I use corrugated cardboard from shipping boxes or several layers of newspaper, topped with 3" of shredded or chipped wood mulch which is readily available to me. I don't use landscaping fabric because of bad experiences with it soon after starting gardening, and because it is a barrier that prevents organic matter enriching the soil. If you do a search on the perennials forum for landscape fabric you will get a ton of negatives; IME roots grow through it and make weeding in the future extremely difficult. Weed seeds will blow in regardless of what you use, and with an all organic mulch weeding is very easy, and as the mulch breaks down, the clematis are fed. I renew mulch every 2-3 years. In a new bed I will sometimes put down corn gluten to stop newly exposed weed seeds from sprouting, but I don't particularly like using chemicals in my gardens that may have a long half-life or end up in the water table or river nearby. I have one bed with many clematis along with shrubs and perennials that is about 15' by 75', and I spend perhaps an hour per month at most weeding it to maintain it weed-free. It was created with a large amount of horse manure incorporated, so it has plenty of weed seeds, but they don't sprout with the cardboard and mulch.

Thanks nhbabs. I like the idea of the cardboard and have some questions about it.. How do you lay the cardboard? Do you have to overlap it? Wet it? What kind of cardboard do you get? Like refrigerator boxes etc? It seems like it would be hard to stay in place. Sorry about so many questions but I usually get in the middle of a project and then realize I don't know enough about it.

I use whatever sized brown corrugated cardboard shipping boxes I can easily get (appliance store, grocery store,etc.) and overlap them 3"-5". If I am doing a new bed I just lay them out and then cut through them when I am ready to plant, but if I am working around established plants I may use smaller ones or cut/rip them into pieces that fit around the plants. In general I do wet them, and if using newspaper layers you have to wet them as you go along so they don't blow away before you get the mulch down. Once the mulch is down,that holds the cardboard or newspaper in place. Here's a bed expansion in process. I edge the beds with a combination of black plastic edging buried to the rim and bricks along the inside edge for the mower wheels to ride on. This keeps the grass from creeping in.

I just use compost as mulch. Never landscape fabric.

No,no,no! A thousand times no! Hate the stuff for all the reasons nhbabs listed. Organic mulches are the way to go. I prefer shredded leaves as have lots in the fall including those my neighbors collect for me. Have also used shredded bark but it is harder to dig in when moving or adding plants.

When using cardboard boxes I remove the tape or I end up picking it out of the soil later as find it does not readily decompose. Also remove any staples.