Lavender is a tough plant and is extremely drought resistant, once established. However, when first starting you lavender plants, don't be afraid to give them a handful of compost in the planting hole and to keep them regularly watered during their first growing season. They spread easily through underground root system. I've had a summer where it died out quickly and didn't produce much. This is caused by an unusually full humid summer.Prune them regularily and in the spring cut them about 1/3 down. This will help growth and spread. If you get complete winter dieback, prune down to new growth. Make sure you wait till you see new growth though or they will give up and not do anything for you