Is your land good for walnut trees?

Most land is.

Walnut trees grow and produce well in a very wide range of places. They are grown as a business in France, Italy, Spain, Turkey, the Balkans, Central Asia, Chile and California. These places have very different soils, rainfall and winters. If your neighbours grow fruit or nuts, walnuts will very probably work on your land too.

So this chapter is not a list of reasons to say no. It is a short guide to the few things worth checking before you plant. Check them, and you will plant in the best part of your field, choose the right variety, and know what to prepare.

Most of what you find will be fine. Some things you can fix easily before planting. Only a small number of problems are serious, and this chapter tells you how to spot them early, while they still cost you nothing.

That is the whole list. Everything else in this chapter explains how to check those four things.

How to check your land, step by step

You do not need to do all of this in one week. Work through it in this order, because each step tells you whether the next one is worth doing.

1. Look at the climate. Is there late frost in spring? Is the summer warm enough?
2. Walk the land. Where is it high, where is it low? Where does water sit after rain? Where does fog settle?
3. Ask what grew here before. Old orchards, old woodland and old vineyards can leave problems in the soil.
4. Dig some holes. This is the most useful hour you will spend. You want to see the soil down to about 1.5 metres.
5. Send soil and water to a laboratory. One test, before planting, tells you what to prepare.
6. Check your summer water. Not the total for the year — the amount you can get in July.
7. Decide, and write down what you decided and why.

Part 1 — Climate

Late frost in spring

This is the one climate question worth real attention. The good news is that it is a question about where you plant, not whether you plant.

Frost in spring is only a problem when the tree has already opened its flowers. A cold night in winter does nothing. The same cold night in April, on open flowers, can take the crop.

A later variety helps a great deal. It does not turn a cold hollow into good ground, so use both: the right place and the right variety.

Is your summer warm enough?

Walnut trees need a warm summer to fill the kernel properly. Cool summers can give thin shells and light kernels.

Very hot summers are also fine, as long as the trees have water. Above about 38°C, the sun can darken the kernels of nuts on the outside of the tree. But this happens almost only when the tree is short of water at the same time. A tree with enough water cools itself, and the damage does not happen.

So in a hot area, the answer is not to avoid planting. The answer is to make sure your irrigation is good. That is Part 3.

Height, slope and wind

• Height above sea level tells you very little on its own. A high slope can be warmer at night than a low valley. Do not use a number you read for another country.
• Where you sit on the slope matters more. Gentle slopes are excellent, because the cold air keeps moving.
• A south-facing slope is warmer and starts earlier in spring. That is an advantage in a cool area and a small risk in a frosty one.
• Wind is mostly fine. Light wind dries the leaves and is good for the trees. Strong, constant wind makes the trees use more water and makes sprinklers work badly, which is one more reason we recommend drip. If you plan a windbreak, first check that it will not block the cold air from draining away at night.

Part 2 — The soil

How deep is your soil?

Walnut roots go deep when the soil lets them — 3 metres or more in perfect ground. In normal ground they reach about 2 metres. And most of the roots, around three quarters of them, stay in the top 70 to 100 cm.

That last number is the useful one. Your topsoil does most of the work. But the trees still need room below it.

What you are looking for

• How far down can roots go before something stops them? Rock, a hard layer, heavy clay, or wet soil.
• Old roots. If roots from a previous crop all stop at the same depth across the hole, something is stopping them there. Find out what.
• Colour changes. Grey, blue or green soil means the soil is wet all the time. Orange or red spots mean it gets wet and dries again.

Most soils in walnut-growing regions pass this easily. Deep loam soils — a mix of sand, silt and clay — are the best. Heavy clay and pure sand both work, they just need more care with water.

Does the water drain away?

This is the one soil question worth checking properly, because it is the hardest to change later.

Walnut roots need air as well as water. When water sits around them, they cannot breathe. In warm weather, roots start to die after one to three days in soil that is full of water. Wet soil in winter does far less harm, because the soil is cold and the roots are resting.

Only a field that is wet across almost all of its area, all year, with nowhere for the water to go, is better used for something other than walnuts. That is rare.

Good practice whatever your soil: water should soak in within a day or two, and water should never stand against the trunk. If your soil takes water slowly, give smaller amounts more often. Longer watering does not push water deeper — it just leaves it standing on top.

Hard layers in the soil

Many good orchards sit on soil with a hard layer in it. These are usually fixable before planting, and this is normal work, not a warning sign.

