How much water do walnut trees need, and when?

Walnuts are deep-rooted and thirsty, and they punish both mistakes.

Too little water at the wrong moment costs you nut size, kernel colour and grade. Too much suffocates the roots and invites crown rot.

How much water an orchard uses

A mature, healthy walnut orchard in a hot climate uses roughly 1,040 to 1,120 mm of water across the season. That figure is for a full canopy in a hot summer. Read it as a scale, not a target, and check it against local advice before you plan around it.

Canopy size drives the number. An orchard with less than half its ground shaded, or one in decline, may use 25 to 50 percent less than a full mature orchard. A young orchard uses far less again — and watering a young block as if it were mature is one of the easiest ways to drown it.

Water use follows a curve through the season. Low in spring, while the leaves are still coming out and the weather is cool. Rising fast as the canopy fills and the heat arrives. Peaking in July and August. Falling away in autumn as the days shorten and the leaves start to go.

Reference evapotranspiration (ETo) is how much water the weather is pulling out of a standard grass surface. It combines temperature, humidity, wind and sunshine, and your national weather service publishes it. Walnuts do not use water at the same rate, so you multiply by a crop coefficient (Kc) for walnuts — low in spring, highest in mid-summer, falling again in autumn.

That gives you the demand. The soil tells you how often you have to meet it.

What your soil can hold

Between one irrigation and the point where the tree starts to suffer, the soil holds a certain amount of usable water. How much depends on the texture of your soil and on how deep the roots go.

This is the share the tree can draw on comfortably, not the total the soil is holding. Chapter 1 quotes the total store for the same soils, which is a larger number for the same ground — the two are measuring different things.

Multiply by your root depth to get what the block actually has to work with. A young tree with 30 cm of roots on a sandy loam has about 17 mm. A mature tree on the same soil with 1.5 m of roots has around 87 mm. That is why the two need completely different schedules, on the same farm, in the same week.

The 24-hour rule overrides everything

On a heavy soil with slow infiltration, this rule beats the arithmetic above. If your soil cannot absorb a full irrigation within 24 hours, apply less each time and irrigate more often. Do not force the full volume in because the calculation says so.

The same applies to run times. Do not run an irrigation set for longer than 24 hours. If your calculation says you need 40 hours, split it across two or three sets rather than leaving the soil saturated.

When to start in spring

Walnut trees do not need early-season water when the soil is already full. Starting too early keeps the soil cold and saturated exactly when root diseases are most active, and it keeps the roots shallow, because they never have to go looking for water.

Delay the first irrigation until the trees show mild water stress. This encourages deeper rooting and does not harm nut quality. Growers who start irrigating on a fixed date in spring are usually watering soil that does not need it.

The word that matters is mild. This is a delay, not a drought.

Knowing when: the pressure chamber

The most accurate way to know when your trees actually need water is to measure the tension inside the tree itself, with a pressure chamber. It measures stem water potential — how hard the tree is having to pull to get water out of the soil.

Knowing how much: soil moisture

The pressure chamber tells you when. Soil moisture sensors tell you how much, and how deep your water is actually reaching.

Tensiometers

These measure how hard the soil is holding its water. Typical points at which to start irrigating:

Left in centibars, because that is the unit printed on the instrument.

Volumetric sensors measure the percentage of water in the soil instead. They need calibrating to your own soil before the numbers mean anything at all.

Install sensors at several depths — around 45 cm, 90 to 105 cm, and 150 cm — so you can see the whole root zone. A mature walnut’s roots work down to 1.2 to 1.8 metres, and that is the depth you are trying to keep supplied.

Which irrigation system should you use?

Drip puts water exactly where the roots are, at a rate the soil can absorb, with very little lost to evaporation. It lets you water little and often, which is what walnut roots want. It lets you feed through the same pipes. And it lets you grow the wetted area as the tree grows.

Lay the line on the surface, not buried

Buried drip is sold as tidier and more efficient. In practice it causes problems that are hard to find and expensive to fix, because everything you need to inspect is under the ground.

