Mike Guye

Joseph Byrne wrote: ... it might be possible to grow an avocado in ground even pretty far north in England ...

If you are anywhere north of London, it's my guess that you'll need a polytunnel or glasshouse to get the number of growing degree days (GDD) up high enough to evoke flowering, i.e.  1300 GDD or more. My gut-feeling is that this is probably true for  both Mexican and other avocado types.  Additionally, you'll need good winter protection to prevent any flower buds that form from being damaged or reverting to vegetative buds.  Good luck Joseph with this project - it could well be several years until seed-grown trees fully transition to the reproductive phase  - it might be quicker if you bought a tree that's already grafted for you, e.g. a grafted Bacon. I think there are some suppliers online for those living in the UK ...

Joseph Byrne wrote:How much does the rootstock matter if the variety grafted onto it is cold hardy mexicola joey etc.
Most commercial avocados will be a cross of hass and fuerte/zutano so probably a decent rootstock
Where can budwood be sourced I know about the company in America but is anyone is uk/Europe selling budwood of the Mexican hardier varieties like mexicola

Hi Joseph, yes I had this same question a while ago.
While it is true, for different plant species, that cold-hardy rootstocks can be used to increase the cold-hardiness of the grafted scion, a survey of the literature revealed that this does not appear to be the case for avocado. The main feature that is exploited with avocado rootstocks is either their increased tolerance of saline soils or their tolerance to root-rot, caused by the pathogen Phytophthora cinnamomi which thrives in cold wet soils and which apparently is a major threat to avocado production.  

That's a wonderful looking potted avocado, Priyanshu - it looks very healthy - it seems very happy on your balcony
This year, did the flowers begin to set any small fruit or did the flowers/fruit abscise prematurely?

All five of my UK avocado blogs on imgur have been updated (Nov 2025) - just scroll back 4 posts for the different links if you want to view these.
For those viewing from the UK, you will need to use a VPN, as imgur is no longer directly accessible to UK users.

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