Pruning a begonia that is 2 metres tall? Begonias are hardy plants, especially those with thick stems that can grow to 2 metres or more. You can prune them by half and if you want, and make cuttings out of the pieces of stem. Good luck!
 

What about your lemons? If the inner pith of lemon, or citrus, is very thick it is usually due to the tree being too dry. Water more around the drip line.