Rosemary Bush Very Susceptible to Dieback if Overwatered

Rosemary dieback from reader

Q. Can you diagnose what is wrong with my rosemary bush?  We have two rosemary bushes which are less than three-feet apart.  One is beautiful; the other has die-back in the center.  The one that has the die-back was replaced last year because the previous one had the same problem.      Both of these plants receive the same water and the same fertilizer.  When I replaced the rosemary plant last year, I did a 50/50 mix of compost and native soil.  I don’t see any pests on either of the plants.  Any suggestions?

A. I looked at the pictures. There are no specific pests that would cause this type of damage on rosemary. The most likely culprits would be physical damage to the stems (broken stems) or root rot or collar rot from either planting it too deeply or keeping the soil around the one with dieback too wet.

Healthy rosemary of reader

            If the soil was heavily amended with mulch or compost and it was planted in this then it is possible it could have been planted too deeply while the other one was not. Sometimes plants will sink in the planting hole if the soil is too "fluffy" at the time of planting.

            This sinking and soil consequently coming in contact with the stem due to follow-up waterings, can cause the main stem or side stems to decay and then entire stems can dieback.

            I would carefully lift the plant out with a shovel and examine the main stem or trunk and side stems and see if they were planted too deeply. Rosemary is very susceptible to collar rot or keeping the soil too wet. If the stems are rotten, dispose and replant in a new location.