Southwest
Yard & Garden
By
Marisa Thompson
 |
| Several Afghan pine trees are stunted and have thinner canopies than others. More photos below. All photo credits Michelle J. |
Question: I have 120 healthy
Afghan pine trees, but about 10 may need fertilizer because they're not as full
as the others. When and how much fertilizer should I apply around the drip line
of a 25-ft tall tree?
- Michelle J., Dona Ana County
Answer: Great question!
Thank you for seeking local recommendations before taking action. Here’s my preliminary
response, followed by further discussion:
Tree decline can be a real
mystery, and your question raised more questions for me than answers. My
response questions mostly revolve around how you care for your trees.
·
What are your irrigation practices? Details here
can be helpful.
o What’s
the primary method you use?
o Do
you flood the area with a garden hose or sprinkler?
o Are
there soaker hoses or drip emitters hidden under the thin layer of pine needles
I see?
o How
frequently do you irrigate these trees in each season?
o Where
do you apply water in relation to the tree trunk?
o Approximately what amount of water is applied with each irrigation?
It can be tricky to estimate an
exact amount of water. Another way to get at this question is to share
specifics about the flow rate (e.g., full-blast hose or 1-gallon-per-hour drip
emitters), how many emitters you’re using per tree, and how long it takes you
to water.
For tips on when, where, how, and how much to water
established trees, check out my 2019 column, “Knowing How Much to Water”, parts
I & II (https://nmsudesertblooms.blogspot.com/2019/04/southwest-yard-and-garden-knowing-how.html).
More questions: In the photos, I see some good-looking pine mulch under some of the trees, although the layer appears too thin to be helpful with controlling weeds. Are the pine needles allowed to stay where they fall as a mulch? Or are they routinely removed? And, finally, have these trees been fertilized in the past, and if so, what was the regimen?
**Important note: Afghan pines don’t typically need
fertilizer, as they’re well adapted to our soil conditions.
Update from Michelle: Due to a well pipe break, half of the trees are watered by drip tubing placed two feet from the trunk, three times a week for an hour. The rest receive irrigation from neighbors on the other side of the wall. The sickly ones received more hand watering along the drip line. Pine needles around the trees had been removed for fifteen years but left where they fell for the last five. As for fertilizer, Michelle fertilized the trees in the second year and annually for five years using an injection method. The affected trees received six fertilizer spikes around the drip line in March and again in October.
With all this in mind, I believe
the answer lies in irrigation (including where the water is applied), not in
fertilization. While fertilizing may not cause or exacerbate the problem, I
don’t think it’s the fix that’s needed. Sometimes fertilizing can push an
ailing plant beyond its capacity. It’s kind of extreme, but I’ve heard before
that giving fertilizer to a dying plant is like giving a Coke to a dying man—it
may offer an initial sugar surge of energy, but it’s definitely not the
answer.
Avoiding fertilizer is
especially important in soils with already high salt levels, which are common
in our region. Check out the short video titled “Unavailability of Water in
Saline Soils” found here: https://westernsoils.nmsu.edu/soils/animations.html#saline. More saline soil resources are linked below.
Watering just two feet from the
trunk is too close for your drip irrigation tubing. Think about where you’d run
if you needed to get out of the rain quickly and trees were nearby. You’d
shelter under the tree canopy, right? That’s a clue that the soil is drier
there, too. Roots proliferate where they can maximize access to precipitation.
That’s why we recommend placing irrigation water strategically at the canopy
dripline and beyond. If watered at the right root location and sufficiently to
reach roots about two feet deep, you shouldn’t have to water as often.
Depending on your soil and environmental conditions, irrigating once every 1-2
weeks during the growing season and once every 4-6 weeks in winter should be
about right.
I checked with three southern New Mexico landscape tree experts for their input.
Longtime local horticulturist
and nurseryman Les Finley responded:
We see this situation often: Afghan
pines along property lines with rudimentary drip systems. As the trees grow,
water becomes scarcer, and competition affects the weaker trees. When the
survivors don't receive enough water to support their defense systems,
secondary pests like borers move in.
These pines appear seriously water-stressed and need immediate irrigation. I
suspect the current system has deteriorated and may have aged out. It should be
repaired, flushed, and audited for adequate delivery to each tree. Ideally, it
should be replaced with a system designed to serve the trees' current and
future needs. Since a well is being used, proper filtration and regular
maintenance are necessary for the drip system to function reliably.
International Society of Arboriculture
Certified Arborist & professional horticulturist John White also suggested
watering where the roots are (meaning at the dripline of the tree, or out where
the furthest branches reach). Watering less often and more deeply with each
irrigation is also important, especially for mature trees. They need deep water
at the roots, not constantly-soggy soil surfaces.
As general things to watch out
for when diagnosing tree problems, White offered these additional
considerations:
1. Confirm that the tree is not planted too deeply. The trunk flare (aka root flare, or where the trunk widens at the root transition) should be visible above ground.
2. Circling roots can girdle the trunk and eventually kill the tree. Sometimes exacerbated by planting too deeply, circling roots can also form in growing pots and worsen over time if not corrected at planting. They may also form in the ground after planting if the surrounding soil is too compacted and/or irrigation is applied only at the base of the trunk.
3. If pines have borers, they’re usually an indicator of underlying issues, like those mentioned above.
4. Everyone wants to apply fertilizer, but it’s not the easy answer we’re looking for.
5. Watering more does not mean every day! We recommend watering deeper and farther from the trunk, but allow drying time between irrigations, perhaps every 10 days. Don’t keep it too wet. Roots need oxygen.
White added that, unfortunately,
in New Mexico’s arid climate, a watering issue is behind many plant problems,
and trees take some time to both show symptoms of water stress and to recover
from it. Water approximately once a week and deeply, and, hopefully, that is
the real help these trees need.
As luck would have it, when I spoke with Jeff Anderson, Doña
Ana County Ag Agent, he had already performed a site visit to see these pines
in person! Anderson confirmed that insufficient irrigation, combined with
excess salt buildup, is the most likely culprit.
One more thing: Good work on leaving
the needles. As they pile up under the trees, they’ll make a nice mulch layer
that can help hold soil moisture, in addition to many other benefits.
Please share updated photos after
a few months. Hopefully, you’ll start seeing shiny new green growth on the tips
of branches by then.
More resources on saline soils:
-
Several NMSU Extension Publications on Soils:
o Circular 656: An Introduction to Soil Salinity and Sodium Issues in New Mexico
o Circular 676: Interpreting Soil Tests: Unlock the Secrets of Your Soil
-
NMSU Learning Games Labs video series: https://westernsoils.nmsu.edu/
For
more gardening information, including
decades of archived Southwest Yard & Garden columns, visit the NMSU
Extension Horticulture page (http://desertblooms.nmsu.edu/), follow us on social media
(@NMDesertBlooms), or contact your County Extension office (https://extension.nmsu.edu/county.html).
Marisa Thompson, PhD, is the Extension Horticulture
Specialist for New Mexico State University and is based at the Agricultural
Science Center at Los Lunas.
Special thanks to Bethany
Abrahamson and Lee Martinez for their input and edits.
More photos from Michell:
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