Success in drying herbs starts with a good harvest – it is important to source fresh plant material free of insects and damaged parts. For more about harvesting guidelines see Chapter 5 in The Medicinal Forest Garden Handbook. If fresh plant material is to be preserved for later use then drying is an economic and effective way forward. Drying is the removal of water in order to prevent further deterioration and maintain key active constituents. In actuality, some water remains in the dried herb but, for good storage, as little as 5-15% moisture content is the ideal aim to stop fungi or bacteria developing.
Plenty of air flow
People often emphasise the importance of heat for drying but the key element of successful drying is plenty of air flow to draw out moisture. Ideally, the drying situation is indoors or a covered area which is out of direct sunlight and is airy. So an even room temperature of 20 degrees C is sufficiently warm for most drying. An alternative may be an airy loft. Additional heat up to around 40 degrees C can speed up drying but needs to be controlled to avoid losing aromatic constituents. The length of time needed for drying can vary from a few days to a few weeks. More detailed advice depends on the plant, part and key constituents. Note that some things do not dry easily – especially solid items such as flowerheads and roots which may rot before the core parts are fully dry. In general, thicker plant parts need longer to dry, and benefit from being chopped up to ensure thorough drying.
For small quantities of herbs you can tie bundles by their stems inserted upside down into brown paper bags and hung on hooks till crispy dry, so they snap when broken. For larger quantities you need a dryer. There is no one-size-fits-all dryer, especially as the amounts of plant material can vary from one situation to another. Whatever the amount to be dried you need as large a surface area as possible so that the plant material can be spread out thinly. For limited quantities there are electric dryers which are sometimes sold for drying sliced mushrooms or fruit rings. Based on perforated trays they provide warmth and propel air through the material with a fan. If possible purchase the version that offers both temperature control and a timer so that you can manage the drying process more effectively.
However the surface area in electric dryers can be rather small and airflow may not be evenly distributed. A more cost-effective option may be a hanging herb dryer which offers more space with large circular ‘shelves’. This can be hung from roof rafters in a loft, providing a good breezy location. Lining the shelves with brown paper or newspaper may make it easier to put plant material in and to remove it when dry.
For larger amounts of herbs it is worth looking at home-made options. For example, making wooden drawers with perforated bases is ideal, as are stackable wooden trays. For reuse, the surface part needs to be easy to brush clean of dried herb, or can be lined with a sheet of newspaper. Combining these drawers or trays with a convection heater providing a flow of warm air probably provides the best solution for larger amounts of herbs.
The best dryer we made so far
For use in the medicinal forest garden, at Holt Wood Herbs, we wanted to be able to cope with harvests that could happen throughout the year, such as buds, leaves, flowers, roots and bark. We were usually dealing with limited quantities of up to several kilograms weight of fresh plant material. So our stacking plastic bakery trays have worked well – each tray is approximately 600 x 400 x 120 mm deep, made of food grade plastic with perforated base and sides. Up to 6 or more trays can be stacked on a small wheeled trolley which is easily manouvered around and a lid placed on the top tray to prevent dust. The trays are loosely lined with brown paper and plant material is evenly distributed in a thin layer. To protect from insects a black plastic sheet surrounds the whole stack and can be held in place with a velcro fastening. To speed up drying we use a small fan under the trolley and a heat pad (such as used for pets), and the black plastic surround ensures that the warmth and air are fed through the trays above.
Quality and keeping
Good colour and aroma are indicators of well-dried herbs. Remember that dry plant material may be difficult to recognise so it is always worth putting in a label with species name, part and date. Check if you can bend the leaves or stems which means there is still too much moisture, they should crumble and break. Some plants which are very high in moisture levels, such as comfrey, lemon balm, nettle, are quite a challenge to dry well. Aim for crispy dry and your herbs should keep for longer. Well-dried plant material lasts 1-2 years before it should be replaced (possibly more for larger pieces such as whole leaves and roots). See The Medicinal Forest Garden Handbook for further details on making preparations and storage suggestions.