How much water do walnut trees need, and when?

Walnut trees have deep roots and need a lot of water. They are harmed by too little water and by too much.

Too little water at the wrong time lowers nut size, kernel colour and grade. Too much water leaves the roots without air, and it allows crown rot (rot at the base of the trunk) to develop.

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 (the branches and leaves of the tree) in a hot summer. Use it as a rough guide, not as a target, and check it against local advice before you plan your system around it.

The size of the canopy decides the amount of water. An orchard with less than half its ground shaded may use 25 to 50 percent less than a full mature orchard. An orchard whose health is declining may also use that much less. A young orchard uses even less. If you water a young block as if it were mature, it is easy to kill the trees with too much water.

Water use changes through the season. It is low in spring, while the leaves are still opening and the weather is cool. It rises fast as the canopy fills and the heat arrives, and it peaks in July and August. In autumn, it falls as the days get shorter and the leaves begin to fall.

Reference evapotranspiration (ETo) is the amount of water that the weather removes from a standard grass surface. It combines temperature, humidity, wind and sunshine, and your national weather service publishes it. Walnut trees use water at a different rate from grass. So you multiply ETo by a crop coefficient (Kc) for walnuts. The Kc is the factor that converts ETo into the water use of walnut trees. It is low in spring, highest in midsummer, and falls again in autumn.

What your soil can hold

The soil holds a certain amount of usable water. This is the water that the tree can use between one irrigation and the point where it starts to suffer. How much water this is depends on the texture of your soil and on how deep the roots go.

This is the share of the water that the tree can use comfortably, not the total amount that the soil holds. The total store is a larger number for the same ground, because the two figures measure different things.

Multiply the figure in the table by the depth of your roots, in metres, to find how much water the block has available. 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 young trees and mature trees need completely different schedules, even on the same farm in the same week.

Never let water stand for more than 24 hours

On a heavy soil (a soil with a lot of clay), water soaks in slowly. This is called slow infiltration. On this kind of soil, the rule about standing water is more important than the calculation above. If your soil cannot absorb a full irrigation within 24 hours, apply less water each time and irrigate more often. Do not force in the full volume of water because the calculation says so.

The same rule applies to the time that a system runs. Do not run an irrigation set (a group of rows watered together) for longer than 24 hours. If your calculation says that you need 40 hours, split the irrigation into two or three sets. Do not leave the soil saturated.

When to start in spring

Walnut trees do not need early-season water when the soil is already full. If you start too early, the soil stays cold and saturated at exactly the time when root diseases are most active. It also keeps the roots shallow, because the roots never have to grow down to find water.

Delay the first irrigation until the trees show mild water stress (the tree has slightly less water than it needs). This makes the roots grow deeper, and it does not harm nut quality. Growers who start irrigating on a fixed date in spring are usually watering soil that does not need it.

Deciding when to irrigate: the pressure chamber

The most accurate way to know when your trees really need water is to use a pressure chamber. A pressure chamber is an instrument that measures how water-stressed the tree is, using a leaf. It measures the tension inside the tree itself. This is called stem water potential. It shows how hard the tree must pull to get water out of the soil.

Deciding how much water to apply: soil moisture sensors

The pressure chamber tells you when to irrigate. Soil moisture sensors tell you how much water to apply, and how deep your water goes.

Tensiometers

A tensiometer is an instrument placed in the soil that shows how wet the soil is. It measures how hard the soil is holding its water. These are the typical readings at which to start irrigating:

The readings are given in centibars (cbar), because that is the unit printed on the instrument.

Volumetric sensors measure the percentage of water in the soil instead. You must calibrate them to your own soil (adjust them so that the readings match the water in your soil) before the numbers mean anything.

Install sensors at several depths: around 45 cm, 90 to 105 cm and 150 cm. Then you can see the whole root zone (the soil where the roots grow). The roots of a mature walnut tree reach down to 1.2 to 1.8 metres. You are trying to keep that whole depth supplied with water.

