In temperate climates like Austria, Persicaria tinctoria can be harvested twice a year, once in early to mid-July and once around 5 weeks later, in mid-to-late August. A third harvest may be possible in autumn, but as autumn in Austria can quickly set in, along with colder temperatures, the indigo concentration may be very low. It is therefore more advisable to leave the plants to go to seed for next year’s crop, as the cycle starts again. Since this year’s long, cold spring delayed the growth of the indigo plants, we harvested at the end of July.
The ideal time for the first harvest is when the leaf to stem ratio is around 1:1 and the rows of Persicaria plants are densely closed. Indican, the precursor to indigo, is found almost exclusively in the leaves of the plant. When a bruised leaf shows a deep blue mark or edge, this is an indication that the indican has developed in the leaves, and they are ready to harvest.
For the first harvest, we cut the plants by hand with a sharp knife, about 50 cm high, cutting the stems several nodes above the ground. The plants will sprout from these nodes for the second harvest, which should be completed before the plants flower.
For the first indigo extraction, we harvested 3 rows (80 cm wide and 10 m long) of Persicaria tinctoria, giving us 120 kg of plant material.