DUH! I meant a Venosa Violacea but am sure most of you knew what I was refering too. Same growing habits just different names so I thought I would correct myself before someone else did.LOL.
Lois again

No, you don't need cement but you should use gravel to protect the wood from the soil and to brace them. I've done this and it works perfectly.

Bought cheap pea gravel from Home Depot and poured some in the bottom of the holes then set the trellis "legs" in and filled up the holes with gravel. They don't move/wiggle at all and it has been a year. They are 8' tall and 2' down in the ground.
They are freestanding, no braces. The legs are 2 x 4's.

Another option is to buy rebar in two to three length pieces, pound them into the ground next to the legs, and then attach the legs of the trellis to the rebar. What you attach the two together with would depend on the side of the trellis legs. I typically use hose clamps when I do this, but you could also use wire of some type.

I would think the weight of a single clematis would not be so great as to require this. I have a 6'X6' trellis where I first planted an Asao clematis, then a Lady Banks rose. We did sink our supports with concrete so that I could have the versality of planting what I wanted on it. It will soon be home to 2 different clematis as well (Palette and Duchess of Albany). The weight bearing capability of the trellis makes this kinds of arrangement possible. I also have another trellis (8'x4') where I did the same thing--built the trellis using concrete, then planted according to the weight bearing capabilities of the trellis--it now houses a cl. hydrangea, precisely because it can handle that kind of weight. I have also added a Lemon Zest honeysuckle and a see a clematis in its future. :) I like to have versatility-but it's a personal decision whether to concrete or not.

I have 3 Clematis on each of my 8' tall and 7' wide free standing trellises described above.

Thanks to all who replied to my question. At this point I think I will try it witout the concrete.I have no idea what rebar is. I was at Lowe's and Home Depot yesterday and forgot to check on it.That will give me a excuse to go back. As if I need one ! LOL.
Lois

Rebar is cheap metal poles used to reinforce poured concrete.

And, for all sorts of creative garden uses....

Pumptoad, rebar is steel poles that do help reinforce concrete and it has ridges on it to help it "catch" in the concrete. In our Lowes, it is found over in the section where you purchase portland cement, redimix concrete, fencing, and similar items.

Here is a link that might be useful: rebar