Broad Beans

Knee height. And just use your finger and thumb to pinch out the topmost growing bit.

Hi All, The reason that you pinch out the tops of the broad bean plant is twofold. 1 to encourage the beans to fill out. 2 to deter the black fly, As black fly are sap suckers they will go for the soft tissue, As opposed to the tough tissue lower down the plant. When you pinch out the tops you remove the soft tissue, And so deter the black fly. hope this helps you.

Richard M. Watkin.

The growth tip of a plant is called the "apical meristem". If you "nip it in the bud" it causes the plant to branch. I've been told that this allows the roots to catch their breath so to speak.