Peter Entwistle

My in-ground avocado tree has been growing very rapidly over the last month. It's been putting out some very fast vertical growth recently. It will have been in the ground for 1 year next month. I'm not sure how much more it will grow this year, but I'm delighted with its progress so far. I recorded a video of it the other day: https://youtu.be/3cPzlipbdEw

Mike Guye wrote:I've just edited my previous post (of 6 months ago) regarding winter 2024/25, for the avocado orchard here in St Leonards-on-Sea - scroll back one post to view my 5 July edit, which was in response to Peter.

Recently, I've had a growing suspicion as to why there only appears to be documented evidence of avocado trees fruiting within the London area, in the U.K. I don't think it's merely due to warmer winters in London, compared to elsewhere in most of the U.K., but also the result of significantly hotter summers, given the mediterranean-like climate of inner London.  This thought is also based on the observation that the non-grafted 'Hass' tree, in the orchard here in St Leonards-on-Sea, has only flowered once and this was in spring 2023 at the 6-year-old stage. That flowering was preceeded by a summer of unusually hot weather in 2022, and a summer of extreme heat generally throughout Europe. At that time, temperatures were frequently above 30°C in St Leonards-on-Sea (unusual for our coastal location); since then, our summers have been significantly cooler, again a possible explanation as to why it hasn't flowered since.

SE1 is the postcode location for the famous London avocado tree that has appeared to fruit annually, since it was around 17-years-old. Below is a screenshot of  the max/min daily temperatures, forecast for the period 9-20 July 2025, comparing SE1 with St Leonards-on-Sea (taken from the BBC website). It gives you some idea of the temperature differences between the two locations, during spells of warm summer weather. Sometimes, inner London can be as much as 9-10°C warmer than down here on the southeast coast. The hour-by-hour data, shown below in the screenshot, represents the weather for the 7 July (today).

Frustratingly, there appears to be no peer-reviewed research specifically stating the heat-unit requirement (number of 'growing degree days' or 'GDD') that a mature avocado tree needs to induce flowering. A figure of 500-1000 GDD is sometimes quoted, but this doesn't seem to be based on any specific evidence for avocado, as far as I can see.

Thanks for the updates about the minimum winter temperatures you received last winter, Mike. It got a lot cooler in my location, at least on a couple of nights; the rest of winter was pretty mild, however.

Interesting idea about the warmer weather being the reason for the flowering on your tree after the summer of 2022, but none since. I'm not sure what it has been like in your area, but so far this year has proved to be much warmer than average here, especially over spring. It will be interesting to see if your tree decides to flower next year. But like you said, there doesn't seem to be much (if any) research about GDD required for flowering on avocados. I suspect your tree may simply not be mature enough yet. Perhaps it flowered early that year due to stress? I think I remember your tree completely defoliated that winter? Someone else who has been documenting growing avocados on YouTube on the southeast coast (Folkestone, I believe) had some flowers on his seedling tree this year (I think he said it's around 12 years old from seed). Interestingly, though, it has only flowered on one section of the tree that he attempted to air layer (which failed). I suspect the air layer attempt has effectively girdled that section of the tree and forced it into flower. He has it growing in the ground, but I think he constructed a greenhouse around it about a year ago. He attempted some hand pollination and has some fruits developing. This is his latest update: https://www.youtube.com/watch?v=tTGZcWJf4lo. Perhaps it may be worth experimenting with girdling a few branches on some of your larger trees?

I now have 3 reasonably sized grafted avocado trees and around 6 smaller ones of various varieties that I have grafted myself, including the London (Julienne) avocado and Joey. I also have the seedling (unknown variety), which I planted in the ground last year; it's growing very fast this year. It's developing some very thick vertical shoots at the moment. I'll post some photos of some of the trees soon. My largest grafted tree, which was labelled 'Fuerte' (although I suspect it may be mislabelled), didn't flower this year, despite having it in the greenhouse most of last year and overwintering it there too. However, it barely grew at all last year due to being kept in the original pot, which was far too small and kept drying out. It has grown pretty well so far this year after repotting it, however. I also have it outside now since I was seeing leaf scorch on the newest leaves when it was in the greenhouse. It was getting far too hot in there, and I don't have roof vents in it (it's just a cheap plastic one). I'm not sure what I will do with them all since they probably won't do well in containers long-term. I don't have enough space to plant them all in the ground and provide winter protection for all of them. Equally, they will soon end up too large to fit inside my greenhouse over winter.

Mike Guye wrote:28 Dec 2024: 4 new pics added for the 7½-year-old 'Hass' tree:  https://imgur.com/a/5gflnlU

Thanks for the update, Mike. It seems to be progressing really well. I agree, it does look like there might potentially be some flower buds forming on it again. Hopefully, the temperatures in your area stay mild enough not to cause any damage to them 🤞.
We had a low of around -3°C here last night, although in the area next to the wall where my avocado is planted I recorded a low of -1.6°C (I have some temperature sensors set up now). I had the avocado and my clementine trees covered separately in frost cloth plant jackets with the incandescent lights turned on, so it stayed above freezing. Looks like we will be getting several nighttime lows of between 0°C and -3°C over the next week.

Mike Guye wrote:

Winn Sawyer wrote: I'm guessing the lack of flowers was at least partly related to the amount of freeze damage, but it's interesting the least-damaged inner branches didn't put out a few flowers

My interpretation is slightly different - I think the flowering in 'Hass', observed in spring 2023, was due to environmental stress. Winter 2022/23 was more severe than last winter (e.g. 26 vs 12 frost days), and was reflected by the the more severe visible symptoms of winter-injury (95% defoliation) at the time.  I think this severe injury triggered the plant to flower, though it produced only three small inflorescences.  

