When and how to harvest Rosemary for stems and essential oil
Most farmers do not harvest rosemary for the first 15-21 months after sowing or plants establishment. From the second year onwards, they harvest once or twice a year.
As a rule of thumb, if you grow rosemary for plant material, you shall prefer young shoots and you may have to harvest two or more times per year, in order to promote new growth.
If you grow rosemary for essential oil, knowing when exactly to harvest is very important for the quantity and quality of essential oil. It requires extensive research and constant “trial and error” procedure. First you have to decide if you plan to produce essential oil from leaves, flowers, stems or all of the above (distill the whole plant). Stems and leaves -when distilled- can normally produce essential oil of higher quantity but of lower quality, compared to the flowers.
Many professional rosemary growers have reported that harvesting stems and leaves exactly before the flowering results in essential oil of superior quality, while harvesting stems and leaves after the flowering often results in smelly product. However, this is just a common reported pattern that should not be followed without making your own research. In other cases, growers wait until some plants bloom and then they collect only the flowers, which -after distillation- give a high quality essential oil, but normally of very low quantity.
In many countries, rosemary is harvested mechanically via machines attached to tractors. Those machines cut the stems in predefined length in order to promote new growth. In developing countries, farmers harvest rosemary by hand. They cut the stems with special knives and they often let them dry in the sun for 2-3 days.
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