Short diagnosis - Before we move into pruning, lets take a moment to talk about the salvia's water regime - from the picture I gather you that what you have there is heavy soil. This means moisture stays in the ground long after watering, which brings me to the bright color of the leaves, salvia mexicana isn't usually that bright. That's not to say it's a bad thing, just keep an eye on it. I have no idea how, when and how much you water, but the Salvia enjoys soil with high drainage ability, so if the soil is heavy you should increase the gaps between watering intervals. Try watering every two- three days. Besides that your salvia mexicana looks great! And you can definitely prune to make it look even better. Follow me...
What would you like to achieve? I have a feeling that you and I have the same goals :-) A strong healthy plant that will bloom and smile back at us, In order to achieve that, follow the colored markings in the picture:
1. Orange circles - The flowers should grow and bloom , this area will soon become blooming clusters. When? after the flowers dry up you can prune them. (Again, keep an eye on the water, to much water often reduces flowering)
2. Red marks - Cutting at these points will prevent your saliva from breaking. These are mostly side stems that are already touching the ground. And the main stems (those that have blooming clusters above them) should be pruned where I marked. When? now.
3. White arrows - Show you the stems you should cut in order to reduce their weight.
4.Purple square - Shows the potential braking point you wish to avoid.
What will happen? At every cutting point, new multiple growths will sprout and the stems will get thicker. This will enable them to support the shrubs new growth stage and your salvia will grow strong.
When to prune? Twice a year actually. Now (spring) is the best time. Salvia Mexicana can actually be pruned throughout summer, but only for maintenance reasons like removing dried flowers, cutting edges or broken stems etc....
The next time you should prune hard or cut back will be in early fall. Winter is not a good time to prune your salvia, but cutting it during fall will allow new growth to sprout in good shape and good time for winter. And the cycle goes on until next spring's prune.
Where to prune? Make the cut just above the node, leaving 1/2 an inch above it. Try to prune only above nodes that already show new leaves growing. It's always better than pruning above nodes that haven't shown new growth, although when pruning salvia mexicana in spring - everything's a go.
Done. I hope that helps, and please send me pictures in four weeks time so I can see how things have developed. :-)
Providing general guidelines on 'how to prune' any kind of plant is important, but there are plants, and there is YOUR plant. So despite there being loads of information on this site, I also provide specific and personalized support. If something wasn't clear, or you're not sure of your plant's name, simply send me a picture with your question. Use the form below and I'll get back to you. In the mean time, see what others are asking.