We may be over-emphasizing the suitablilty of certain plants to act as companion plants for clematis :-) If you are sufficiently addicted to growing these vines so that your collection becomes significant, you will find that many plants make good neighbors for clems. With the number of vines I grow and the variety of their locations, they have a wide range of companions, from trees and shrubs to all manner of perennials and groundcovers and they all seem to coexist well.

There is no reason hostas and clematis cannot be neighbors. Many hosta cultivars are extremely sun tolerant and most will be surprised to know that an established clump of hosta is quite drought tolerant. And if the area at the base of the clematis vine is rather shady (in my garden, because of the density of the plantings, this is pretty much a given for most of my clems), any hosta would be quite at home. And I think too much emphasis is placed on the need for shallow rooted companions. If you commonly grow clems through trees and shrubs, these support systems offer both a dense, shallow immediate root structure as well as often quite deep and far reaching root systems. It may take a bit more care and attention to get a clematis established under these conditions but given that, they will thrive with some pretty significant root competition.

My take on this issue is consider the suitablility of the location for the clematis and potential support structure, take the time to dig a proper planting hole and plant carefully and then combine with whatever other plants work for that area/composition. My only caveat would be to avoid perennials that require frequent dividing, as once it is established, you don't want to unnecessarily disturb the clematis roots.

It also depends where the hosta is in relation to the clematis roots. I have clematis weaving through anything available, and in some cases that would be hostas, but ones that are planted 2-4' away from the clematis. Often my goal is to get the vine up higher and into the sun, so the clematis roots and the hosta are fairly shaded, but the majority of the vine is higher up in the sun winding through a plant. Some of it just ends up on the hosta sometimes.

Where I live, it typically stops raining around mid-June, and when/if we get an occasional summer storm, it doesn't rain enough to make a difference. Sometimes we can go for weeks with no rain. I cannot imagine trying to keep a hosta looking good in full sun under drought conditions. It's hard enough to keep most hosta in shade looking good during a drought.

Gardengal is right, other plants, including trees and shrubs, can and will grow well side by side with clematis, often times developing a symbiotic relationship, but I wouldn't want a densely packed hosta rootball next to a clematis. Following the advise of someone on the hosta forum, I lifted my hosta this spring to loosen the soil ball. You do this by banging in on the ground a few times. My hosta looked so much better this year. I also agree that you wouldn't want to plant something that needed to be divided frequently.

For similar color and form (in a way), and a bonus would be winter interest is dwarf evergreens. There are a lot of really nice varieties available on the market. I have one called Sherwood Frost. It's leaves are dark green and the tips of the plant are a creamy-white. It's very pretty although it is cone shaped and gets 4-5 feet tall. I also have a yellow-green dwarf evergreen that has a more rounded shape, And there are gold, bright green, near-chartreuse, and dark green varieties. Many of these dwarfs are arb-type evergreens (sorry, I'm horrible with names), and do well in the same soil conditions as clematis. Some of them will even do well with only 4 hours of sun.

Another plant to consider is sedums if your clematis is in a sunny location. Sedums are very drought tolerant yet they have done well this year with all the rain. The taller ones can help hide bare legs, if that should happen to your clematis, and they bloom late adding color to the fall garden. The shorter varieties make nice ground cover.

Move south and you will find there is no thing as a drought tolerant hosta. Even when we haven't had drought as bad as the ones we have had the past two years, hostas tend to dry up down here without sufficient moisture. They do tend to return when the moisture comes back, but they are rather ratty looking by midsummer at the latest. I am sure that things are very different the further north you go and especially in the PNW where the weather conditions are more similar to that in England.

We are similar to England except in summer. It typically stops raining here by the 4th of July and doesn't rain regularly until late September or early October.

Fall is typically drier and warmer than spring here.

We had several really hot spells that were out of the blue and there is fried foliage all over. I went on a big tour of nurseries over the weekend and they are mostly in the same boat. Loads of crispy leaves.

I toured the Rogerson Clematis collection too and though I can see the potential for greatness in the future right now they have loads of fried baby Clems. I didn't feel so bad about my own after the tour.

I used to have very bad luck with hosta even though they do grow really well here. The Hosta Forum helped me because they suggested mine were planted too deeply and not getting enough water. So, I raised them up and planted new ones shallower and watered each with the hose as well as sprinkers and soaker hoses and it did make a huge difference. They actually got bigger when they came up this year instead of smaller.