Knowing How Much to Water, Part II

May 4, 2019

Question:

How much should I be watering my trees?

- Multiple Gardeners from All Over NM

Answer (Part 2):

In last week’s column, we learned about how the type of soil in your garden affects tree water requirements. Now we will focus on other considerations, such as rates of water movement, tree species, age, canopy size, and seasonal fluctuations in water needs.

Plants take water up from the soil through their roots all the way to the leaves where it is released into the air. Transpiration is the process by which a plant loses water, primarily through pores in the leaves called stomata. This is a necessary process that involves the use of about 90% of the water that enters the plant through the roots. The other 10% of the water is used in chemical reactions, like photosynthesis, and in plant tissues. Transpiration is necessary for mineral transport from the soil to the plant tissues, for the cooling of the plant through evaporation, for moving sugars and plant chemicals, and for the maintenance of turgor pressure. The amount of water lost from the plant depends on several environmental factors such as temperature, humidity, and wind or air movement.

For an entertaining 2-minute video on how transpiration works during ‘normal’ conditions versus hot, dry, windy conditions (aka ‘New Mexico normal’), visit this week’s column online at Desert Blooms.

Mary Irish and Judith Phillips include a simple table on how much and how often to water landscape plants in their book, “Arizona & New Mexico Getting Started Garden Guide.” In this table, trees in New Mexico are divided into two categories: cool desert-adapted and high water use. The recommended number of days between waterings differ depending on the time of year, partly because tree water needs increase when temperatures are higher. In winter they recommend watering established, cool desert-adapted tree species once every 45-60 days. Intervals of 14-30 days between waterings are suggested in the spring and fall and shorter intervals of every 7-21 days in the summer. More water may be needed for trees that are newly planted (less than three years), especially in the hottest, driest times of the year.

Slow soaks at long intervals between irrigation events are best for tree root systems. Remember that for mature, happy trees the hard-working, actively-absorbing root hairs are more concentrated in the top two feet of soil depth and extend out beyond the canopy, even two to four times the height of the tree. So be sure to water deeply enough and not just at the base of the tree trunk. For younger or neglected trees, it is most important to apply water both where you know where the roots are and where you want them to grow.

Drip irrigation is a great method for watering trees as long as enough water is applied to really soak down into the soil and the placement of emitters will need to be modified each year to accommodate root and canopy growth until the trees are established. Similarly, low berms of soil can be used to build a basin around the dripline of a young tree if watering with a hose, but these too will need to be expanded as the tree grows.

Certainly, some plant species are bigger water guzzlers than others. Several municipalities offer lists of recommended trees and other plants for their specific climates. If you know of a reference you think might be useful for others please share it with me via on social media (@NMDesertBlooms).

More and more communities in New Mexico are adopting incentives for water-wise gardening, like rebates (aka “tree-bates”), for the inclusion of low water use plant species and installation of drip irrigation systems with controllers at Xeriscape.

It is tricky to figure out when to water, how much, and how to deliver water to the trees in your garden. Try not to get discouraged. Selection of species that grow well in our climate with minimal extra water is an excellent first step. Mulches and groundcovers are also great tools for conserving soil moisture so that less water is evaporated directly from the soil into the air, but I will save those topics for another week.

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:

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

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!