Which irrigation system should you use?

Drip puts water exactly where the roots are, at a rate that the soil can absorb, with very little lost to evaporation. It lets you water in small amounts, frequently, which is what walnut roots need. You can also apply fertilizer through the same pipes. As the tree grows, you can make the wetted area (the soil that the irrigation water reaches) larger.

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.

These faults can all affect a drip line:

• a blocked emitter (the small outlet on a drip line that releases the water)
• a root growing into a line
• damage from rodents chewing the line
• a leak.

With surface drip, you walk along the row and you can see them. With buried drip, you see a tree that is not growing well, and then you have to guess where the fault is. Buried systems also cost considerably more to install. The extra efficiency is not worth the loss of visibility.

Why not the other systems

Design the system for the mature orchard, from the start

Sizing the system for young trees is the most expensive mistake in this chapter, and growers make it in year one.

Your first-year trees may need one line and two emitters each. That is fine. But the pipes that carry the water must be able to supply four, six or eight emitters per tree in ten years.

If the pipes cannot do this, you will find out in year five that your orchard needs more water than your system can physically deliver. The fix is to replace the control head and the mains, in an orchard full of trees. The control head is the equipment at the start of the system, where the water is filtered and controlled.

Start at the pump and design every part for the water that the orchard will use when it is mature. The extra cost at installation is small, and the cost of a system that is too small is enormous.

Choosing the pump

You choose a pump using two numbers: flow and pressure. If either number is wrong, the orchard suffers for its whole life.

Start with flow. Decide which blocks will run at the same time, then count every emitter that will run in them.

For example, a block of 2,000 trees with four emitters per tree, each emitter releasing 4 litres per hour, needs 32,000 litres per hour. This is 32 m³ (cubic metres) per hour.

Do this calculation for the mature orchard. Include the extra emitters, or the second drip line, that you expect to add later. Then check the result against your peak daily requirement (the most water that the orchard needs in one day). The system has to deliver this water within the hours that you really have available to irrigate.

Then calculate the pressure

The pressure that your emitters need is only part of the total. The pump must also overcome:

• the vertical lift from your water source
• differences in height across the orchard
• friction in the mains, submains and laterals
• losses through the filter, the valves and any fertigation equipment (equipment that feeds fertilizer through the irrigation water)
• the minimum pressure that the emitters themselves need.

Added together, these are the total dynamic head. Give your supplier the flow and the total dynamic head. Then choose a pump whose normal working point (the flow and pressure at which the pump normally runs) is close to both.

Do you need a reservoir?

You do not always need a reservoir (a tank or pond that stores water). If your well, canal or other source can reliably deliver the flow that your system needs while it is running, pump the water 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 takes water from the reservoir at the higher rate that the irrigation needs.

For example, suppose that your well produces 20 m³ per hour and your block needs 60 m³ per hour for eight hours. You can calculate the storage that you need like this:

Reservoirs are also useful in these cases:

• Water is available only at certain times.
• A low-yield well (a well that produces little water) needs many hours to recover.
• You have to collect water from several sources before you irrigate.

Reservoirs also have costs. A reservoir takes up land. It loses water by evaporation and by seepage (water leaking into the ground). Open water grows algae and collects sediment (small particles that settle in the water), so your filtration has to work harder. You may also need a second pump. Build a reservoir when your water source makes it useful. Do not build one because you have chosen drip.

Grow the system with the trees

A young tree needs a small wetted root zone (the part of the root zone that the water reaches). A mature tree needs a large one. The system should change as the tree grows, and you should build it so that it can change.

• Emitter numbers increase as the tree grows. One or two emitters 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 (the roots and the soil around them), close to the trunk. Otherwise the tree gets no water. But this is a starting position, not a permanent one.
• Adding a second line is one option. Adding more emitters is another option. Which one suits you depends on your soil.