A blocked emitter, a root growing into a line, a rodent chew, a leak — with surface drip you walk the row and see it. With buried drip you see a tree that is not thriving, and you start guessing. Buried systems also cost considerably more to install. The extra efficiency does not pay for the blindness.

Why not the other systems

Design the system for the mature orchard, from day one

This is the most expensive mistake in the whole chapter, and it is made in year one.

Your first-year trees may need one line and two emitters each. That is fine — but the pipework carrying that water must be able to carry four, six or eight emitters per tree in ten years. Otherwise you will discover in year five that your orchard needs more water than your system can physically deliver, and the fix is replacing the control head and the mains in an orchard full of trees.

Design from the pump forward for what the orchard will eventually use. The extra cost at installation is small. The cost of getting it wrong is enormous.

Choosing the pump

A pump is chosen from two numbers: flow and pressure. Get either wrong and the orchard suffers for its whole life.

Start with flow. Decide which blocks will run at the same time, then add up every emitter running in them.

A block of 2,000 trees with four 4-litre-per-hour emitters each needs 32,000 litres per hour, which is 32 m³ per hour.

Do this for the mature orchard, including the extra emitters or the second drip line you expect to add later. Then check it against your peak daily requirement: the system has to deliver the water the orchard needs within the hours you actually have available to irrigate.

Then work out the pressure

The pressure your emitters need is only part of it. The pump also has to overcome:

• the vertical lift from your water source;
• differences in height across the orchard;
• friction in the mainline, submains and laterals;
• losses through the filter, the valves and any fertigation equipment;
• the minimum pressure the emitters themselves need.

Added together, these are the total dynamic head. Give your supplier the flow and the total dynamic head, and choose a pump whose normal working point sits close to both.

Do you need a reservoir?

Not necessarily. If your well, canal or other source can reliably deliver the flow your system needs while it is running, pump straight into the system.

A reservoir helps when your source gives enough water overall but gives it too slowly. The well runs continuously at a low rate and fills the reservoir; a second pump then draws from the reservoir at the higher rate the irrigation needs.

Say your well produces 20 m³ per hour, and your block needs 60 m³ per hour for eight hours:

Reservoirs are also useful where water is only available at certain times, where a low-yield well needs many hours to recover, or where you have to collect several sources before irrigating.

They cost something too. They take up land, lose water to evaporation and seepage, and open water grows algae and collects sediment, which increases what your filtration has to do. You may also need a second pump. So build a reservoir when your water source makes it useful — not because you have chosen drip.

Grow the system with the trees

A young tree needs a small wetted root zone. A mature tree needs a large one. The system should change over that period, and it should be built so that it can.

• Emitter numbers increase as the tree grows. One or two per tree at planting may become four by the third or fourth year, and six or eight on a mature tree. The exact number depends on your soil and how far water spreads through it.
• Move the emitters outward as the roots expand. For a newly planted tree the water has to go over the root ball, close to the trunk, or the tree gets nothing. But that is a starting position, not a permanent one.
• Adding a second line is one option, more emitters is another. Which suits you depends on your soil.

When a second drip line is worth it

A second lateral, on the other side of the row, does two things: it wets more soil, and it increases how much water the system can apply per hour.

• On sandy soil it is often necessary. Water in sand moves down far more readily than sideways, so a single line gives you a narrow, deep strip and leaves much of the root zone dry.
• On finer soils one line may be enough. Clay and loam spread water sideways well, and the bulbs from a single line may already overlap into a continuous wetted strip.
• It also buys you capacity. More emitters means more litres per hour, which means fewer running hours to meet peak summer demand. On a block struggling to keep up in July, that matters as much as the wetted area.

Check the wetting pattern yourself

What you find should decide your emitter number, your emitter spacing and whether you need a second line. There is no single correct emitter spacing. Sandy soil needs closer emitters, more of them, or a second line to build enough wetted volume. Finer soil spreads water further and needs fewer.

