Japanese indigo, (Persicaria Tinctoria), Is one of the most easily grown sources of the beautiful indigo blue dye.
It is also one of the hardiest of the indigo bearing plants.
This listing is for 50+ seeds.
Despite my best efforts, occasionally a few immature seeds may wind up in the packets.
I add plenty of extra seeds to each packet, just in case...:)
These seeds are often best started indoors a few weeks before the final date of frost in your area.
This seems to be the best way to give Japanese indigo enough time to mature, especially for parts of the country with shorter growing seasons.

Usually these Japanese indigo seeds will sprout in about 10 to 21 days, depending on the temperature, and amount of moisture in the soil.
They seem to germinate more readily when not covered with much soil.
In fact, pressing the seeds firmly onto the soil surface and maintaining moisture is the best method I've found for good germination.

I prefer to start them in containers so that I have more control over the temperature and moisture requirements.
I find that using a spray bottle is a good way to keep the soil moist without dislodging the little seeds.

When they're planted directly outdoors they may face a few natural dangers. The seeds may be washed about by heavy rain, dried out quickly by sun and wind, and they seem to be a favorite snack for sparrows...:)

When all danger of frost has passed, plant the Persicaria, (or Polygonum), seedlings outdoors in a spot where they'll receive many hours of sunshine each day and give plenty of water.
Persicaria tinctorium seedlings grow very quickly, and given enough space, each plant will spread like a little bush, several feet tall and wide.

Growing and dyeing with indigo is a fascinating process!