While using the RHS "Plant Selector" to research plant culture http://apps.rhs.org.uk/plantse... I started to wonder why H4 is the only designator for outdoor hardiness without protection. I thought there was a significant difference in climate between say north east Scotland and south west Wales?

The link didn't show any info Evan, I'm not sure where you found the info.

As far as I know there is H1-4 with H4 being the hardiest, so that includes the coldest parts of Britain. I usually see these ratings on sites selling Rhododendrons or Camellias but also on 'hardy tropicals' sites.

http://www.rhododendrons.co.uk...

I can't find the link now Janet. What I'm referring to is the Resilience/Hardiness of a plant. The Plant Selector uses H1 thru H4 (Hardy) for hardy throughout the UK, correct? If there is a significant difference in climate throughout the British Isles how do you know when something that won't survive in the northeast UK, (say an H3) would survive outdoors in the south?

http://www.rhs.org.uk/Plants/R...

a bit on the issue of the hardiness 'H' designations in the UK

Did you see the link I posted Evan? It gives some answers to your question, other than that it's a matter of trial and error. There's always exceptions to the rule but eventually a hard winter like the last one we had will kick those exceptions into plant heaven. We get an exceptionally cold winter every 20 years give or take a little, at times down to -20C. Some winters can be mild with often 8C in the day and night, one winter we had only 4 frosts down to -5C.

For example: Plants which are said to be hardy to -5 or -6C but have endured worse ..

I had a Pittosporum 'Eilsa Keighley' in the ground for about 10 years, it was about 10 feet tall and very happy. Dead.

I had Pittosporum Tom Thumb in the ground for the same time, it was a nice mature shrub. Dead.

I had Acacia dealbata in the ground for the same time. It was huge maybe 35 feet tall, perhaps too big as it was smothering other plants. Almost dead! The trunk was very thick but the tree was frosted so much the leaves died, and the trunk did too as it turned from a bue-grey colour to a funny brown-red colour. But, the roots travel far and there's masses of them, now I have plants growing off the roots all over the place! I also had seeds germinating and growing, it has made plenty of seed before but they never grew on the ground. Nature was making sure that tree lived but I now have a problem!

I have several Phormiums, one died but it had ground cover around it so was maybe kept too wet for very hard frosts. Pink Stripe was a large plant, slapping my front window in high winds, it had a huge base but the plant looked dead. It started to regrow from the base but I decided this was my opportunity to dig it out. Others I have in a very sandy place, almost sandstone in places, they had the leaves killed too and regrew from the base so all was not lost. Pampas grasses I have were killed back, chopped the plants down to the base and they have regrown, same with Cordyline australis I grew from seed and were looking great, I lost one Cordyline and two Pampas grasses. If we had another very hard winter this time I doubt they would live as they were weakened and haven't had time to grow strong again.

My Trachycarpus fortuneii which I grew from a seedling survived! Most of the leaves were half frosted, and many of those which grew this year were scorched but it's quite a big palm now and has been in the ground since 2004 when it was about 5 years old. It's really the only palm which will stand a chance here.

Janet, no I didn't review your link. That clears it up nicely. I must have misread the reference I found at RHS. It's so sad to read about your loosing older/mature shrubs and trees. 30 years ago I had purchased 2 Chamaecyparis lawsoniana for my dad. One died 10-12 years ago, the other is doing fine. It was a hard winter but the more exposed one survived.

Unfortunately I can't follow your link Janice, it indicates the "file is damaged". Is this link to a pdf file?

Sorry Evan. It works fine for me.

It's probably a link to a pdf file. Not the links fault. My old program. Have to update.

Yes, Evan it is a PDF file.
It's a reprint of an article from an RHS publication last July, about how they are working on updating the Zone designations.
I'll see what I can figure out for you...

Janice, thanks for the offer but don't trouble yourself. I've been needing to take care of this for quite a while. Now's the time.

edited to add:
Link works fine now. Didn't realize it would be that easy to fix.

Very informative article Janice! Thank you.

Evan, yes it is sad to lose so many plants but I guess nothing lasts forever. I lost many bulbs I had been growing in greenhouses for years too, Arisaema, Eucomis, and a lot of Crinums I had grown from seed, some were an unknown species from Africa but I got the seeds from a grower in Australia. I had several different Crinums I kept in greenhouses which were hardy there until last winter, although the more tender ones I bring inside over winter so they survived. Heartbreaking to say the least! I also had many Gladioulus species which I grew from seed from South Africa, lost the lot!

You should have seen the pile of dead stuff I had for burning, it would have filled one of those huge containers they use at recycle centres!

So is the greenhouse unheated and the solar gain and wind protection adequate for most winters?

You're welcome, Evan!

Yes the greenhouse's' are unheated, and were adequate for most winters. I grew the Crinum in 2005 and they had lived there since, even as young plants. Winter 2009/10 was not a good one, but they all survived no problems. Having them under glass keeps winter wet off too, wet combined with cold is often a killer.