As much as I love to stand and admire this beautiful deciduous tree when I see it providing shade in parks and large gardens, the best thing about walnut trees for me has to be the walnuts themselves. Walnuts are the third most consumed nut in the world (after peanuts and almonds, if you’re curious) and they’re definitely my favorite. Yet, you may notice that your walnut tree isn’t producing the delicious walnuts you’ve been expecting. What gives?
On average, walnuts are not produced until a tree is 7 to 8 years old. Subsequently, they may not produce nuts every year as they are biennial bearing, generating mostly flowers one year and nuts the next. Other factors, such as pollination, shade, drought and disease will also affect yield.
There are plenty of nuances though! It really is rewarding when you make a small change that results in a big improvement in your trees. It’s incredibly cool to be able to collect a huge crop of walnuts time after time. Read on to find out more.
By the way though – technically, the fruit of the walnut tree isn’t a nut! They are drupes – a seed, encased within a shell, which is itself has a fleshy soft surrounding, kind of like a peach. But to keep things simple I’m going to refer to them as nuts in this article.
There are two main types of walnut tree that grow edible nuts, and together they’re the most commonly planted worldwide. The black walnut tree, Juglans nigra is the one most often seen in the United States, though it’s grown as often for its dark, durable timber as for its nuts. English or Persian walnut, Juglans regia, is common in Europe and the Far East, but is grown commercially in California. Where there are differences between the two, I’ll outline them below.
How old are walnut trees when they start producing walnuts?
Walnut trees sold from nurseries are often grafted, meaning that an upper part of one walnut tree was attached on to the roots of another in order to combine some of the qualities of both. The grafted varieties produce nuts a bit earlier than non-grafted varieties, such as those grown entirely from seed. Grafted walnut trees may produce nuts after 5 to 6 years, compared with 7 to 8 years (or often longer) for non-grafted trees.
The heaviest walnut production starts when the tree is about 30 years old.
Do walnut trees produce nuts every year?
Walnut trees are biennial bearing, much like mangos, apricots and avocado. This means that if they have used their available energy and nutrients to make nuts one year, they focus on flower production the next year. There won’t necessarily be no walnuts harvested during the blossoming years, but there will be fewer. If your tree isn’t bearing many nuts, take note of whether it has been producing abundant flowers. That may be a clue that next year will be much better for walnut yield.
Are walnuts self-fertile?
Walnuts trees are partially self-fertile, meaning they don’t need pollen from another tree to produce their fruit. Wind carries pollen from the male to the female parts of the same tree. However, higher walnut yields can be expected if there are other walnut trees planted nearby.
Commercial walnut growers will take advantage of this by strategically planting rows of walnuts where the prevailing wind direction will ensure maximum pollination.
You can tell apart the male and female flowers that will both be present on a single walnut tree – the male flowers are long, slender catkins and the female ones, and the females are tiny, more traditionally ‘flower-shaped’ blooms on the tips of the branches.
Could temperature changes prevent walnut production?
Walnuts, in keeping with other fruit trees, need both cold and warm weather in the course of a year to produce fruit. When the temperature drops below 45°F (7°C), this signals a walnut tree to stop growing and enter dormancy for the winter. If the weather is unseasonably warm, the tree’s yearly cycle is thrown out of sync, and it may not produce any flowers (and subsequently, no nuts) the following summer and fall. Chandler et al estimated in their study of walnut trees (in 1937!) that most need between 400 and 1500 hours below this temperature to enter dormancy and go on to produce flowers, followed by walnuts.
Here is a link to a map provided by the US Dept. of Agriculture, where you can check how many chill hours you’ll usually receive in regions throughout the United States.
On the other hand, walnuts need about 200 frost-free days during the year for their growing season. In general, this means most walnut trees will grow well between the warmest and the coolest regions – in the US that means from ‘USDA hardiness zones’ 4 to 9, corresponding to regions with average daily minimum temperatures from -30 to -25 °F/-34.4 to -31.7 °C, up to 25 to 30 °F/-3.9 to -1.1 °C. You can check your hardiness zone on the USDA’s map here.
Sun, soil, and water requirements for walnut production
The growth and overall health of your walnut tree is like a barometer for walnut production. If your walnut tree is growing poorly, walnut production will be poor as well.
Walnut trees thrive in full sun, so a walnut tree growing in a shady spot is unlikely to do well or produce as many walnuts. So it’s best to plant them about 50 trees away from trees and other structures. There’s another reason to do this though – tree parts of walnut trees (black walnut in particular) are famous for secreting juglone, a chemical that is toxic to other plant species, to reduce growing competition in its vicinity.
Unfortunately, they develop a deep ‘taproot’ so are difficult to transplant to another site unless they’re very young. They therefore prefer deep soil, ideally ‘loam’ (well-balanced soil that has components of sand, silt, and clay), and usually do best in slightly acidic soil, i.e. pH levels of 5.5 to 6.5.
