Why is my dwarf Buddleia undwarfing?

My 5 years old Buddleia Davidii Nanho is growing full sized branches with full sized flowers. I don't think these are grafted and the big stems grow straight from old wood.

What's going on please?

The bottom part is as it should be, about 60cm tall with small flowers. The top part is about 2m tall with flowers three times bigger than the proper ones.

I'm not complaining, just curious

 

Reversion?

 

Thank you @pete , I'll look that one up but I don't think the shrub is particularly religious

 

If its a dwarf grown from a cutting of a kind of "sport" on a normal plant it could just have formed a normal shoot on your dwarf plant and reverted back to type.

 

Seedling growing through the original plant? If not…

The Nanhoensis type is naturally small and not a sport, highly stable and unlikely to sport more vigorous shoots. I've never heard of a Buddleja being grafted, so a suckering rootstock is not the answer either. Nanhoensis is very distinctive, gracile rather than short.

But I think the plant you have is actually 'Operette', a colchicine induced aneuploid of 'Opera'. This cultivar is one third the size of 'Opera', and I think you might be seeing a reversion. The photos look remarkably like these two plants.

 

Thank you, I have learnt a thing or two this morning. Considering that the plant was bought as part of heavily discounted 3- pack "lucky dip" (one dead, another ugly and this one) it is earning its keep.

I'll chop it well down next spring and see what happens.