Our penstemon  plant shoots turned brown, wilted and died off, can you give a reason for this and how to prevent it happening again?

The plants have most probably been affected by a wilt disease.

These tend to be soil-borne infections, gaining entry through roots that perhaps have been damaged by hoeing.

There is no real effective control, apart from removing the plants and some of the surrounding soil.

You could try spraying the plants with a systemic fungicide early on in the year as the new growth commences, failing that you will have to resort to the method described above.