Thinning Apple Fruitlets in our Air-Pot Mini-Orchard

Back in 2017 I set up a bit of an apple growing experiment[1] using Air-Pot containers. Air-Pots are widely used by plant nurseries to grow young trees of all sorts until they’re ready for planting out. But would it be feasible, I wondered, to grow containerised apple trees in Air-Pots on a long-term basis? Full of my usual enthusiasm[2], I went ahead with a batch of trees I’d ordered from Grow at Brogdale, and wrote about the setup back in February 2017.

And that’s been pretty much it on the subject ever since. The trees had a fairly decent first year, producing a couple of apples each, but since then… nothing. For the last two seasons they didn’t even put out blossom, which of course meant no fruit, and I was on the verge of deciding to knock the idea on the head and plant them out in open ground instead.

Then, having spent quite a lot of time over the past year or so reading academic research papers on apple tree growth and development, I realised that it probably wasn’t so much the trees or the growing setup that was causing the poor performance; my custodial care regime was a much more likely reason. Watering them every couple of weeks and feeding them when I remembered really wasn’t cutting the mustard. I clearly had to up my game.

Last Autumn, I gave the trees a really good nitrogen feed just as the leaves were turning, to assist with N storage in their root system over winter. This, theoretically, would boost the early development of leaves in spring, which in turn would give the trees enough energy to initiate their blossom buds.

I’m sure it wasn’t this alone – the lack of blossom last year would have helped with the early development of this year’s blossoms buds anyway, and other conditions may just have been spot-on – but the results have been good. All three of our trees – ‘Cornish Armoatic’, ‘Herefordshire Russet’ and ‘Blenheim Orange’ (which I’ve learned since 2017 is a very vigorous cultivar, probably not at all suited for container growing) – were covered in gorgeous blossom in late April and into May. Much of that blossom was successfully pollinated, and many fruitlets were set.

Air-Pot grown ‘Herefordshire Russet’ (left) and ‘Cornish Aromatic’ (right) trees in blossom, 2nd May 2021

Which is fantastic, except that now the trees are carrying too many fruitlets to be able to develop them all into good-sized, nicely ripened apples by Autumn. It’s a simple question of mathematics and the massively over-simplified version goes something like this: each tree has a certain amount of energy (in the form of sugars, which are produced at a rate determined by the tree’s ability to photosynthesise, which in turn is dependent on the amount of the required nutrients and water available to the tree[3]) which it requires for everything from tissue growth (leaves, shoots, roots, bark etc.), to wound healing, to fruit development.

When it comes to fruiting, if the amount of energy the tree can produce has to be divided between a large number of fruitlets, then each fruitlet (depending on its position in the tree canopy, how many leaves are nearby and a host of other factors) will receive a smaller amount of the available energy. Reduce the number of fruitlets and you increase the amount of energy available to each, which will result in larger (more energy for cell division = more fruit mass), riper (more sugar content and ethylene production = enhanced ripening) and therefore tastier fruit at harvest time[4].

Plus, whilst this year’s fruits are developing, the tree is also initiating[5] next year’s potential fruit buds, and this is another significant drain on its energy reserves. Reducing the number of fruitlets this year therefore increases the likelihood of fruit buds developing and blossom opening next year. It also helps to lessen the risk of the tree entering a biennial bearing pattern[6]

In summary: it’s vitally important to properly thin out fruitlets, and it’s a good idea to do so at this time of year, before the tree invests too much redundant energy in developing fruitlets that are already destined for the snip.

Fruitlet clusters before thinning: two fruitlets on the cluster on the left, four on the cluster on the right.

The method is incredibly simple. Examine each cluster of fruitlets in turn and pick the fruitlet that looks like it’s best-formed, or growing on the strongest stem, or is best-placed (i.e. growing nicely out into a space, rather than towards the stem, or in a direction that will cause it to be crowded as it grows). Then, with your sharpest, cleanest secateurs[7] snip off all the other fruitlets in the cluster.

The same fruitlet clusters after thinning: one well-formed, strong-stemmed, well-placed fruitlet per cluster.

Yes, all of them. I know, it’s incredibly tempting to leave two per cluster, just in case something goes wrong and the ones you’ve selected don’t develop properly or just drop off and then you’ve lost the lot, right? But by doubling the fruit load on the tree, you’ll only be decreasing the average amount of energy per developing fruit and therefore increasing the likelihood that fewer fruits will develop well. Far better to be strong, be ruthless, and stick to one per cluster[8]. You might lose a few fruit later in the year, to birds, squirrels, drought (if the weather is overly-dry and you get the watering regime wrong) or even disease or disorder (I’m keeping an eye open for bitter-pit in particular), but those are just the usual breaks in any orchard situation. Again, over-insuring by keeping multiple fruitlets is more likely to result in smaller, inferior quality fruits at harvest time.

Of course, I’m talking about hand-thinning small, easily manageable potted trees. It’s not really practical to hand-thin large, open-ground trees in an orchard situation, although I’d suggest that, if you have the time, thinning the fruit clusters you can reach is still a good idea, if only to increase the chance that the fruit on the accessible branches is of better quality when you come to pick them. Fruits growing out of safe reach are likely to come down as windfalls and sustain damage on the way anyhow, so they’re more likely to be juiced or cooked than eaten fresh.

How about you? Do you practice hand-thinning or, if you’re an orchardist on a larger scale, either chemical or hormonal thinning? Have you been able to control biennial bearing as a result? Have you noticed any real difference in fruit size or quality on thinned versus un-thinned branches (that’s another mini-trial waiting to happen). Do let me know, via the comments (which you’ll find after the footnotes).


1 I use the word ‘experiment’ in the full realisation that it wasn’t, and isn’t, in any way scientific…
2 …but lacking some of the knowledge I’ve accrued since…
3 In the case of potted trees, the situation is further complicated by the lack of a mutually beneficial mycorrhizal network between trees. In an open-ground situation there’s a very good chance that mycorrhizal fungi will have developed hyphal networks between multiple trees and will be ‘managing’ the tree’s energy availability by transferring essential nutrients between them to maximise the availability of photosynthates (see Merlin Sheldrake’s superb book Entangled Lifereviewed here – for much more on the astounding impact of mycorrhizal networks). But potted trees stand in isolation, with the mycorrhizae in their pots limited to accessing the nutrients that are either within, or added to, the growing medium by the orchardist. Which of course means that an effective care regime is even more essential.
4 This is obviously much more important for dessert or culinary apples than for cider apples. It’s unlikely you’ll thin out cider apples at all, as they’ll all be swept up, scratted and juiced to make the golden liquor, so their individual size and appearance isn’t really a huge concern.
5 This might not be the correct technical term for the very earliest stages of fruit bud development. It’s a while since I read up on this particular aspect of top-fruit morphology so I’m a bit hazy, but I intend to refresh my memory and write a few posts on the subject later in the year.
6 Biennial bearing is where the tree has ‘on’ and ‘off’ years, producing large amounts of (most likely smaller and inferior quality) fruit in the ‘on’ years then resting and producing nothing at all in the ‘off’ years. Fine for the tree from a seed-dispersal point of view, but not so good for the orchardist looking for a reliable annual crop.
7 Niwaki Otaksune secateurs are my tool of choice, but feel free to pick your preference.
8 Okay, unless there’s only one cluster of fruitlets on a good-sized stem or branch, in which case you can probably get away with leaving two. But even then, just one is probably a better bet.

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