The first Woad seedling growing in an egg carton ~ Yay! Recycle!
Woad is a biennial plant - meaning that each plant only lives for 2 years before it dies completely. In it's first season of growth, this plant will form a rosette of leaves that lay close to the ground. They will usually overwinter without dying down. It's in this first season that you want to harvest the leaves for indigo, since in the second season, the amount of indigo is usually low.
In the second growing season, in addition to the lower growing mound of leaves, the plant will send up several flower stalks. I have read that these flowering stalks can sometimes grow as tall as five to six feet! Leave them be and they will produce seeds. Lots of seeds.
I'm going to let one plant overwinter (okay, maybe two) so I can see the flower spikes next season. I don't think I'll let them set seed or even plant it again because Woad does not produce a decent amount of indigo to make it worth my while. I have a small yard! I also don't want a lot of seeds to 'get away' as the plant can be invasive if you're not careful.
Dyer's Knotweed, on the other hand, contains a lot more indigo than Woad. Although not as much as it's tropical counterpart, the 'true' Indigo plant (Indigofera tinctoria), you can still get deep rich color. I also think it's an attractive plant, growing on upright stalks that resemble (to me anyway) the common annual Vinca, just without the flowers on the tips. Dyer's Knotweed does flower but to me the flowers are quite insignificant.
With Dyer's Knotweed, you can cut the stalks down close to the ground and strip off the leaves to obtain the dye. The plant will then send up a new shoot - sometimes you can get up to three harvests a year, but I think two is more typical. This plant usually grows from one to two feet in height.