Rosemary Dying Usually Soil Problem

Q. Can you tell me what is killing our rosemary hedge, by inches?  The plant is 15 years old.  Other rosemarys of the same age on the property are still thriving.  Any help would be appreciated.  Picture attached. Thank you

A. There are a few insects such as spittle bugs that get on rosemary but there are not that many diseases that affected except some of the soil borne fungal diseases. In other words, these are disease organisms that are already present in the soil but they need opportunity in order to take advantage of rosemary. When soils remain wet, this stresses rosemary and makes it susceptible to these types of diseases. The usual advantage these diseases get is because soils are from plant stress because soils are kept too wet from frequent irrigations or the soils do not drain water fast enough before the next irrigation.

Rosemary dying is usually a soil or irrigation and drainage problem.

These diseases attack the roots of the plant and cause dieback of the top similar to what appears to be drought. It appears to be drought because the roots are dying and they can no longer take up water to the tops so, in fact, it is drought but drought caused by too much water present around the roots.

Like many Mediterranean plants, rosemary does not like wet soils in the summertime. They can tolerate wet soils as long as there is drainage and the soil has enough time to dry out between irrigations. These types of diseases frequently start at low spots in the irrigated area where water collects or puddles. Usually plants growing where the water has drained to low spots remain healthy.

Plants that have extensive root rot from soil disease organisms frequently will pull from the soil fairly easily or are loose in the soil when they are pulled from side to side.

The solution? The usual solutions are to water less often or improve the drainage in the soils or both. Rock mulches around rosemary will cause the soil to become more compacted and not drain water well. Organic wood chip mulches help to keep the soil "fluffy" and improve drainage. Organic wood chip mulches help to keep soils more moist so the frequency of irrigation typically has to be less. 

If this problem was caused from soil disease organisms it will be difficult to reap plant rosemary in that soil again. You might have to remove the soil from that spot and replace it with an amended soil before you replant another rosemary in that same location.