I have three young clematis sieboldii which have really taken off while I was on holiday. Two of them now have one long shoot approx 2ft. Should they be pinched out or just planted as they are?
I am no expert, but cutting off a bit of it now seems a bit risky to me, I would plant them as they are and let them get on with it. I have a couple of them and they seem fairly bomb proof to me!
Clematis need planting 3 or 4 inches below where they were planted in the pot. I think it stops clematis wilt, well helps to ;D. I think Sieboldii are a bit tender depending whereabouts you are in the country.
Sieboldii are not very hardy so make sure they get winter protection.
I wouldn't pinch them out either. They'll naturally grow more shoots as they get older.
Planting deep is good advice as it encourages the production of new shoots from below ground and also helps the plant recover better if th etop does die off from clematis wilt - but it won't prevent the plant getting it.
Clems are hungry plants and thirsty too but don't like to be wet so make sure you use good quality compost, add some slow release fertiliser to it at planting time and give them a top up every spring as well as occasional liquid feeds from March to June.