Do you know your own soil pH? You can get a pH tester cheaply and it comes in handy all the time. The one I use is Rapitest’s meter (check price on Amazon now) – it’s the only one I’d really trust for accuracy since it uses the proper ‘logarithmic’ scale. Here’s a pic of mine:
They also do not like drought! In reality, most young walnut trees do need some irrigation if you want to see walnuts, especially in the first couple of years. And since they have deep roots, that means a long soak every week or two, especially when the weather’s warm. Avoid blasting the base of the tree with a hose – sprinkle under and out to the edges of the area shaded by the canopy, where the roots have developed to collect water that drips from the leaves above.
Be sure to always apply a high-quality organic mulch like this around the trunk (Amazon affiliate link). This will help moisture permeate into the soil and slowly decompose, acting as a kind of slow-release fertilizer. Avoid piling it up around the trunk – leave a 2-inch gap so it isn’t actually touching. UK readers – you can get an ideal equivalent product that’s well priced on Amazon here.
How do animals and birds affect walnut yield?
If you have a walnut tree, there’s a good chance you’ll see squirrels in it. Walnuts are an exceptional source of protein for squirrels, so be aware that it might be these critters who are responsible for reducing your yield. While walnut shells are incredibly hard (especially the black walnut), chewing through them will pose no difficulty to their strong jaws.
Some gardeners attempt to deter squirrels by providing a ‘buffet’ of other nuts, such as peanuts or acorns, in easier reach, in the hopes that the creatures will take the path of least resistance and ignore the walnuts.
Birds have more trouble getting into walnuts, but they have developed some exceptional methods. But a 1997 study by Cristol and Switzer, published in Behavioural Ecology described how American crows have learned to crack open walnuts by dropping them from great heights…
OK, I know you’re desperate to know: the crows knew that black walnuts were harder to crack, so tended to drop them from higher altitudes than the English walnuts, and they aimed for asphalt, rather than soft ground. Clever crows!
Which walnut cultivars are best for producing walnuts?
‘Broadview’ is a compact black walnut tree that is one of the best for producing a reliable yield in favorable conditions. ‘Chandler’ is a black walnut tree with a thinner shell – since the black walnut is especially good to eat, but also especially hard to crack, this is a popular variety.
‘Lake’ is a popular cultivar of the English walnut with reliable nut production.
Pest and diseases affecting walnut production
The two best-known diseases that can affect walnut production are walnut blight and Thousand Cankers Disease (TCD).
Walnut blight is caused by Xanthomonas juglandis and causes black spots to appear on green areas of the tree, particularly the leaves and drooping catkins, including the fruit. It can invade inwards and even affect the quality inner walnut itself. Walnut farmers use chemical preparations to control the disease. It’s important to prune off any parts of the tree that exhibit these signs of infection as soon as they’re identified.
TCD causes yellowing and loss of the foliage at the crown of the tree and dieback of small branches. The characteristic boreholes in the tree trunk are caused by walnut twig beetles which carry the fungus Geosmithia morbida and infect the tree. This disease is fatal to the tree once it takes hold.
In 2010 there was a sizeable TCD outbreak in Texas, causing the loss of many black walnut trees, and by 2013 it was found on mainland Europe infecting both black and English walnuts.
Have a look at your walnut tree for signs of disease. Do the leaves look healthy, or are they yellowing, small, or curled up? Are there boreholes in the trunk, or black spots on the leaves or flowers? If you think your tree may be infected by a pathogen, I wouldn’t hang around – consult a trained arborist as soon as you can. The sooner, the better. They’ll be able to tell you if the tree can be saved, and can get to work removing disease tree parts and initiating a tailored treatment plan.
Will fertilizer help?
It very likely will – if you haven’t been applying fertilizer you should start. I recommend fertilizer spikes – they’re slow-release (impossible to get it wrong or overdo it) and can be used for almost any size of tree. Jobe’s are great (particularly the ones for nut trees, designed specifically to stimulate a good crop), as are Miracle-Gro’s. Check the current price on Amazon (affiliate link).
How can I increase walnut yield?
- Choose a site for your tree in open sun, in well-drained, deep soil.
- Deeply water your walnut every one to two weeks in warmer seasons
- Apply a slow-release fertilizer
- Deter or distract squirrels and crows by providing alternative food
- Plant at least one other walnut tree on your land – ideally consider the wind direction, so that pollen can be carried from one tree to the other.
- Consult an arborist if your tree has yellowing, curling leaves, bare branches, black spots, or pits in the bark.
Do walnut trees ever stop producing nuts?
Walnut trees commonly live for 130-150 years, though many live much longer – up to 400 years. While they don’t produce their best crop for the first three decades, after that they can be expected to keep producing walnuts for their natural lifespan.
Have you ever been curious – can trees die old age, or go on living forever? It’s surprisingly difficult to find the answer to this, but I’ve broken it down and answered it in this post!
If you’ve found this post helpful, please have a look at my other articles (link to home page). I put my absolute all into giving you guys helpful and practical advice that you can rely on. You might be interested in the other posts in the fruit and nut tree category. Have a look!
Zeynel Cebeci, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
hedera.baltica from Wrocław, Poland, CC BY-SA 2.0 https://creativecommons.org/licenses/by-sa/2.0, via Wikimedia Commons
Jeffrey Beall, CC BY-SA 2.0 https://creativecommons.org/licenses/by-sa/2.0, via Wikimedia Commons