• A plough pan is a compacted layer made by machinery, usually 30 to 50 cm down. Very common. Ripping breaks it easily. Just change how you drive afterwards, or it comes back.
• A clay pan is a natural tight clay layer. Ripping alone often fails here, because the clay closes up again behind the tine. A slip plough or a deep mouldboard plough works much better, because it mixes the clay layer into the rest of the soil instead of just cutting through it. In one trial, ripping gave 11% more yield, a slip plough 17%, and a deep mouldboard plough 42%.
• A hard pan is a naturally cemented, rock-like layer. If it is thin, deep ripping can break through it into good soil below, and then the field is fine. If it is thick and continuous, that part of the field is better used for something else.

Normal working depths are 45 to 90 cm for machinery compaction, and 90 to 180 cm for natural hard layers. Deep, even soils on river flats often need nothing at all.

Two things to avoid if you are levelling the field: do the main earth moving one year and the final levelling the next, so the ground settles. And never bury cut branches, tree stumps or turf under the fill. Buried plant material rots without air and produces gases that can kill young trees.

Soil pH and lime

Walnuts are not fussy here. They produce well from about pH 5.5 to 7.5. The best range for taking up nutrients is 6.5 to 7.2.

• Below 5.5, ask the laboratory for a lime recommendation and spread lime before planting. This is straightforward.
• Above 7.5, especially on chalky or limestone soils, iron and zinc become harder for the tree to take up. This is very common around the Mediterranean and it is managed every day, with well-placed fertilizer and leaf sprays.

One honest point: you cannot change the pH of a deep chalky soil. There is no amount of sulphur that will do it economically. So on chalky ground, do not plan to change the soil — plan to work with it. Ask for a calcium carbonate test so you know how much lime the soil contains, and plan your trace element feeding around the result.

Salt in the soil

Walnuts are more sensitive to salt than many crops, so it is worth one laboratory test. In most of Europe and inland Turkey this comes back clean and you never think about it again.

Salt matters mainly in dry regions, near the coast, and where land has been irrigated for many years with hard water.

Ask for the right test. For soil, the number is ECe — this comes from a standard laboratory method called a saturated paste extract. A quick “1:1” soil and water reading is not the same thing and cannot be compared with the values below. And soil salt (ECe) is not the same as the salt in your irrigation water (ECw). Ask for both.

These figures are a guide rather than walnut-specific limits. They come from work on young walnut seedlings and on almond and plum, not on mature grafted walnut trees. Treat the middle column as a reason to ask, not as a measured threshold.

If your numbers are in the middle column

• The effect is gradual, not sudden. Growth slows by roughly 18 to 21% for each 1.0 dS/m above 1.5. So a number a little over the line means slightly slower trees, not a failed orchard.
• Salt washes down. In most cases, winter rain and a little extra irrigation move salt below the roots. This works well on soil that drains, and badly on soil that does not — one more reason drainage is the first thing to sort out.
• Wash it down gently. Several small waterings work far better than one large flood. A big flood runs down the large channels in the soil and leaves the salt behind in the small ones.

If your water is above about ECw 1.0 dS/m, talk to a local adviser before you plant. The amount of extra water needed to keep salt moving rises quickly above that point, and it is worth doing the sums first.

And one useful thing to recognise in the field: salty soil looks like dry soil. The trees droop, but the soil feels moist. Trees on salty ground need watering more often, not less.

Testing your soil, simply

• Divide the field into areas that look different. Different colour, different slope, wet corners, old creek beds, an old farmyard. Test each area separately. Do not mix them into one sample — an average hides the problem.
• Take samples at several depths, not one. At 0–30, 30–60, 60–90 and 90–120 cm. If salt or wet ground is a question, go down to 1.5 m. A good topsoil sample tells you nothing about what is at one metre.
• Ask the laboratory for: soil texture, ECe and pH, calcium, magnesium, sodium, potassium, chloride, boron, calcium carbonate, organic matter, nitrogen, phosphorus and potassium, and the trace elements zinc, iron, manganese and copper.
• Send your irrigation water at the same time, to the same laboratory, and ask for both reports to be read together. Soil salt and water salt only make sense side by side.
• Always ask which method the laboratory used, and put it on the report. Different methods give different numbers for the same soil.

Part 3 — Water

How much water will the trees need?

A grown walnut orchard uses roughly 1,050 to 1,100 mm of water in a year in a warm, dry climate like California or inland Spain. In a cooler, wetter place like western France or the Black Sea coast, much of that comes from rain and you irrigate far less.

The proper way to work out your own figure is:

ETo is a number your national weather service publishes. It says how much water an open field loses to the air each day where you are. Kc is a factor that adjusts it for walnut trees at each point in the season:

These crop coefficients are Californian. No equivalent European or Turkish series was available, so use them to get started and then check them against local advice. If your variety opens its leaves later, move the whole table later by the same number of days.

