Which way does the wall face and what is the state of the soil? Clematis is going to need some sort of mesh attached to the wall in order to climb up and it will climb, I've never heard of trying to train one to grow horizontally and am not sure this is feasible. First think though is aspect and soil oh, and are you expecting to just plant one clematis?

From what I have observed in Colorado, the more vigorous species will grow in all directions, but the large-flowered hybrids are pretty obstinate about growing straight up. Maybe corrugated wire mounted in a fan pattern?

Thanks for your replies. The wall faces east, gets sun from dawn until about 2 pm in midsummer. Soil is clay but will be improved. I've trained clematis (group 3) over a fence previously. Being horizontal encourages shoots upwards and lots of flowers. My question really was asking if it will be ok to lay the trimmed paving slab over the roots. I plan to put 2 clematis of the same pruning group in this position. ( depending on opinions of the GOY members.

Yes that should be fine so long as you can get enough water to the roots all the time.

The slabs are only 2 feet wide, yet a mature clematis can get up to 8 feet wide. You'll be creating a major obstruction people will have to walk around. Reconsider this project. Each of those slabs are placed exactly according to specification. Any variation in size or depth is another tripping hazard.

Another idea might be to fasten planters to the wall with special hardware. You can grow all kinds of trailing and flowering plants without upsetting the pathway. Certain plants would be quite happy just growing from the cracks in the wall - thyme for example & many succulents.

Depends on the variety of clematis - if its a large species one like montana, then its not a good idea because it will form a very large trunk over time, but if its one of the hybrids (usually with large flowers), it'll be fine. Being under paving slabs is fine, its usually a damper place to be and also protects the roots from the sun. Not sure about the viability of training it horizontally though.

Thanks again for input. Re training clematis horizontally. I have 'trained' 2 at the moment. One is an evergreen around an obelisk , basically tied it in on the side arms and then gradually tied in in an upward direction . This has resulted in the obelisk completely covered and hundreds of flowers. The second one is a group 3 so gets a good prune in spring. I tie in 1 or 2 stems horizontally ( looks like a wide fan) with other growing vertical. This results more wall being covered in flowers.

So do you train the prune group 3 one every year - I'm assuming you must do so because of the necessity to prune it back to about 8 inches every winter. I admire your dedication...

Hi Bamboo,
At the moment I do but I am retired and have the time. I'm waiting for a climbing rose to cover more of this wall.

Barbarak, if the rose is slower than you would like, try feeding it a few teaspoons of Epsom salts right after the first flush of bloom. That usually makes the climbing canes more vigorous and numerous.

Thanks Tunbrethil for the rose advice. I think I probably over prune aswell.

Generally, climbing roses should have highly selective pruning. In most modern varieties, during the main pruning in winter, the main canes should be trained horizontally to cover the wall, and the side shoots that come from them should be pruned back to 3-4 buds. Canes that are more than 3-4 years old should probably be removed, since they don't often bloom well, and their removal helps stimulate the growth of new ones. Once blooming ramblers should be pruned severely in spring, right after bloom--the more vigorous the variety, the more severe the pruning.
During the summer, new canes can be tied into their proper directions, and those that are stubborn can be pinched or lightly pruned back.