The main reason for conifers going dry and brown in the centre is over-dry conditions.  Some, like Chamaecyparis and Thuja, need a regular supply of water if they are to grow well, and they can be badly affected by dry, windy conditions.

 

To ensure that your conifers do not develop dry foliage, keep the plants well mulched with thick layers of compost, peat or pine-bark chips (this will help retain soil moisture) and then water them deeply and regularly.  In very hot, dry or windy conditions, watering the foliage will also help prevent it turning brown.