(Last year the Sunshine Coast Spinners and Weavers Guild had a project growing and dyeing with Japanese indigo. (See “Sea Foam Blue” blog on October 5, 2013.) Some members did some further dyeing and obtained a darker blue using Spectralite in a technique described by Rebecca Burgess in her book “Harvesting Color”. This week at our meeting we shared seeds and we’ll no doubt have another blog in the fall about our further explorations with dyeing with Japanese indigo. Heather)
I suggest starting and treating Japanese Indigo much the same as tomato plants. I assume that they like it warm for starting and growing, as they are a tropical or semi-tropical plant. I usually use a starting mix with good drainage and often sprout the seeds between damp paper towels to ensure viability before investing time and energy. Once the seedlings have a few leaves they should be transplanted into a richer soil mix. I prefer a good potting soil with a little bone meal added. You can make your own mix with compost, vermiculite, sand, peat moss, soil and a pinch of bone meal, but this in not sterile which can cause problems.
Keep the plants well watered and in a warm sunny humid location. I put a plastic bag greenhouse around plant growing lights, in a sunny window near the wood stove. You do have to watch out for mould and other problems such as damping off, depending on the soil used and other conditions. In other words, these little plants do require a certain amount of baby sitting at this stage! If they grow well you might need to transplant them into larger pots and add some fertilizer etc. Now it will just be a matter of playing wait and see with the weather.
Once outdoor temperatures are warm the plants can be transplanted out into the garden. Some years, here on the West Coast, that can be as early as the end of May. Choose a sunny spot with rich soil and good drainage. I usually supplement the soil with compost, bone meal and a light application of all-purpose fertilizer such as 6-8-6. If the weather is stubbornly cool and wet and the plants are just too big to keep in the house in pots, you might try planting them in large pots in the greenhouse or otherwise out in the garden under some clear plastic protection. Last year my Japanese Indigo plants stayed viable in the garden until we got our first frost. They might not have been making much blue pigment at those temperatures, but they certainly are not as sensitive as tomato plants. No fear of blight either!