Dyer's woad (Isatis tinctoria L.) is an herb used as a source of blue dye called indigo. This short-lived perennial grows 1 to 4 feet tall with a 3 to 5 foot taproot. Bluish-green leaves are covered with fine hairs and grow in a rosette shape. Dyer's woad begins growing this rosette in the fall it is planted. It shoots up 7 to 8 woody stems with four-petaled yellow flowers that are 1/8 inch wide in the spring. The new fall growth may need protection in order to survive severe winters.
Clean out any weeds and dead foliage from around the dyer's woad plant. Old leaves and garden debris can act as a safe place for plant diseases and insects.
Water the ground well before the temperature drop down to freezing. This ensures that there is enough moisture in the soil to prevent the dyer's woad from drying out during the winter.
Mulch after the ground begins to freeze once a couple of hard frosts have hit. This will help the dyer's woad stay dormant and not put on new growth due to warm soil. New growth is susceptible to damage by winter temperatures.
Spread 3 to 6 inches of straw or hay mulch around the dyer's woad plants. This will help insulate the soil. Do not pile the mulch on the plant crowns and leaves heavily since this will only encourage rotting. Keep the mulch layer to the depth of 3 inches directly over the plants.
Remove the mulch once growth begins again in the spring or when all danger of hard freezing has passed.