How to prune cassia floribunda
I have a number of Cassia Floribunda, planted one year ago, when they were about 30 cm tall. I would like them to grow up into a more bushy structure rather than spreading sideways as they are now. I have tried tying one of the branches to a stake to encourage it to grow vertically. The other branches are too thick and hard to train upwards. How can I prune the plants to make them grow upwards into a bushy shape?
In case it is relevant our garden is in Portugal, 20 km North of Lisbon.