Long rows and slopes need proper hydraulic design. Water pressure falls along a long lateral and changes with elevation, so emitters at the far end or at a different height discharge different amounts from those at the valve. The answers are correct pipe sizing, sensible zoning, pressure regulation and, on sloping ground, pressure-compensating emitters. Get this right in the design — you cannot fix it later by adjusting a valve.

Watering young trees

Young trees are not small mature trees. They have a tiny, shallow root system that cannot reach deep moisture, and the water you apply is concentrated in a small area rather than spread across the orchard.

Little and often, and over the roots

A young tree in high summer may need only 20 to 40 litres a week — but it needs that in several small applications, not one large one. Water it like a mature tree and most of what you apply runs straight past the roots.

Never leave more than about a week between irrigations for a young tree, and preferably water twice a week in small amounts. Their reserve is measured in days, not weeks.

Work over the wetted area, not the hectare

Knowing how much of your orchard floor you are actually wetting changes the arithmetic completely:

• Two drip emitters per tree wet roughly 2 percent of the orchard floor.
• Four drip emitters per tree wet roughly 15 percent.

Your soil decides the shape of the wet patch

The same emitters make very different shapes:

This is why the same irrigation plan behaves differently on two blocks of one farm. On sand you need more emitters or a second line to wet enough soil. On clay you reach the 24-hour standing-water limit sooner.

A practical layout for the first two years: three emitters per tree — one smaller emitter about 15 cm from the centre of the trunk, and two larger ones about 60 cm either side of it. That puts water onto the small root system and starts widening it. Then move outward from there.

Estimating a young orchard’s water use

This is the most portable method in the chapter, because it does not depend on anyone else’s climate.

So a third-leaf orchard shading 35 percent of the floor uses about 70 percent of what a mature orchard on that site would use. Above about 50 percent shading, treat the orchard as using full mature water. Measure the shade at midday in midsummer, take your mature-orchard figure, and scale it. It works anywhere.

These figures are from a hot Californian summer, so treat them as a shape and a scale rather than a schedule for your farm. Where they disagree with the canopy-cover method above, trust the canopy-cover method — it is based on your own trees. Note also how much the number changes from one year to the next: a schedule that suited your trees last summer will underfeed them this summer.

Root depth grows too. A first-leaf tree has roughly 15 to 90 cm of root zone, a second-leaf tree 60 cm to 1.2 m, and a third-leaf tree 90 cm to 1.5 m. Recalculate what your soil holds each year, because the depth keeps changing.

Distribution uniformity is rarely the problem in a new orchard, because the system is new. It becomes a real problem once a system is more than about ten years old and has worn.

Harden young trees off in autumn

Young trees that keep pushing soft new growth into autumn get damaged by the first frosts. Soft, actively growing wood has no defence against cold.

When water stress costs you most

Stress does not cost the same at every point in the season.

What years of under-watering do

Under-watering once is recoverable. Under-watering repeatedly is not, until you fix it.

The damage compounds because two things happen at once. The nuts get smaller, and the shoots stop growing. Fewer new shoots means fewer places to carry a crop the following year, so each dry year makes the next one worse.

The good news is that walnuts recover. When those trees were returned to full irrigation, yields came all the way back within two years — through a burst of strong shoot growth in the first properly watered year, which rebuilt the fruiting positions the tree had lost. If you have had a dry couple of years, your orchard is not finished. It needs two full seasons of proper water.

The signs of too much water

Over-watering does not look dramatic. It looks like a tree that is somehow not thriving — and growers often respond by watering more.

Saturated soil pushes the air out. Walnut roots need oxygen, and without it they suffocate. Wet, airless soil is also where Phytophthora root and crown rot does its damage, and that kills walnut trees quickly. Water moving past the deepest roots carries your nitrogen away with it.

Watch the leaves

• burnt leaf tips;
• thick, leathery leaves;
• distorted leaves and distorted leaf edges;
• yellowing;
• a bronze cast to the leaf;
• purple veins.

