When cooler weather arrives, one of the first things many California homeowners do is turn down or turn off the irrigation.
Usually, that is exactly what we should do.
Plants use less water as temperatures fall, evaporation slows, and seasonal rain begins to replace irrigation. Leaving a summer watering schedule running through winter can waste water and create overly wet soils potentially harming the plants.
But shutting irrigation off can create a different problem:
we stop paying attention to the trees.
Winter rain in California is rarely perfectly consistent. A soaking storm may be followed by several weeks of mild, dry weather. Some winters bring long gaps between meaningful rain events. And while the surface of the garden may still look damp or the lawn may remain green, the deeper soil around a tree can gradually become dry.
For trees, those dry periods matter.
Your Tree May Have Been Using More Irrigation Than You Realized
Many homeowners think of their irrigation system in zones:
the lawn zone,
the shrub zone,
the drip zone.
Trees do not recognize those boundaries.
A mature tree growing in or alongside a lawn may have spent years extending roots beneath that irrigated area. The lawn sprinklers were not installed specifically for the tree, but the tree may still have been benefiting from that water.
When lawn irrigation is suddenly reduced or shut off, the tree loses part of its water supply too.
UC researchers specifically caution that trees growing in turf can become water stressed when irrigation is substantially reduced, particularly when there is no meaningful precipitation to replace it.
This becomes especially important when lawns are allowed to go dormant, converted to drought-tolerant landscaping, or irrigation systems are shut down for winter.
The lawn may be able to brown and recover.
A mature tree represents decades of growth.
Trees Need Less Water in Winter but Not Necessarily No Water
A tree’s demand for water decreases considerably during cooler weather. Deciduous trees lose their leaves, growth slows, temperatures fall, and water loss from the canopy declines.
But “less” and “none” are not the same thing.
In California’s mild Mediterranean climate, the soil rarely behaves like frozen winter ground in colder parts of the country. Many California native and Mediterranean-climate plants actually resume root and vegetative activity during the cool, wet season.
If winter rains are regular and substantial, they may provide everything an established tree needs.
During an extended dry spell, however, supplemental irrigation can become appropriate again.
UC landscape trials follow this exact principle: winter irrigation is generally turned off during normal wet conditions, but researchers may irrigate during unusual periods of winter drought. UC Landscape Plant Irrigation Trials
That is a much better model for homeowners than simply choosing an arbitrary “off” date for the irrigation controller.
Why Underwatering Matters
A tree under water stress does more than simply “feel thirsty.”
As soil moisture declines, a tree begins conserving water. It closes tiny pores in its leaves, reducing water loss, but also reducing photosynthesis. Growth slows, and prolonged stress can eventually lead to wilting, leaf loss, branch dieback, and greater vulnerability to pests and other problems.
There is another reason not to wait until a tree looks obviously distressed:
visual symptoms often arrive late.
By the time a mature canopy is noticeably wilting, browning, or thinning, the tree may already have been responding to limited water for some time.
That is why winter tree care is less about waiting for obvious symptoms and more about noticing dry spells and checking the soil.
Winter Moisture Is Part of Preparing for Summer
This does not mean we can “fill up” a tree with water in January and expect it to coast through August.
Trees do not work like storage tanks.
But winter and spring moisture determine the condition of the soil and the tree as it enters the dry season.
Trees tolerate irrigation reductions better when their root zones are adequately moist before water is reduced. Soil texture, rooting depth, root health, species, and existing soil moisture all influence how quickly water stress develops.
the condition in which a tree enters summer matters.
Allowing the root zone to repeatedly dry excessively through winter and spring can add stress before our hottest, driest months have even begun.
Maintaining appropriate moisture during unusually dry winter periods gives the tree a better starting point.
A Little Rain Is Not Always a Deep Watering
One of the easiest mistakes to make is assuming that because it rained, the tree was watered.
It depends on the storm.
A short rain may moisten the upper surface while leaving deeper portions of the root zone much drier. Runoff can make this even more pronounced on slopes, compacted soils, and areas surrounded by pavement.
Rather than judging by the appearance of the soil surface, check underneath it.
UC Davis has recommended checking soil approximately 6 to 8 inches deep near the tree's drip line. If the soil at that depth is dry and crumbly, supplemental water may be warranted. UC Agriculture & Natural Resources
You do not need specialized equipment.
A small trowel, soil probe, or screwdriver can often tell you much more than looking at the irrigation controller.
Deep Watering Does Not Mean Watering at the Trunk
Another common misconception is that tree roots are concentrated directly around the trunk.
For an established tree, they extend much farther.
Most absorbing roots are relatively shallow and spread outward through a broad area of soil, often extending beyond the canopy. UC Davis describes much of a mature tree's root system as occupying the upper several feet of soil and extending beyond the drip line.
So putting a hose directly against the trunk for a few minutes is not really “deep watering.”
The goal is to apply water slowly over a broad portion of the root zone and give it enough time to infiltrate.
Depending on the property, that may mean:
bubblers,
slow drip irrigation,
a soaker hose arranged through the root zone,
or a deep-root watering attachment used carefully in multiple locations.
You do not need the fanciest tool.
You need water to reach the soil where the active roots actually are.
There Is No Universal Winter Schedule
This is probably the most important thing for a homeowner managing their own irrigation to understand.
There is no correct rule that says:
Water every two weeks.
Or:
Turn everything off in November.
Tree water requirements vary enormously according to species, age, canopy size, soil texture, rooting depth, rainfall, slope, exposure, and the surrounding landscape.
UC irrigation guidance emphasizes that these factors all influence both the volume and frequency of irrigation a tree needs. UC IPM
A large established tree growing in deep clay soil may retain moisture far longer than a recently planted tree growing on a sunny slope.
The weather matters too.
Three weeks without rain in cool December weather is very different from three weeks without rain during an unusually warm February.
The schedule should follow the tree and the soil, not simply the calendar.
Watch the Weather. Then Check the Soil.
Winter irrigation can actually be very simple.
When rainfall is regular, let it do the work.
When a meaningful dry stretch develops, check the soil beneath your important trees.
If the root zone still feels moist, leave it alone.
If it is becoming dry, provide a slow, thorough watering and then allow the soil to dry appropriately before watering again.
The goal is not constantly wet soil. Overwatering can deprive roots of oxygen and cause serious root problems of its own.
We are looking for appropriate moisture, not maximum moisture.
Don't Turn Off Your Attention
Reducing irrigation for winter is smart water management.
Forgetting about the trees until spring is not.
California's winter rains normally do much of the work for us, but our rainfall comes in pulses. Paying attention during the spaces between those storms can make a meaningful difference for trees that will soon be asked to withstand another long, hot, dry summer.
Start with three questions:
When did we last receive meaningful rain?
Is the soil actually moist below the surface?
Where was this tree getting its water before I turned the irrigation off?
Those few observations can tell you far more than the date on the irrigation controller.
Turn the irrigation down for winter. Just don't turn your attention off.