How large is your arbor? Grapes take a lot of space, and to get good fruit, they need to be spaced about 6'-8' apart. You probably won't get good fruit for 3 years, but after that you will get a lot of fruit. It's best not to let them fruit for a couple years. When you allow them to fruit, the vine energy goes into the fruit and stunts the vine. The only flowers I've ever seen in the vineyards here are roses planted at the end of the rows, NOT to compete with the vines. Not sure about the root system of the Clematis. Good luck! Sounds very pretty!

Hey fellow gardenstater! I live in OH now but grew up in NJ near Asbury Park! lol To help with you question... I am trying to do the same thing here with a new clem Henryi, and several others and my John Cabot rose which is coming in a couple of weeks. I am also constrained by space because they are against a trellis. Since 'Haku Ookan' is a group 2 clem you might want to consider the two side by side? Think in terms of not only water and nutrient requirements, but which way will be easier to prune both? Clems don't really need to have "their roots shaded" as what they really need is Moisture! The roots shaded by mulch or low growing plants (or the rose for that matter) is only there to keep the clem and rose moist. If you can't plant them side by side I would tend to think in terms of planting the clem in front of the rose. Of course, it is all up to you. Our gardens should be done the way WE like them :) I think at least a ft is ok for the spacing if you are training it to go through the rose. But be careful digging as all roots are easily sliced through. Mine are also going to be about a ft away from eachother. Here is a pic of the general plan: (there is actually a lot more space than what is shown as the trellis is 20 ft long and 6 ft high.) But I'm not that great at drawing lol.

My mistake was buying many climbers when I first started my garden. It is my opinion that most climbers are just over sized HT's. Make sure what you pick as flexable canes and few thorns. Most of my climbers are fishing pole canes with no flex and would seriously injure a person. I am going to SP Compassion for this reason. If I were to buy a climber, it would be Maman Cochet climber. I am broke at the moment and this is on my list for x-mas from Vintage. But that would be my perfect pergola..

I'm a fan of the small-flowering clematis, viticellas in particular because they never wilt for me. Very dependable and I appreciate the easy pruning. Polish Spirit is a beast! Very vigorous. (here on a trellis over a gate) I could not find a close-up of my arbor, but it has Venosa Violacea on one side and Julia Correvon on the other. They meet in the middle but don't cross over :) But they would probably cover your lovely arbor too much! Mine is just a simple arch.