Optimising flavour in rosemary

However it’s grown, variety, light quality and availability are the main factors influencing rosemary flavour.

Optimising growing conditions can help improve the flavour of rosemary, and these are our top recommendations:

• Use appropriate varieties for your environment
• Maintain daytime temperatures below 29°C during growth
• Use deficit irrigation to improve the flavour
• Mild salt stress can improve flavour, but extremes will damage yield
• Maximise light availability and quality, using supplementary lighting with high UV-B, low red:far red ratio and high light intensity
• Store cut herbs in cool, dark conditions

There are several species of rosemary, but only Rosmarinus officinalis is grown on a commercial scale. There are different classifications of rosemary based on the chemical composition of the oil, and these are broadly specific to global regions. These include 1,8-cineole (eucalyptol) types, camphor, myrcene and verbenone types.

Rosemary naturally grows in well-drained and alkaline soils, and can be exposed to certain levels of drought and higher temperatures without significant damage. The growing season for rosemary typically runs from March to November in the UK, but potted herbs can be grown under glass all year round.

Flavour in rosemary is primarily the result of monoterpenes, which are produced by the plant to protect it from UV damage and high temperatures. There are around 100 compounds in rosemary essential oils, many of which impart some flavour, but there are 11 main constituents. All the main flavour compounds are monoterpenes and strongly regulated by light and temperature conditions.

Eucalyptol, borneal and camphor are the main components of rosemary flavour. Verbenone-containing varieties of rosemary are considered to have the best taste, with verbenone thought to be one of the characterising flavours. However, it’s not found in all varieties and its presence is not essential to a recognisable rosemary flavour.

Essential oil content and composition can directly affect flavour, with low oil leaves seen as watery and insipid, which may unmask fewer desirable flavours.

These are the factors you can influence during the growing process to ensure your rosemary has a desirable flavour.

Choose the right rosemary varieties for your growing conditions, depending on whether you’re opting for potted, cut, glasshouse or field production. High verbenone content (10–15%) may improve the flavour without the need to change growing techniques or compromise yield.

Ideally, use a separate fertiliser regime from other plants growing on the site, and one with a comparatively higher iron content (particularly if you’re growing rosemary on high pH soils), as it will increase the contents of verbenone and improve the flavour profile. Rosemary also responds well to nitrogen fertilisation. Avoid over-fertilisation and high irrigation, which could damage the growing plant, while under-fertilisation can affect the plant’s ability to produce flavour compounds.

Research using naphthalene acetic acid (NAA) and cytokinins can lead to smaller plants with higher essential oil yield, which, at a higher planting density, could be used to create a better flavour. Benziladenine (BA) may increase the overall content of essential oils and can be used in a range of herb crops.

Overwatering limits growth in rosemary, while some deficit irrigation can be used to improve flavour but in excess may substantially reduce yield and adversely affect flavour. Rosemary is moderately salt-tolerant, and salt stress can improve its flavour, but more research is needed to understand its effects.

Using supplementary lighting with high UV-B, low red:far red ratio and high light intensity will help develop a strong typical flavour, even if only used for a short period of time during the day. Glasshouses cut out much of the UV, so supplementary fixed UV lamps or lights attached to booms can be used to improve flavour.

Reducing the effects of short days helps maintain flavour quality in rosemary. This is particularly important for glasshouse growers during winter months, when supplementary lighting will be needed to maintain at least a 12-hour day for flavour development and normal crop growth.

Where possible rosemary should be grown at temperatures in the range of 16-25 °C for optimal flavour. Temperatures below 20 ˚C will result in higher camphor, and temperatures above 20 ˚C will result in higher eucalyptol. This is hard to control for field-grown rosemary and expensive to achieve in glasshouses.

Unlike most plants, the flavour of rosemary can be improved after flower initiation. Varieties that initiate flowering more quickly, and for a longer period of time, are a good choice for maximising the duration of high flavour quality in cut rosemary. There are minimal differences in flavour between older and younger leaves on the same plant.

Long days and falling temperatures at the end of summer lead to a higher accumulation of verbenone in verbenone-rich varieties of rosemary, increasing flavour. Rosemary for the cut market is harvested several times per crop. A second cut of a rosemary plant leads to a more intense flavour, but further cuts can increase bitterness. The crop from first and second cuts have a greater oil content, but initiation of flowering increases oil content and may counteract this.

Store rosemary in temperatures between 0-5˚C to minimise flavour compound losses for cut and pot rosemary plants. Up to half the flavour compounds are lost 36 hours after picking in substandard conditions, so try to limit times between harvest and marketing.

Consumer perception

Field-grown rosemary has a more intense flavour characterised by pine, floral, menthol and bitter notes. Pot production is characterised as having more sweet and grassy-green attributes. Multiple harvests from a rosemary plant will lead to higher intensity of flavour, pine and floral notes, but it also increase bitterness which may not be desired by the consumer.