Mike Guye

Douglas Campbell wrote:Old article on cross species grafting of Persea.
http://avocadosource.com/CAS_Yearbooks/CAS_42_1958/CAS_1958_PG_102-105.pdf

This early work, which is fascinating in itself,  has been superceeded by the widespread use of Mexican (var. drymifolia) rootstocks (especially in Mexico and parts of California), due to their vigour & adaptibility, disease resistance, compatibility and seedling abundance (for seedling rootstock).  

The exceptions are:
(1) the humid tropics (Central America, Caribbean):  where West-Indian (var. americana) type avocado rootstocks are used as they are more tolerant to moisture & heat.
(2) commercial orchards: where clonal rootstocks (like Dusa, Toro Canyon, Zentmyer) are often used, selected from Mexican or Guatemalan (var. guatemalensis) genetic backgrounds for disease resistance and uniformity.

Greg Alder's article on rootstocks is of interest here: https://gregalder.com/yardposts/avocado-rootstocks-what-do-they-matter/

That's a really nice example, Peter, of a healthy young avocado tree that you have growing against a wall.
Avocado are meant for the open ground, as Peter's photos & video show, where roots can wander at will, not being permanently imprisoned in pots.

My avocado blogs have just been updated; the next time will be at the end of the growth season in late autumn (~ Nov 2025), where I'll post measurements (height, trunk diameter) and any other interesting observations. The links for the avocado orchard are as follows:

Hass (Guatemalan x Mexican hybrid): https://imgur.com/a/5gflnlU
Bacon (Guatemalan x Mexican hybrid): https://imgur.com/a/FmRvs7d
Fuerte (Mexican): https://imgur.com/a/0XuODou
Del Rio (Mexican): https://imgur.com/a/dAbixIe
Daughter (descendent of Gainesville), Joey (a.k.a. Brazos Belle) & Wilma (all 3 varieties are Mexican): https://imgur.com/a/FuxWI29

Edit (26 Jul 2025): apologies for any confusion that arose for the Winter 2024/24 table of data, in the links I gave above. The years in the 'Summary' are now corrected, and any links from it to photos are also corrected.

Priyanshu Uniyal wrote:

Mike Guye wrote:..
St Leonards-on-Sea....917
Inner London..............1365
Axarquia.....................2800-3200
...
With a much lower GDD, here in St Leonards-on-Sea compared to inner London, I suspect that regular flowering/fruiting may out of reach.

Interesting, how much was the GDD level for the year when your avocado tree got flowers (As compared to London. ) ?

Hi Priyanshu, that's a really good question.

Unfortunately, I'm unable to carry out the GDD calculations you mention, as I only record temperature data, in the avocado orchard, during the winter periods.

However, it's possible to get an approximate idea by using published archived weather-data for St Leonards-on-Sea  vs. SE1 for 2022 for example, or comparisons between years in St Leonards itself, but it wouldn't give the real situation in the orchard. The orchard site has its own microclimate which can be quite different from the official local weather data. Thank you for the research paper link - unsurprisingly it reports that progressively higher heat levels (GDD) are needed as fruit development goes from the early stages through to the later stages.

I got chatGPT to do an approx calculation for 'degree days' or 'growing degree days' (GDD), based on published weather data for St Leonards-on-Sea, inner London (location of fruiting outdoor trees), and Axarquia (productive avocado growing area in Spain). Using 10C as a baseline, and published historical monthly weather data, the results are as follows:
 
St Leonards-on-Sea....917
Inner London..............1365
Axarquia.....................2800-3200

The values would be a bit higher and more accurate if the A.I. model had used max/min daily temperatures, but the above is enough to give a clear idea of the large differences in heat that the trees are exposed to on an annual basis for the three different locations.

Growing avocado trees here in St Leonards-on-Sea is relatively easy, provided frost-protection is given for the first 3 years. However, the key here is getting them to flower and fruit successfully.

I strongly suspect the limitation to flowering/fruiting is the amount of GDD that the trees receive.  With a much lower GDD, here in St Leonards-on-Sea compared to inner London, I suspect that regular flowering/fruiting may out of reach. However, I'm still interested to see how the five younger Mexican trees progress (all representing different 'named' genotypes) as they are still only at a very young stage (20 months to 6 years). Again, I suspect that they, like 'Hass', will need a significant amount of summer heat to complete the annual vegetative/reproductive cycle.  

PS. Winn, have you seen any reliable published data for the GDD level required for avocado?

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 my orchard, 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.

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 (3 Jan 2025). Not unheard of here by the coast. Guessing around –5°C.  The seafront was forecast for around –2°C, 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.

Edit 5 July 2025 ...
Datalogger analysis shows that winter 2024/25 was the mildest winter in the avocado orchard for the last 5 years.  There were only two days with a frost below –3°C, i.e. 3 Jan (–3.84°C) and the 10 Jan (–3.45°C); my 'guess' above was colder than it actually was. The 8-year-old 'Hass' tree showed minimal winter damage, injury restricted to some apical leaves. It appeared to hold onto all its leaves throughout winter and spring, remaining largely green and visibly healthy.

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

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.  

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.

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.