In a hurried search, found more in Japanese than in English, so I used Google translate for rough copy, then [edited glaring omissions] — you could probably try using Chrome or other auto translating browser.
All of these references mentioned that flax is best grown in cool summer regions like higher altitude or northern countries.
— very often, well-researched blogs are best source of information — the blogger posted photos that may be useful for reference, too
...According to this website for a flax SEEDS company, DTM for flax is about 120 daysフラックス(亜麻)を収穫しました。 - ながくて風土
I harvested flax — long and climate
https://blog.goo.ne.jp/hudo-mau/e/a791c ... 2f2ae60952
[Apparently Flax stems are cut-harvested] after flowering (about July) when taking fiber. Put it in water to rot the fiber and beat it to get it.
When harvesting seeds, [the flax are] harvested after ripening, and about August is appropriate. I am a little early, but [the seed pods might pop and scatter] and [massed weeds are a getting to be a mess] ...! Seeds can be dried and stored for use.
At one of those ask a question website, I found a long, somewhat discouraging answer to someone asking when to harvest flax for fiber to be woven into a fabric. But a few key points (to me anyway) —一年草亜麻の育て方|亜麻の里
(How to grow annual flax | village of flax)
https://www.amanosato.jp/news/ama_20160426_01.html
In Hokkaido and the Tohoku region (latitudes 43.2°N to 47.3°N), flowers will bloom even after sowing seeds until mid June or so. In the case of other areas, I do not know because I have not [tried growing], but I think that it is fine if it is about the same time. Because flax is a plant that is more dependent on [daylength] (the ratio of day and night time) than temperature. However, if the season is late, the height is low and the number of flowers tends to decrease.
That last part was particularly interesting to me because I ran into the single-ply thread not being strong enough to weave with before — so that was an “ah ha!” moment.[after] the flowers [bloom] and [before] the stems die, the whole stem is pulled out, 20 to 50 stems are bundled upside down and dried, and then the seed[pods] are [removed].
To [obtain] the fiber next, you need to know the following:
1. There are many species [that can be grown for] fiber from stems, but when taking out fiber from plants, [only about ] 8% to 10% of the weight of one stem
[represents] the fibers [that can be made into] thread.
...
5. Next, [in step 4 (with spinning wheel), one] can only make a single[-ply] thread (tanshi). [To weave a fabric with just a single-ply thread, [there are two methods — applying] Sizing (natural glue = water soluble) [and] twisting two single[-ply threads] into a double[-ply] yarn. If you want a twin thread, [] this can be [made] by changing the direction of rotation of the [spinning wheel].
Oh! I’m thinking in Step 5, I might be incorrectly using the terms ‘single-ply’ and ‘double-ply’ as well as the words ‘thread’, ‘yarn’ and possibly missing ‘rope’. But I woke up in the middle of the night and got caught up in this, and now my brains are exhausted just as it’s getting light outside I’m going to go back to sleep for another hour or so. If it was confusing, let me know, and I’ll try to fix that later. Or if you understood and can correct the wording, please do because this is going to bug me.