic and growth processes. This food is first available to the newly formed leaves and twigs. As the season progresses and new growth declines, carbohydrates are directed to the small branches, then larger branches, and finally to the trunk to support growth in these portions of the plant and for storage in these plant organs. Finally, as autumn approaches, greater quantities of carbohydrates are available for the root system. At this time the air is becoming cooler and slowing metabolism in the upper portions of the tree. However, the temperatures in the soil remain warm well into the fall and winter. Where the temperatures are higher, metabolism is greatest. Where metabolism is greatest, more carbohydrates are needed. All this activity below the ground in the autumn results in much new root development. Some root growth occurs in the spring as roots develop to supply water and nutrients to the upper parts of the plant, but autumn is when the root system enlarges.

In newly planted tree, development of the root system is of great importance. So, the apparently counterintuitive autumn season is really a good time for root growth and a great time for planting trees and shrubs. Instead of supporting development of leaves, fall planted trees devote their energy to producing roots, which results in better development of leaves the next spring.

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.

Links:

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