By Joesoap
United Kingdom 
Princess Diana, (Clematis) is alleged to reach height in 10 yrs of, 8 feet and width of 6 feet. But it is advised that should be reduced in height no later than February each year. Does that mean the root system is by then so strong as to produce this amount of growth,?? Puzzled Joe
21 Jul, 2016
Answers
Derekm, Thank you for your response,I did'nt realise it was group 1.
Fantastic website/help. Joesoap.
22 Jul, 2016
Hi Joe. Princess Diana is a texensis clematis, and is in pruning group 3, not 1 as advised above. This means that you should prune hard early each spring (to about a foot or so above soil level).
22 Jul, 2016
If you remember the rhyme "If it flowers before June, don't prune" (only lightly to keep in check) if it flowers after that, hack it back in Feb., as it flowers on new growth. Have a look at the following site:
https://www.sarahraven.com/articles/how_to_prune_clematis.htm
22 Jul, 2016
Hi, my apologies for the wrong information, {senior moment}I knew it was Clematis group 3, but I stupidly jumped to the conclusion that, as it was a late flowering1. it must be pruning group 1, completely forgetting that the groups are based on their pruning needs, sorry, Derek.
22 Jul, 2016
Hi, welcome to GoY, C 'princess diana' will reach 6 to 12 ft in a lot less than 10 years, unless pruned, it's pruning group 1, which means removing crossing shoots, and any dead or diseased shoots, prune in late winter, whilst the plant is dormant, it flowers on current years growth, so you can control the height if you wish, Derek.
21 Jul, 2016