Another reason to deadhead these are that you do not want them to spread outside your garden. They can be quite bad for the local flora with displacement and changing the ground conditions when they are outside their orginal range of the Alps.
Yes, Laburnum trees grow very quickly, however their eventual height is not huge, once they are about 10 years old they usually reach their full size and don't get much bigger. Both Vivax and Paulownia are very large plants so they would be best in their own containers.
