Avocados and Frost in Europe (plus info on the cold-hardiest avocados and how to grow them)

Peter Entwistle wrote: Wow, the Hass seedling has really thickened up nicely now, Mike!

Yes, around a 2 cm increase in mean trunk diameter at ground-level, compared to last year - it's very vigorous. Next spring, this tree will need to be pruned hard to control its size & shape, but that wont prevent its continued increase in trunk girth.

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.

Those are looking great! 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.

This is reminding me that I should post an update on the Cascadian avocado thread to show the amount of regrowth I've gotten on my various outdoor trees.... maybe tomorrow! Need to take a bunch of photos.

Mike Guye wrote:

Peter Entwistle wrote: Wow, the Hass seedling has really thickened up nicely now, Mike!

Yes, around a 2 cm increase in mean trunk diameter at ground-level, compared to last year - it's very vigorous. Next spring, this tree will need to be pruned hard to control its size & shape, but that wont prevent its continued increase in trunk girth.

I like to do most of my pruning "for shape/size" right after they finish flowering and the first major fruit abscission in early summer, rather than risk removing branches earlier in spring that might have set fruit. Often only some of the branches set flowers, so it would be a shame to accidentally remove all of those.

Blog now updated for two 'Bacon' trees, now 2 years 10 months old:
https://imgur.com/a/FmRvs7d
   

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.

Winn Sawyer wrote: I like to do most of my pruning "for shape/size" right after they finish flowering and the first major fruit abscission in early summer, rather than risk removing branches earlier in spring that might have set fruit. Often only some of the branches set flowers, so it would be a shame to accidentally remove all of those.

Yes, that's wise Winn. I do much the same, waiting for the buds to break before doing any pruning just to check if there are any flower buds there. My situation is quite different to yours though, as I have trees that haven't been grafted  - timescales from germination to flowering are going to be significantly longer than for your grafted collection. Any flower buds on mine would be a real bonus at this early stage...

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 ...

The remaining UK avocado blogs have now been updated to August 2024:
Del Rio (2-years-old) ..................................... https://imgur.com/a/dAbixIe
Daughter, Joey, Wilma (10½-months-old) ... https://imgur.com/a/FuxWI29

NB. Wilma is also known by the name Brazos Belle.

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 ...

That does make some sense, but the problem with that theory is that the almost invisible dormant buds "decide" whether they are going to be flowers when the flush that created those buds is finishing the previous growth cycle in fall. No new flower buds can form on new growth in spring, they only come from the dormant buds that overwintered. So usually you cannot get flowers at all if the existing dormant buds on growing tips are all damaged, only vegetative buds will form adventitiously on older wood.

Your tree is larger than many seedlings I've seen flowering. One of my own seed-grown trees that's got a trunk half that diameter flowered for the first time this year (no fruit set). Once an avocado tree is "mature" (has flowered once) it typically remains that way and does not revert to a pubescent stage, though many do fall into biennial bearing patterns.

Winn Sawyer wrote: That does make some sense, but the problem with that theory is that the almost invisible dormant buds "decide" whether they are going to be flowers when the flush that created those buds is finishing the previous growth cycle in fall. No new flower buds can form on new growth in spring, they only come from the dormant buds that overwintered. So usually you cannot get flowers at all if the existing dormant buds on growing tips are all damaged, only vegetative buds will form adventitiously on older wood.

Thanks Winn. You’ve made a very valid point which made me look up when flower primordia are actually initiated which, as you state, is normally towards the end of the previous growth season.  As you rightly say, these buds will already be ‘committed’  to flower the following spring, irrespective of what happens during the winter period, unless of course the flower buds are actually killed by freezing temperatures.

In spring 2023, the three buds that gave rise to small inflorescences on the 'Hass' were apical buds (not ‘dormant buds’ in the botanical sense of the word).  The previous growth season during 2022, that gave rise to these flower buds, was very favourable for avocado growth generally. It was an exceptionally warm summer with three heatwaves, temperatures topping 34°C on one ocassion. Maybe such warm weather favoured the development of flower primordia. Perhaps, I would also have seen flowering in spring 2024 had the growth season temperatures in 2023 been as favourable as those in 2022.  Of course one can only speculate - it’s impossible to know with any degree of certainty.

I suspect that the prolific annual flowering/fruiting of the famous 30-year-old Southbank avocado tree (London, UK) is not only the result of the milder winters in a large built-up and sheltered urban environment, but also due to more favourable higher summer temperatures, compared to my cooler coastal location here in St Leonards-on-Sea.

I just want to check language-use here. Winn, when you use the term  ‘dormant buds’  do you mean the botanical sense of the word, i.e. inconspicuous buds that normally only open following a very stressful event, e.g. complete defoliation, or do you mean normal buds that are ‘overwintering’ ?

Winn Sawyer wrote: Your tree is larger than many seedlings I've seen flowering. One of my own seed-grown trees that's got a trunk half that diameter flowered for the first time this year (no fruit set). Once an avocado tree is "mature" (has flowered once) it typically remains that way and does not revert to a pubescent stage, though many do fall into biennial bearing patterns.

Do you think a reason that one of your seed-grown trees [i.e. a tree that hadn't been grafted] flowered at a smaller size than the 'Hass' was because it was glasshouse-grown, i.e. it had more favourable (warmer) growth conditions, including an extension of the growing season?  I'm sure genetic differences play a significant role too. I was surprised to see 'Hass' flowering at such a young age (6 years), when seed-grown trees  are normally expected to flower anytime between 4- and 20-years-old, and of course sometimes never flowering at all.