Two more points that change the total. Young trees use much less — the orchard reaches full water use only when the canopy shades about 55 to 60% of the ground at midday. And grass or a cover crop between the rows uses water too, which can add 30 to 40% to the total. That is worth knowing if water is tight.

The month that matters is July

Here is the most useful number in this chapter.

How to work out what you need

1. Take the water the trees use in your peak month, from the table above.
2. Take away the rain you can count on in that month.
3. Divide by how well your system delivers water: about 85 to 95% for drip, 75 to 85% for sprinklers, 65 to 80% for flood or furrow.
4. Turn depth into volume: 1 mm over 1 hectare = 10 m³ of water.
5. Divide by the hours a day you can actually run the system.

Then check the same figures against a hot, dry year, not just a normal one.

How much water your soil holds

Your soil stores water between irrigations. Deep soil is like a bigger tank.

Multiply by the depth your roots really reach. Clay holds the most, but it holds it tightly and gives it up slowly, so the difference in practice is smaller than the numbers suggest.

Do not let the soil dry right out. Water again when about half the stored water has been used. On drip, an extra watering costs almost nothing, so keep the soil comfortably moist through spring, and save the deeper soil moisture for July, August and harvest.

Check your well or your supply properly

You do not need paperwork for this. You need one honest measurement.

• Measure the flow at the rate you will actually use, not a quick short test. A well that gives a good flow for ten minutes may not hold it for eight hours.
• Watch the water level while the pump runs, and see how quickly it comes back afterwards. That tells you whether the well can keep going day after day in July.
• Ask what happens in a dry summer. Neighbours who have farmed there for years will know.
• Take a water sample for the laboratory while the pump has been running for at least 30 minutes. Fill the bottle right to the top so no air is left inside, or the result changes.
• If you take water from a canal or reservoir, the question is timing, not total. Can you get the water in July, when you need it?

A storage pond is very useful. It lets you collect water steadily and use it quickly when the trees need it most.

Can I grow walnuts without irrigation?

In a place with wet summers, yes, it can work. In a place with dry summers, it is not a good idea for a commercial orchard.

Walnut trees survive without irrigation. But the crop is small and the kernels are often poor. Where walnuts are dry-farmed successfully, annual rainfall is 635 mm or more, and the rain falls through the growing season rather than only in winter.

If you are in a dry-summer region and thinking about planting without irrigation, our honest advice is to plant a smaller area with good irrigation instead. It will earn far more.

Which irrigation system?

We recommend drip irrigation for almost every new walnut orchard. It puts measured water straight where the roots are, wastes very little, lets you feed the trees through the same pipes, works on sloping ground, and is easy to control block by block.

• Dirty water is drip’s weak point. Sand, lime and iron block the small holes. Match your filter to your water, and flush the lines regularly.
• Design for the grown tree, not the young one. A mature tree needs roughly 40 to 60% of the soil around it wetted, and may need up to six drip points on a loamy soil. Many orchards need a second drip line added as the trees fill out. Plan for it from the start.
• If frost protection is part of your plan for a particular block, drip cannot do it. Sprinklers can. Decide that before you buy the system.
• Lay your irrigation blocks out to match the soil. Keep similar soil inside one block, so each block can be watered to suit it. This is easy now and very hard once the pipes are in the ground.

Do not let the trees go short

Walnut trees do not tolerate being kept short of water. It is worth knowing exactly what it costs, because the damage builds up slowly and it is easy to miss.

In a three-year trial, trees were given 100%, 66% and 33% of the water they needed:

• Year one: almost no difference. Even at one third of the water, yield fell only about 10%.
• By year three: yield was down about 35% at two thirds water, and 50% at one third water. Less water means less shoot growth, and less shoot growth means fewer places to carry nuts the following year.
• Recovery took two full seasons of proper watering, even though the trees looked healthy again straight away.

So a supply that covers two thirds of what the trees need costs you about a third of the crop by the third year. That is why it is worth checking your July figure before you plant, rather than after.

Watch the trees, not just the calendar. Put soil moisture sensors at two depths at each measuring point — one at 45 to 60 cm and one at 90 to 120 cm — with at least two points every 16 hectares, and an extra one in any part of the field that behaves differently.

Part 4 — Putting it together

A simple check before you plant

Most fields will sit in the first or second column of this table.

The short version

Walnut trees are more adaptable than most people expect. Growers produce good crops on sandy soil, on clay, on chalk, at sea level and at 1,200 metres.

Everything else — depth, hard layers, pH, nutrients, moderate salt — is either fine already or fixable before you plant, and much cheaper to fix now than later.