using & re-growing woad

 I have a woad plant that looks just like the "Woad plants in their
first year" photo on Wikipedia. [1]

I'd like to try to make some dye from the leaves [2], but I'd also
like it to produce seeds next year.  (AIUI, woad normally flowers &
seeds the second year.)  Does anyone know how much foliage I can clip
off & still have it survive to next year?

[1]
https://secure.wikimedia.org/wikipedia/en/wiki/Isatis_tinctoria
[2]
http://www.woad.org.uk/html/extraction.html

Thanks,
Adam