Mike Guye wrote:
I just want to check language-use here. Winn, when you use the term  ‘dormant buds’  do you mean the botanical sense of the word, i.e. inconspicuous buds that normally only open following a very stressful event, e.g. complete defoliation, or do you mean normal buds that are ‘overwintering’ ?

This is a good question! I'm referring to all the buds on the tree that are not actively growing. As you correctly pointed out, the flowers generally only form on the terminal buds of the previous flush on each branch. However, I have noticed that sometimes a long-dormant bud on older wood under the canopy will grow just one or two leaves in summer, and that "terminal" bud will often flower the next year even though there was no clear stem extending from the branch.

If you have not seen it previously (I may have sent it to you?), I highly recommend this publication from the agriculture ministry of Western Australia. Here are some excerpts:

The first aspect is to consider what triggers flower initiation. The general consensus is that a period of low temperature (below 20°C) and short day length (less than 10 hours) is required to initiate the transition from vegetative bud to floral bud (Buttrose and Alexander 1978, Nevin and Lovatt 1990, Salazar-Garcia et al. 2006).

The term ‘irreversible commitment to flowering’ is used to describe the time when the apical bud becomes committed to reproductive growth. Generally, this is achieved after the accumulation of about 28 days of conditions suitable for flower initiation (Salazar-Garcia and Lovatt 2002, Salazar-Garcia et al. 2006).

The statement of ‘irreversible commitment to flowering’ can be a little misleading. It implies that once achieved, the bud will continue to develop as a floral bud regardless of conditions. However, certain events can arrest further development of the floral bud. For example, a moderate frost event on 17 June 2006 resulted in what appeared to be significant damage to buds (Figure 5), even on shoots with only minor leaf burn.

Based on the requirement of 28 days below 20°C and the temperature conditions normally experienced in the South-West, it could be anticipated that irreversible commitment to flowering had occurred prior to the frost. However, after the frost event the majority of buds that would have been expected to flower actually developed into vegetative growth in the following spring, with generally only a few weak late flowers. Therefore it would seem that the period of extreme cold temperature had either damaged the developing flowers or almost totally inhibited their further development while promoting vegetative growth.

Avocado flowers are borne on new season growth, that is, shoots produced during the previous season’s vegetative flush. Therefore, growth of shoots is required to produce buds that can develop into flowers. In the South-West of Western Australia, three vegetative flushes are normally observed – a spring flush, summer flush and autumn flush, similar to New Zealand (Dixon et al. 2008).

Flowers can develop on any of the flushes, but the spring flush reportedly provides the greatest contribution in Mexico (Salazar-Garcia et al. 2006) and New Zealand (Cutting 2003). These were both in minimally irrigated orchards that resulted in a strong spring flush — that is, a greater number of shoots produced per branch, compared to later flushes.

Salazar-Garcia et al. (1998, 2006) observed that under Californian and Mexican conditions, crop load did not have a significant impact on the number of shoots produced.  The percentage of floral to vegetative shoots produced from these shoots the following flowering period was affected in California but not in Mexico. In California, the ratio of inflorescences to vegetative shoots was significantly higher after a light crop as compared to a heavy crop. What was not reported was the length of the shoots produced or the total number of flowers produced as a result of differing crop load. However, Salazar-Garcia et al. (1998) reported that in California the return flowering after a heavy crop was less intensive than after a light crop.

Dixon et al. (2008b) estimated that a shoot producing at least six panicles gave the best initial fruit set. This was estimated to be a shoot of about 150 to 200mm long. Dixon also noted that in a heavy flowering year, there was a higher percentage of initial fruit set per inflorescence than in a light flowering year. Unfortunately, what was not reported was the total number of shoots produced in each year, to determine the impact of crop load on the total number of flowers, rather than just the impact on the individual shoots.

I should add that the precise thresholds they cite were generally for Hass specifically, so other cultivars (including seedlings of Hass) likely have different specific temperature and daylength thresholds, but they probably all follow a similar basic pattern.

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

Peter Entwistle wrote:
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).

It underlines that, for non-grafted trees, the wait until regular flowering occurs may be a long one!
I suspect the two events, i.e. hard pruning & subsequent flowering, were not causally related, merely coincidental.  

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

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.

Peter Entwistle wrote: Hopefully, the temperatures in your area stay mild enough not to cause any damage to them.

Well, we appear to have had a very hard frost in the early hours of Friday morning (3rd Jan 2025). Not unheard of here by the coast. Guessing around -5C.  The seafront was forecast for around -2C, but we're on a steep north-facing slope which often makes the microclimate cooler than the official local weather forecast in winter. The 'Hass' and 'Bacon' leaves haven't fallen limp yet (a good sign), but then visible symptoms of frost-injury often only show a few days later. The trees always recover though, based on previous years, even if they lose practically all their leaves.

I wont know how severe any of the frosts we have this winter will be, with any degree of accuracy, until I analyze the field-datalogger later on (1 Jun). My max/min Hg-in-glass thermometer showed -7C, but I don't believe it (not very accurate) - I should've calibrated it against melting ice, before using it, to determine the 'systematic error' ...