Why are my hazelnuts empty? Can I fix it? All about blank filberts!


blank empty hazelnuts filberts

Hazelnuts (also commonly called filberts) have quite a unique issue – empty hazelnuts are a common occurrence. In a given season, some of the hazelnuts will grow normally, but some will be ‘blanks’ instead! This is frustrating for hazel owners – what’s that all about, and can it be prevented? I’m here to explain it all in practical terms.

As a general rule, all hazelnut trees produce some empty or ‘blank’ hazelnuts. Certain cultivars of hazelnut appear to produce more blanks than others. Genetic or environmental factors can interfere with fertilization, or cause the kernel inside the shell to fail to develop properly.

It’s important to know a little about this, because finding quite a few empty hazelnuts can indicate a problem with your tree. Read on!

Having lots of experience growing hazelnuts (you can also call them ‘filberts’) I’ve taken an interest in blank nuts for some time. Luckily because of my background, I can dig into the science aspect too, and there’s been some heavy-duty research that’s been done on the topic – it’s of huge commercial importance to hazelnut farmers after all.

I found references to blank hazelnuts in scientific writings that are over 150 years old!

And oddly, in Shakespeare as well….

Her chariot is an empty hazelnut, Made by the joiner squirrel or old grub,
Time out o’ mind the fairies’ coachmakers.

Mercutio, in his strange speech about Fairy Queen ‘Mab’, from Romeo and Juliet, 1597

What are blank hazelnuts?

Blank hazelnuts are those which look normal on the outside but which are hollow inside, lacking a fully developed kernel (which is the edible part). Some scientific studies have used definitions like ‘nuts with a kernel occupying less than a fifth of the shell’, but there’s no universal definition.

What causes blank hazelnuts?

Nobody has precisely nailed this down in the world of arboriculture, but we do have quite a bit of research that gives us some strong clues.

Empty hazelnuts seem to occur in three different ways:

  1. the hazelnut ovule (the reproductive part) isn’t properly fertilized by pollen
  2. the kernel starts to develop but fails to grow, and remains shriveled
  3. the kernel develops normally, but is consumed by pests

I’ll unpack these.

1. Lack of fertilization

To begin with, unlike many other trees, hazelnuts are self-incompatible. More than one hazelnut tree is needed to successfully make nuts.

They have both male and female flowers on the same tree, but pollen from the male flowers cannot successfully fertilize the female flowers. However, partial fertilization can occur, resulting in nuts that fail to form properly. That’s just one reason.

In large part, it relates to the unusual way in which flowers of the Corylus genus are pollinated. In most plants, when the stamen of a flower is pollinated, the ovule is fertilized a few days later, but in hazelnut trees, it doesn’t happen until late May/early June, which is about 3 months after pollination!

hazelnut corylus male and female flower catkins
I pulled both of these from the same tree. At the top we have the male catkin (flower) and at the bottom, the female! But they can’t successfully fertilize eachother – they need another tree.

During this long period, a lot can go wrong. Studies have found that temperature changes can affect the proportion of blank nuts. So fertilization may not properly complete if, for example, there’s a particularly cold or dry spring, which results in empty nuts.

Some variation in the number of fertilized nuts from season to season is a normal phenomenon in hazelnut trees. This is sometimes known as ‘biennial’ or ‘alternate bearing’ and is a common phenomenon in many fruit and nut trees including walnut, cherry, plum and even in pine cone production. In the same way, natural variation in the number of blanks from year to year has been observed in hazelnuts. Read more about that in my article here.

There appears to be a genetic predisposition for some types of hazelnut trees (and certain pairings of hazelnut varieties or ‘cultivars’) to produce more blanks as well – see the section on cultivars below.

2. Kernels that fail to develop internally

Equally, environmental factors may play their part during the development of a nut growing from a successfully fertilized ovule. According to the published literature, moisture stress, cold temperatures or lack of nutrients may prevent the kernel from reaching full size. Even if it has reached more than 50% of its expected size, the kernel can shrivel and die before the nut is harvested, leaving an empty nut.

Research has also shown that light availability has a role. Plucky researchers have removed leaves on certain branches of hazelnut trees to see if those branches produced more blanks, and they did!

