Less Growing Days highlight hardiness issues

Has anyone noticed the decidedly Autumnal feel in the morning and evening now that the days are getting shorter? This is a sure sign that growing days for many plants are starting to reduce.

Sad to think that very soon I will be starting to protect some of my exotic and tropical plants in the garden so they can survive the English winter. Hoping that we will not have a repeat of the last two winters when many previously categorised “hardy”plants were destroyed by continual low temperatures which reached as low as – 12c in some parts of the UK.

sun on bananna leaf

Growing tropical, exotic and mediterranean plants is a rewarding experience but making sure they survive our wet and sometimes cold winters is one of the biggest challenges for UK gardeners.This is complicated by the fact that many parts of the UK experience completely different weather patterns during the winter.

Cornwall, located in the SW of England, is one area that seems to provide conditions that enable these plants not just to survive our winters but actually do extrmely well. Perhaps this is why Phil from Trebrown Nurseries in Liskeard is such an expert on the subject. There is an excellent plant hardiness map on their site although it only goes to 2000 and does not yet include the record low temperatures of the harsh 2009/2010 winters. http://www.trebrown.com/hrdzone.html

Here is Phil sharing his thoughts on hardiness and growing days :

The map is very much an underestimate due to the data being an average record of 40 years.
If we’d only focussed on, let’s say all the years post 1979 then the map would be very different, as the climate is much warmer today.
 
We plan to revise it in 2010/2012 for records up to 2010. It will be interesting tosee if the records from the last decade show significant changes to the map.
Of-course the map doesn’t really tell us much regarding what can be grown here. No zone map can be accurate due to the fact that it doesn’t take into account the number of growing degree days for a particular latitude. This would be different for different species. UK summers are short in comparison with other similar world-wide zones. Although our summer day length is long, they seldom reach high enough temperatures to count as Growing Degree Days (GDD) for truly tropical plants. A GDD for a particular species is a day that can be counted as a day that that plant grew. Quite often a zone 9 or zone 10 plant will put on no growth whatsoever during the whole year here in Cornwall.
We have a very strange climate here in the UK. There is very little change between our
cool summers and our mild winters. Very little temperature swing compared to almost everywhere else in the world outside the tropics. For this reason plants can manage to pull through our winters but then sit waiting for the summer heat which almost never comes. Therefore we should be looking for plants that can put on growth at low temperatures, high elevation tropical plants make reasonably good candidates. It’s not
possible to make a map to reflect this information, because every plant has it’s own ideal temperature Growing Degree Day.
At some stage we will include the minimum GDD temperature for each species contained in the species’ growing information on our site. We have quite a lot of information on file here, and all this information will eventually be placed on the Trebrown.com website.
Currently there is an interesting article about Growing Degree Days (GDD) on our server here:http://www.trebrown.com/documents/climate/climateadaptedness.php
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- who has written 863 posts on My Climate Change Garden.

I am not an experienced gardener - more of an enthusiastic amateur who learns by trial and error and is keen to "manage" the effects of the weather on my garden. Writing this blog is my passion and I hope that it will continue to grow, allowing global gardeners to communicate about the effects of climate change on our plants and the future of our gardens.

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