When a second drip line is worth adding

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

• On sandy soil it is often necessary. Water in sand moves down much more easily than sideways. So a single line gives you a narrow, deep strip, and much of the root zone stays dry.
• On finer soils one line may be enough. Clay and loam spread water sideways well. The wet patches from a single line may already overlap and form a continuous wet strip.
• It also increases capacity. More emitters give more litres per hour. So you need fewer running hours to meet the peak summer demand. On a block that cannot meet the water demand in July, this matters as much as the size of the wetted area.

Check the wetting pattern yourself

What you find should decide the number of emitters, the distance between them (the emitter spacing) and whether you need a second line. There is no single correct emitter spacing. Sandy soil needs emitters closer together, more emitters, or a second line, to wet enough soil. Finer soil spreads the water further, so it needs fewer emitters.

Long rows and slopes need proper hydraulic design (the design of the pipes and the water pressure). Water pressure falls along a long lateral, and it changes with the height of the ground. So the emitters at the far end, or at a different height, release different amounts of water than the emitters near the valve. The answers are:

• correct pipe sizing
• reasonable zoning (dividing the orchard into sections that are watered separately)
• pressure regulation
• on sloping ground, pressure-compensating emitters (emitters that release the same amount of water at different pressures).

Solve this problem in the design. You cannot fix it later by adjusting a valve.

Watering young trees

Young trees are not small mature trees. A young tree has a tiny, shallow root system that cannot reach deep moisture. Also, the water you apply is concentrated in a small area, not spread across the orchard.

Water often, in small amounts, over the roots

In midsummer, a young tree may need only 20 to 40 litres a week. But it needs this water in several small applications, not in one large one. If you water it like a mature tree, most of the water runs straight past the roots.

For a young tree, never leave more than about a week between irrigations. It is better to water twice a week in small amounts. The water reserve of a young tree lasts for days, not weeks.

Calculate for the wetted area, not for the hectare

The amount of orchard floor that you really wet changes the calculation completely:

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

Your soil decides the shape of the wet patch

The same emitters make very different wet patches in different soils:

This is why the same irrigation plan works 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 limit for standing water sooner.

A practical layout for the first two years is three emitters per tree. Use one smaller emitter about 15 cm from the centre of the trunk, and two larger emitters about 60 cm on either side of it. This puts water onto the small root system and starts to widen it. Then move the emitters outward from there.

Estimating a young orchard’s water use

This is the easiest method in this chapter to use in any region. It does not depend on anyone else’s climate.

Growers commonly count the age of a young orchard in growing seasons instead of years. The season when the trees were planted is “first-leaf”, the season after that is “second-leaf”, and so on. These names do not count leaves.

For example, a third-leaf orchard shades 35 percent of the floor. It uses about 70 percent of the water that a mature orchard on that site would use. If the shade is above about 50 percent, treat the orchard as using the same water as a mature orchard. Measure the shade at midday in midsummer, take your figure for a mature orchard, and multiply it by that share.

These figures are from a hot Californian summer. Use them to understand the pattern of the season and the size of the numbers, not as a schedule for your farm. Where they disagree with the canopy-cover method above, trust the canopy-cover method. That method is based on your own trees. Notice how much the numbers change from one year to the next. A schedule that suited your trees last summer will give them too little water this summer.

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

Distribution uniformity means that every tree receives the amount of water that you intended. Poor uniformity is rarely a 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 is worn.

Harden young trees off in autumn

Young trees that keep producing soft new growth into autumn are damaged by the first frosts. Soft, actively growing wood has no defence against cold. To harden off a young tree means to make it stop growing, so that the new wood can mature before the frost.

When water stress costs you most

Water stress does not cost the same at every point in the season.

What years of under-watering do

The trees can recover from under-watering that happens once. They cannot recover from repeated under-watering until you fix it.

The damage gets bigger each year, 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.

Walnut trees recover from this damage. When those trees were returned to full irrigation, yields recovered completely within two years. In the first properly watered year, the trees grew a lot of strong shoots. These shoots rebuilt the places where the tree carries its crop (the fruiting positions) that it had lost. If your orchard has had a couple of dry years, it is not lost. 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 not growing well, for reasons that are not clear. 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 this disease kills walnut trees quickly. Water that moves past the deepest roots also carries your nitrogen away.