Deficiencies in various nutrients have been given the blame. Researchers have looked at nitrogen, potassium, boron and zinc defiencies as possible causes. Studies that have applied these nutrients to the trees have gained mostly variable results – adding boron seemed to help reduce blanks in one study, but not others.

3. Hazelnut pests and blanks

Certain pests that famously affect the hazelnut tree can burrow into the hazelnut and devour the kernel – especially the infamous filbertworm

How common are blank hazelnuts

It’s normal to have some blank hazelnuts. On a healthy tree, around 10% of nuts will typically be blanks, but studies have found over 50% blanks in certain seasons on some trees.

When do blank hazelnuts fall?

As a general rule, hazelnuts that are empty or ‘blank’ tend to fall a little earlier than ‘sound’ hazelnuts. They’ll drop from the tree in August, while sound nuts tend to ripen and fall in September or October.

Hazelnut cultivar and blank nuts

Certain cultivars (varieties) of hazelnut trees have, for many years, been paired together as a recipe for a successful crop for hazelnuts, having good genetic compatibility as fertilization partners. It follows that certain pairs are also less genetically compatible. And certain cultivars seem to simply have a genetic predisposition to produce more blank hazelnuts.

For example, the cultivar ‘Segorbe’ produces few blank hazelnuts, while another cultivar, ‘Fertile de Coutard’ produces much more – even though they’re both French cultivars of Corylus avellana, the common or European hazelnut.

‘Negret’, ‘Barcelona’, and ‘Tonda di Giffoni’ have been found to have a higher-than-usual tendency to produce blanks.

One study quantified this in detail, finding that the cultivar Daria produced blanks 4.5% of the time, which was compared to 25.7% in ‘Corabel’.

What can I do to prevent blank hazelnuts?

Here’s where it gets practical.

Remember that a majority of the factors involved here are difficult for everyday tree owners to control, such as –

  1. the weather (particularly the temperature) in a given year
  2. the genetic predisposition of certain cultivars, such as ‘Barcelona’, to produce blanks

However, here are some things you can do!

  1. Regular moisture availability is implicated in kernel formation, so keep your hazelnut tree regularly watered! How much watering do I mean? Read more in my article here about watering hazelnut trees.
  2. As mentioned above, we know from scientific research that more blanks form when there are fewer leaves on the tree (to collect the light and convert it into growth-feeding carbohydrates) so grow your hazelnuts in a sunny spot and prune the branches to ensure that light penetrates well.
  3. Consider the addition of a slow-release fertilizer such as Jobe’s fruit and nut fertilizer spikes. There’s been conflicting evidence about how beneficial this can be, but in everyday gardening, soil is so often deficient in basic nutrients.
  4. Read this article!

How to increase hazelnut yield

So what we all want is fewer blanks, and lots of hazelnuts on our trees to harvest! How do we hands-on gardening folk do this? Read this article to find out, in practical terms.

References

Lagerstedt, H.B., 1977. The occurrence of blanks in the filbert Corylus avellana L. and possible causes. Economic Botany31(2), pp.153-159.

Baldwin, B.J., 2015. The growth and productivity of hazelnut cultivars (Corylus avellana L.) in Australia.

Liu, J.F., Cheng, Y.Q., Yan, K. and Liu, Q., 2012. An investigation on mechanisms of blanked nut formation of hazelnut (Corylus heterophylla fisch). African Journal of Biotechnology11(30), pp.7670-7675.

Erdogan, V. and Mehlenbacher, S., 2000. Interspecific hybridization in hazelnut (Corylus). Journal of the American Society for Horticultural Science125(4).

Silva, A.P., Ribeiro, R.M., Santos, A. and Rosa, E., 1996. Blank fruits in hazelnut (Corylus avellana L.) cv.‘Butler’: characterization and influence of climate. Journal of Horticultural Science71(5), pp.709-720.

Solar, A. and Stampar, F., 2011. Characterisation of selected hazelnut cultivars: phenology, growing and yielding capacity, market quality and nutraceutical value. Journal of the Science of Food and Agriculture91(7), pp.1205-1212.

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