As a recent Santa Fe New Mexican article points out, the yellowing leaves on various trees around town, most notably cottonwoods along the Santa Fe River west of St. Francis Drive, are not a picturesque sign of early autumn. Rather, these trees are exhibiting symptoms of prolonged drought stress. Cottonwoods are moisture-loving species that typically thrive on flowing riverbanks; they may be among the first trees to visibly suffer in extremely dry conditions. But all our trees, even the most drought-tolerant, are in need of deep watering starting now.
The National Weather Service predicts the next three months in our region will be hotter and drier than usual. This is following a very hot and dry summer. We’re also expected to have a warm, dry winter. Under these circumstances almost all plants will need supplemental irrigation to survive, but trees in particular will require regular deep watering before they go into dormancy, and occasional deep watering during the winter if it is as dry as expected. I spoke with local arborist Tracy Neal about guidelines for doing this effectively and responsibly.
While some horticulturalists recommend tapering off irrigation in late summer to “harden off” native and drought-tolerant species in preparation for dormancy, Neal believes this advice is misguided, especially when plants are already stressed by ongoing severe drought. Trees push out a lot of root growth during late summer and fall to store nutrients for winter, so keeping the soil adequately moist is as important at this time of year as it is in high summer.
It can be tricky to know where to water trees because their root systems are often vast and complex. Most tree roots are in the top 18 to 24 inches of soil and can spread well beyond the canopy (drip line), depending on the age of the tree and other factors, such as how much moisture the surrounding areas receive. Roots will reach out to water sources; if you irrigate regularly near the canopy, then that’s probably where most of the feeder roots will be. If you rarely water a certain tree but frequently water a bed 10 or 20 feet away, your tree’s roots will likely be sharing that bed with whatever you have planted there.
The thickest, oldest roots, closest to the trunk, tend not to take up a lot of water, so it’s not efficient to water there. Instead it’s the finer roots, which live in the top 8 to 10 inches of soil and at varying distances from the trunk, that are most efficient at absorbing water and nutrients. A rule of thumb is to water the area starting two feet from the trunk to up to 10 feet beyond the canopy.
Neal recommends watering to a depth of 18 inches in order to create a reservoir that will rise up as the shallower soil starts to dry. Irrigating to this depth can take a long time, depending on your soil, and may require repeated applications over a couple of days. You can measure the depth of moisture by using a moisture meter, a piece of rebar, or even an extra-long screwdriver. Where the device goes in easily, the soil is moist; when you hit a hard area, you’re at the level where the water stopped. This method is less reliable in very loose, sandy soil, which is easily penetrated even when dry.
If you don’t have a drip-irrigation system set up in the area of a tree’s feeder roots, try a soaker hose, a bubbler, or a low sprinkler. Avoid evaporation by watering in the morning, before 10 a.m., or the evening, after 6 p.m. If you’re using a bubbler or a sprinkler, you’ll need to move it around to get a wide area of soil as evenly moist as possible. Once you’ve figured out how long it takes to wet the soil to an adequate depth, you can use a timer. Download the City of Santa Fe’s Eye on Water app (www.santafenm.gov/eye_on_water) to determine how much water you used after irrigating thoroughly.
Neal recommends giving established trees a good soak every two weeks from now until mid-October. Newly planted trees (those planted within the past three to five years) may need more frequent watering. Once deciduous trees go dormant, they need less water, but that does not mean no water. During a cold, moist winter, when the soil freezes deeply, no additional watering is required. But in a warm, dry winter the soil may not freeze; the microorganisms that help trees process nutrients stay active, and they need water to survive. Most experts, including Neal, suggest irrigating once a month in such conditions.
Sarah Baldwin is a freelance editor and writer who has lived and gardened in Santa Fe for more than two decades. She became a Master Gardener in 2017 and serves as editor of the Santa Fe Extension Master Gardeners (SFEMG) newsletter.