USDA Zoning

[stephenprudence]

I know USDA zones dont really mean much in this country and there are plenty of problems with USDA zonation, and it is more of a ornamental appeal in the UK. However I wonder does the USDA zone refer to the average minimum low of a winter, or does it refer to an absolute all time low temperature for the place in question?

[Las Palmas Norte]

Average annual minimum temperature is how the USDA plant hardiness zones are calculated.
So as you've already stated, not overly applicable. It does however have it's roll, albeit a small one really.

Cheers, Barrie.

[stephenprudence]

Cheers Barrie!

[Devonian Jungle]

Average annual minimum temperature is how the USDA plant hardiness zones are calculated.
So as you've already stated, not overly applicable. It does however have it's roll, albeit a small one really.

Cheers, Barrie.

Hi,
I think it is calculated by an absolute low at winter, because if it is calculated by average annual minimmum then orlando would be a zone 11+ due to its hot summer temperatures. Orlando is actually zone 9b/10a according to USDA because many plants can't cope with the winter lows. Here in Devon, we can only just grow 10a plants (Ficus Elastica, Yucca Elephantipes, Monstera etc) outside all year, but other 10a plants would die (coconut palms:( ) so I guess that UK zoning is completely different. I believe that it is the rainfall, shorter summers and daylight hours that change our zoning. Maybe we could create our own, official zone system for the UK?
Anyone up for it?
James

[Dave Brown]

The USDA zoning is only based on the coldest expected temps for an area and then match plants that will withstand that cold. The problem with transporting the zones to NW Europe or even NW Pacific., is that most of Continental USA gets a hot summer. We don't.

A Coconut can withstand -0.9C for a short period, but needs 21C to grow. As they cannot go dormant they will die if they cannot grow for several weeks. The UK outside it could only grow for 3 months of the year, or 2 months this year

Also the zone levels are much further south on the US side of the Atlantic so Zone 8 to 9 which is what we have in Kent at 51.5Deg north, is down at the 30 to 35 dec north in the US, so the sun is always about 16 to 21 deg higher...... or 2 months ahead in spring and 2 months behind in autumn. Even thought the winter lows are the same the sun is always higher and they have a 6 months of above 21C..... would be nice.

[palmking]

I read it's based on the average winter low over the last 5 years. Useful in a changing climate for accomodating smaller incremental increases or other discrepancies over a shorter period of time.Of course, a USDA 9a climate experienced in Orlando, would be much different to that in the UK, where winter temperatures will often quickly recover to the mid 70'sF -24C after any frost has subsided, with much lower frost duration overall.

The met office averages by comparison as most will know are based on currently 1970-2000 data and until recently were still using 1960-1990.

Saying that, relatively speaking, much of continental Europe may experience daytime Zero or Sub zero temperatures, whilst much of the UK rarely, if ever experiences this. The last time I can recall a Daytime Sub zero temperature was around 1988. I still have the data somewhere.

An good exception to the 'hot' continental summers in the States, would be San Francisco, which due to the frequent fogs has a tempered average similar to the climate in Manchester but the higher light/UV levels and milder winter temperatures allow for some great growth of exotics etc.,

Sorry, i'm rambling a bit.

Paul

[turtile]

The 1990 map is based from 1974-1986 (still the standard zone map in the US). The lowest recorded temperatures are used for the averages.

The airport near my house recorded 8F last year and 16F the year before that. For a two year map, the average would be taken of the two lows (12F Zone . The lowest temperature from the airport was -13F in 1987. The 1990 zone map has most of the lowest recorded temperatures of all time. The ferry recorded (closer to wear I live but I don't have complete data) -11F in 1982.

[Alexander]

To my opinion to use that USDA Zoning system as for what to plant in your garden is totally useless! According to USDA criteria the most westerly part of Norway has the same USDA zone as Orlando. Well we all know that lots of plants you can grow in Orlando would die in Alesund and vice versa.
Its time we get a better system. A system wich also takes into account the summertemperature, ammount of rain, sunshine etcetera.
You could very well compaire the climate of Vancouver with that of coastal England. Very similair But Vancouver versus Los Angeles is very differend.

Alexander

[stephenprudence]

I think something with a zoning about what can grow naturally, or what can naturalise in a specific area should be trialled. So for example if a plant from a higher zone rating was able to grow from seed outside in a place and be proved so then that would give the place a higher zone rating. This would take a while though and is probably unworkable.

What we would find though is that the UK is near the bottom of the list in being one of the lower zones if we took everything into account places like the inland Netherlands and Germany would probably rank higher than us due to their abundance of summer heat.

So perhaps the UK in a summer heat zone would be maybe the lowest zone (or one of them), and for winter survival would be in towards the top. Certainly wouldnt come up the same as Florida though!

[palmking]

Stephen, you suggestion would then counterbalance. Summer temperature deficiency against winter high temp advantage. Although several factors would need to be addressed. Soil modification with added drainage and rain canopies afforded by tree cover higher up above a bed gives one serious advantage. Large plants next to smaller ones, dries up soil quicker and affords protection to smaller less hardy specimens. Reflective heat from fencing or a stone wall, south facing raises temperatures. Use of decorative stones on the surface traps heats.Mirrors placed in a garden can direct much needed light at the times of day direct sunlight has passed an area of your garden.

I take the USDA system lightly but useful for an approximation of a plants lower temperature tolerance.

Although continental Europe does have higher summer temperatures in general, the reduction in frost frequency and duration and higher winter daytime and nighttime temperatures obviously benefit us and is evident in places like cornwall and the scilly Isles that lack even the summer temperatures that I experience. Higher light levels and UV radiation compensates here, compared to around a 5-10 % drop in either levels compared to my location.

Direct sunlight reads much higher temperatures than Shade temperatures and a plant in direct sunlight will experience a longer duration in necessary temperatures for photosynthesis and therefore in growth rates and flower production.

Paul

[turtile]

So perhaps the UK in a summer heat zone would be maybe the lowest zone (or one of them), and for winter survival would be in towards the top. Certainly wouldnt come up the same as Florida though!

Here is the AHS Heat Zone Map that I posted in anther topic. It's zones are based on the number of days over 30C.

http://www.ahs.org/pdfs/heatmap.pdf

[Dave Brown]

So perhaps the UK in a summer heat zone would be maybe the lowest zone (or one of them), and for winter survival would be in towards the top. Certainly wouldnt come up the same as Florida though!

Here is the AHS Heat Zone Map that I posted in anther topic. It's zones are based on the number of days over 30C.

http://www.ahs.org/pdfs/heatmap.pdf

turtile, that was the map for good growing of Sabal minor if memory is correct Able to stand more cold but requiring much more summer heat than we can provide.

Stephen, this is why 'The Subtropical Garden' book was my bible. New Zealand is a slightly milder version of here, in terms of temeperature, but even then you have to realise that Aukland is the same latitude as the Costa del Sol in Spain, and the southern tip of South Island is about mid France