Going into 'reproductive mode' is a common response of plants exposed to unusually extreme  environmental conditions for the plant species concerned, whether that's caused by  temperature, drought, a severe attack by insect pests, etc.  Though 'Hass' did suffer significant injury last winter, it was probably not enough to trigger flowering in spring 2024 - I may have to wait a few more years until the tree is naturally mature enough for flowering to occur on a regular basis ...

I have been told by a friend who knows the owner of the large London Avocado tree (the one in Southwark) that it only started flowering after it was cut back heavily around 8 years ago or so. The tree is around 25-30 years old I believe so for at least 17 years it didn't flower at all. But since that severe pruning, it has been flowering yearly (or at least most years). My friend Joe discusses this briefly in this video: https://youtu.be/SwRHGRIVw9Q?t=90

Mike Guye wrote:
Around 6 months ago (scroll back), you were considering using a Raspberry Pi or Arduino set-up for monitoring temperatures. Did you try that, following Winn's guidance for the Pi? I took the easier option, buying a datalogger and thermocouples  from  https://www.omega.co.uk, but you might get cheaper if you shop elsewhere as they seem expensive.

Yeah, I was (and still am) considering something along those lines. Although, I might check out a few different options, including an off-the-shelf solution. I might be fine with the software side of that, but I'm not very good with electronics. But it would definitely be a great option for my greenhouse as I also like to keep track of temperatures in there as I store a lot of my citrus trees in it which need to be kept frost-free.

Mike Guye wrote:This year, I've been a bit delayed in getting my UK outdoor avocado-growing blogs up-to-date. However, Hass & Fuerte have now been updated, up until August 2024 (see links below).  There may be one more update in November, focusing on growth-rates towards the end of the growing season.

Hass (7-year-old tree)...    https://imgur.com/a/5gflnlU

Fuerte (5-year-old tree)... https://imgur.com/a/0XuODou

Blogs for Bacon, Del Rio, Daughter, Joey and Wilma are in the process of being updated and will post here when done.

Wow, the Hass seedling has really thickened up nicely now, Mike! The Fuerte is also coming along really well too. I can't wait to see how all the others are doing.
That reminds me I need to get something to log the temperatures in my garden over winter, which I can place near my avocado (unknown Mexican type) and Clementine trees. Is there anything you would recommend, Mike? I guess I'd need to cover any thermometer with a Stevenson screen to have accurate daytime temperature readings.

I have uploaded a video as I mentioned yesterday. We'll see how it goes over the winter, but hopefully, with some protection during the cold spells, it will make it. Of course, I will continue to give updates on how it gets on.

Brett Lyons wrote:I may well decide to construct a shelter around the seedling I got from Mike Guye as I'm not sure I can fit it into my summerhouse lol

Yeah, it's definitely worth trying something like that, Brett. I'm sure it will do well as it's got to a pretty decent size 👍

I planted this avocado tree in the ground in my garden earlier today. It was purchased as a 'grafted' avocado tree, but it turned out not to be grafted, unfortunately. Based on the strong scent of liquorice I get when I crush the leaves, I would guess it's a seedling of a Mexican avocado type. Of course, there's no way to know for sure which avocado it’s from. I planted it in a mound in the border in front of a brick wall in a southwest-facing position. Half of the root ball is above grade, to hopefully help prevent root rot. The soil in my garden does seem to drain pretty well, so hopefully, it will be fine. I plan to protect it in a similar way to my clementine tree over winter, which I showed in a previous post. That method worked out really well last winter, so I don't see why it wouldn't work for an avocado. I may graft some named varieties onto it in the future, as it will likely take a very long time to produce since it's just a seedling. I will post a video later in the week to show it in more detail.

I also have another avocado tree from the same source, as well as a grafted 'Fuerte' avocado that I have growing in my greenhouse, although the one I have outside has done better. I think it's been too dry, and I haven't been watering the pots enough inside the greenhouse. I'm also growing a number of seedlings from both Hass and Bacon avocados, which will be overwintered indoors. I'll do a separate post and update video on my YouTube channel about the London avocado grafts I did (now known as the 'Julienne' variety).

Thanks for the detailed tutorial, Winn. This will be very useful for me when I look into setting up some sensors in my garden!

Winn Sawyer wrote:
I have a DIY setup for that, using Raspberry Pi Zero W computers with thermometer chips plugged into their GPIO headers. It would require a wifi signal and some type of waterproof case, but I'd be happy to share the hardware design that I use, and the (pretty simple) python code for sending the temperature to a website, as well as the (pretty simple) PHP code on the website that receives the measurements loads them into an SQL database.

In fact, the most complicated part is the javascript code for displaying the data, which uses the Chart.js library, but that should be pretty easy to copy and tweak.

You can see the end result here with a few options for the time series, though only registered members can create custom time series:

https://www.drymifolia.org/data.php

It might be a little off-topic to discuss that here in much depth, but if there's enough interest I could start a new thread in an appropriate section on here.

That would be very interesting to see, I was considering using a Raspberry Pi, or Arduino. I am actually a software engineer, so I do like tinkering with stuff like that, although I'm not so good at the hardware side, so I would be interested to see more detail on that.
Please let me know if you start a new thread.