Watch the leaves

• burnt leaf tips
• thick, leathery leaves
• distorted leaves and distorted leaf edges
• yellowing
• a bronze colour on the leaf
• purple veins.

None of these signs is proof on its own, and several look like other problems. But if a block shows several of these signs while the soil stays wet, the cause is clearly too much water. There is also a practical cost. Over-watered trees grow more wood that you have to prune, and more weeds that you have to manage.

Your irrigation water may already supply nitrogen

Many irrigation waters carry nitrate, especially groundwater (water from underground) in farmed districts. This nitrate is fertilizer, and it reaches your trees whether you planned it or not.

For example, an orchard receives 1,000 mm of water that contains 10 mg of nitrate-nitrogen per litre. This gives the orchard about 100 kg of nitrogen per hectare across the season at no cost, before you spread any fertilizer. Growers who ignore this routinely apply far more nitrogen than their trees need. Ask for nitrate to be included in your water analysis. Then subtract the nitrogen that the water gives you before you decide how much fertilizer to buy.

Salty water and leaching

Walnuts are sensitive to poor water quality. There are two different problems. The first is the total amount of salt in the water. Salt makes it physically harder for the roots to take water out of the soil. The second is specific ions, which are particular substances dissolved in the water: chloride, sodium and boron. These damage the tree directly. A water source can be low in total salt and still contain enough boron to damage your trees.

Leach only when you need to. Leaching means applying more water than the trees use. The extra water pushes the salts that have collected in the soil down below the roots. You need to leach when the salinity (the salt level) of the root zone is approaching the level that walnuts can tolerate. The same applies to each of those specific ions.

• As a working figure, add 15 to 20 percent to the water that you would otherwise apply.
• Do the serious leaching in the dormant season (the trees’ winter resting period, without leaves). 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 soak into the soil and drain away. Without infiltration and drainage, you are only waterlogging the block.

Keeping the system working

Even the best schedule is useless if the water does not reach the trees. Drip is efficient, but it is easy to block. Emitters can become blocked in three different ways, and each way needs a different solution:

• Mineral deposits, for example calcium carbonate that builds up inside the emitter.
• Physical particles that passed through the filter.
• Biological growth: algae and bacteria growing inside the lines.

To prevent blockages, you need proper filtration and routine treatment of the lines. The treatments that are available and permitted are different in each country, so check locally before you buy anything.

If the pressure falls badly along a lateral, or between high and low ground, this is a design problem, not a maintenance problem. The solutions are pipe sizing, zoning, pressure regulation and pressure-compensating emitters.

Can I grow walnuts without irrigation?

Walnuts are not a drought crop (a crop that grows well with little water). A mature orchard in a hot climate needs roughly 1,000 mm of water in a season. It needs most of this water in July and August. In most walnut-growing regions, these are the months with almost no rain. Rain that falls 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 when two metres of soil are full, they supply around three weeks of peak demand.

Three weeks is useful and worth having, but it is not a whole summer. A shallow or sandy soil holds a fraction of that amount. It gives you days instead of weeks.

What happens if you plant anyway. You will get trees, but you will not get a commercial crop. An orchard that is short of water loses more each year. First the nuts become smaller, then the shoots grow less, and then there are fewer places to carry a crop next season.

The summer shortage also falls exactly in the kernel filling period. In this period, water stress turns into dark, shrivelled kernels, off-grade nuts and kernel mould. You would spend thirty years managing an orchard to produce a crop that nobody wants to buy.

Settle the question of your water supply before you order trees, not after. Water is the one input that you cannot substitute, borrow or postpone, and it 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. We will work through the calculation with you, and we will tell 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. Two items are not: the dig-and-look check and the uniformity check. These are the two checks that tell you whether the rest is working.