On a weather forum a thread posed the question if anyone thought the extreme south of the UK, like the Scilly Isles, could be successful with commercial citrus production based on the very mild winters. The crop in question would be lemons, since they may require less summer heat.

I believe you guys on here are much more knowledgeable about citrus than most, so was wondering what you thought based on the climate there.

I was hoping to make a yes or no poll question to see if there was a consensus, but I couldn't get the poll function to work.



The average weather conditions at Scilly below. Notice the lack of any heat, and the very poor annual sunshine hours. By contrast places in the US South get around 2700 hours per year. California gets over 3000 hrs of sun per year.

A very interesting question... From looking at the average temperatures there, I do not believe there is enough annual heat (growing degree days) to support commercial citrus. I'm sure some of the UK members could provide a much better idea.

I base my conclusion on comparisons between the weather in the Scilly Isles and in other cool weather citrus growing areas.

For reference, the small commercial citrus growing regions in New Zealand are some of the coolest (temperature) areas with a commercial citrus industry.
Here's a really good link describing the growing areas there:
http://ceventura.ucanr.edu/Com_Ag/Subtropical/Citrus/Miscellaneous/New_Zealand_Citrus_/

Quoting from the above link:

Quote:

Temperature extremes are rare. Some radiation frosts can cause damage to fruit. High annual rainfall is common to New Zealand citriculture, so much so that some farms have no irrigation systems. However, this wet environment is also conducive to severe rind blemishing, fungal diseases. In the citrus growing districts, the maximum January temperatures (summer) average around 24 degrees C, with the average minimums at 13 C. July maximums (winter) are around 14 C with average minimums are 5-6 degrees C. Annual rainfall ranges from 42 to 67 inches. One major problem for New Zealand citriculture is the low accumulation of annual heat units. These are necessary in order to obtain adequate sugar (Brix) levels . Orlando, Florida produces 3700 annual heat units, Riverside 1700, Valencia, Spain 1600, but in New Zealand annual heat units range from the low 700's to the high 800's. It is obvious that certain varieties of citrus like grapefruit and the Valencia orange will not do well with this low production of heat, and other varieties like lemons and Satsuma will do very well. Of particular importance is the choice the proper rootstock to help obtain good interior fruit quality.

You can get an admittedly non-precise calculation of the citrus GDD (base 55F / 13C) using this calculator at weather.com: http://www.weather.com/outdoors/agriculture/growing-degree-days/
The base 13C GDD in Scilly U.K. can be estimated by averaging the base 50F and 60F annual data from the weather.com link, to get an approximation of GDD (base 55F), which is followed by conversion to deg. C.
Performing these calculations, the GDD for Scilly UK was approx. 420 in 2012 and 478 in 2011 (both in deg. C). By analogy, the same calculations for the citrus regions in New Zealand for 2011 and 2012 are ~1000-1200 GDD (in deg. C). There seems to be about half (or possibly less) of the available heat units in Scilly as compared to the commercial NZ citrus areas.

Thanks. Great information there. I'm going to look over that link you provided. I'm very interested in climate and what subtropical veg can be grown. I'm hoping some forum members from NW France, the UK, etc. respond to this.

Citrus trees, including lemons, do not (cannot) produce any growth when temperatures are below 55.4% F. (13C). The island you point out has 6 months of temperatures below the absolute temperature required for any growth at all. Further, there are three months that are above the minimum growth temperature, but they is so close to the minimum that the growth would be minimal. Unfortunately, a commercial venture would be a financial disaster in my opinion. - Millet

Tresco Abbey Gardens in the Scilly Isles is home to many plants that cannot be grown elsewhere in the UK. This includes some fruiting citrus.
However, the idea of growing significant quantities commercially is a non-starter. The largest island is only 6 square kilometers in area, mostly coastal rocks and wind-swept granite uplands. Yes, it is the Atlantic winds that are the great problem for any agriculture. Only a tiny proportion of land is sheltered enough to grow trees of any significant size. Temperatures do allow citrus to survive in sheltered spots, but lack of warmth and sunshine hours mean they don't really thrive or produce sweet fruits.
Spain and Italy produce vast quantities of excellent citrus at low prices. These are easily transported by truck and rail (through the Channel Tunnel) to the UK market. Transport from the Scillies is costly and can be unreliable in winter, although it is true that cut flowers are grown and flown out commercially. These are mainly early daffodils, grown in narrow fields protected against wind by high hedges.
Mike/Citrange UK