No, it's not too late. Go ahead and plant it out. Sooner is better than later so it can root in before the hard freezes come. I doubt it would survive in the pot all winter.

Thank you Bathgate,I did not know whether to keep it in a cool room over winter,and plant it in spring,but I will plant it out now,thank you once again.

What you have bought is what nurseries call liners which they would pot-on before selling to garden centres or the public. I would suggest that you pot it on into something bigger and overwinter it in a sheltered spot. The only time that I would want to plant it out in the garden would be in springtime.

Agree with Jimmy, 9cm is too small, pot it up into something a bit bigger and keep it sheltered over winter.

I disagree with above. Actually it's fine to plant out now while there's time to root in. The pot is prone to freezing & breakage which would not be good for your buddleia. Pots also dry out very quickly. That's not good either. You'll have to babysit this plant all winter if it's in a pot. Don't let it freeze, don't let it dry out, make sure it has drainage, keep it sheltered, don't let those crazy winter winds knock it over. It will also deplete the nutrients in it's little pot very quickly. All this can be avoided if planted in the ground. You really don't have to think about it if it's in the ground.

I know pots are very popular in the UK. I rarely ever use them. They aren't necessary. I just have a couple ornamental pots on my patio and for houseplants. That's it. It will quickly outgrow its pot then you'll have to transplant again which is very stressful for plants and disrupts its growth cycle. The tag from the nursery says to plant out in August-September. Guess what? It's September now.

I agree with instructions from the plant nursery.

Tri-colour Buddleja is actually three Buddlejas planted in one pot. 9cm. seems rather small for three shrubs. What ever you decide to do, don't leave it too long. There is not going to be much food left in the compost for three shrubs.

Owdboggy's right; this link here

https://www.yougarden.com/item-p-510198/tricolour-buddleia-butterfly-bush

offers a tri colour buddleia plant in a 2 litre pot, which would be ready to plant out right now - it also mentions the fact that other places supply them in 9cm pots, and that is too small to plant out now, though if you lived in London or on the west coast where its warmer, I might not be saying that.

Thank you all so much for your help,I do not want to lose it,it was a birthday present from my 3 year old grandson. I will have a look at that web site Bamboo,thank you all once again,this is a great site for getting answers.