None of these is proof on its own, and several look like other problems. But a block showing several of these signs while the soil stays wet is telling you clearly. There is a practical cost too: over-watered trees grow more wood you have to prune, and more weeds you have to control.

Your water may already be feeding the orchard

Many irrigation waters, especially groundwater in farmed districts, carry nitrate. That nitrate is fertilizer, and it arrives whether you planned it or not.

So an orchard receiving 1,000 mm of water at 10 mg per litre gets about 100 kg of nitrogen per hectare across the season, free, before you spread anything. Growers who ignore this routinely apply far more nitrogen than their trees need. Ask for nitrate on your water analysis, and subtract what the water gives you before you decide what to buy.

Salty water and leaching

Walnuts are sensitive to poor water quality, and there are two different problems: overall salt, which makes it physically harder for the roots to pull water out of the soil, and specific ions — chloride, sodium and boron — which damage the tree directly. A water source can be low in total salt and still carry enough boron to hurt you.

Leach only when you need to. Leaching means applying more water than the trees use, so the extra pushes accumulated salts down below the roots. You need it when root zone salinity, or one of those specific ions, is approaching the level walnuts can tolerate.

• As a working figure, add 15 to 20 percent to the water you would otherwise apply.
• Do the serious leaching in the dormant season. Extra water in winter is much less likely to cause waterlogging and disease than the same water applied to warm soil in summer.
• Leaching only works if the water can get through and get away. Without infiltration and drainage you are just waterlogging the block.

Keeping the system working

The best schedule in the world is useless if the water is not arriving. Drip is efficient, and it is also easy to block. Emitters clog three different ways, and they need different answers:

• Mineral deposits, such as calcium carbonate building up inside the emitter.
• Physical particles that got past the filter.
• Biological growth — algae and bacteria growing inside the lines.

Prevention means proper filtration plus routine treatment of the lines. What is available and permitted differs by country, so check locally before you buy anything.

If you find pressure falling badly along a lateral or between high and low ground, that is a design problem, not a maintenance one — pipe sizing, zoning, pressure regulation and pressure-compensating emitters are what fix it.

This is also the practical argument for keeping the drip line on the surface. Every one of these faults — a blocked emitter, a split line, a rodent chew, a root intrusion — is something you can find by walking the row and looking. Buried, they show up as a tree that is quietly failing, and you have to guess where to dig.

Can I grow walnuts without irrigation?

Growers ask us this often, so here is the honest answer.

Walnuts are not a drought crop. A mature orchard in a hot climate wants somewhere near 1,000 mm of water in a season, and it wants most of it in July and August — the months when most walnut-growing regions get almost no rain. Rainfall that arrives in winter, when the tree is dormant, does not help in August unless your soil can store it until then.

Your soil is a reservoir, and you should know how big it is. Deep soil stores winter rain and releases it through the summer. Two metres of loam holds roughly 165 mm of usable water. At the peak of summer a mature orchard uses about 7 to 8 mm a day, so a full profile is around three weeks of peak demand. Three weeks is real and worth having. It is not a summer. A shallow or sandy soil holds a fraction of that, and gives you days rather than weeks.

What happens if you plant anyway. You will get trees. What you will not get is a commercial crop. An orchard short of water repeatedly loses more each year — smaller nuts, then less shoot growth, then fewer places to carry a crop next season. Worse, the summer shortage lands exactly on the kernel filling period, which is when water stress turns into dark, shrivelled kernels, off-grade nuts and kernel mould. You would be spending thirty years of orchard management to produce a crop nobody wants to buy.

Sort out your water before you order trees. Not after. Water is the one input you cannot substitute, borrow or postpone, and it is the one thing that decides whether the orchard was worth planting. If you are not sure whether your supply is enough, tell us your rainfall pattern, your soil depth and whatever water you can access, and we will work through it with you honestly — including telling you if the answer is no.

Mistakes worth avoiding

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Checklist for the season

Almost everything on that list is cheaper to settle before the season than during it. The two that are not — the dig-and-look check and the uniformity check — are the two that tell you whether the rest of it is actually working.