Some indigo species like woad (Isatis tinctoria) can be planted directly in the field, but most indigo species need to be grown in a greenhouse and later transplanted into the field, especially in a harsh climate like here in northern Austria. The best time to plant your seeds in a greenhouse is about 6 weeks before the last threat of frost in your area.
Spring has been a long time coming this year, and Indigofera arrecta and Indigofera suffruticosa have only carefully ventured to the surface next to the tomato plants.