Among the fibre plants, nettle ranks as high-abundance low-investment.
It's free to take and grows plentifully in the wilderness, becoming available to harvest between late Fallow month until the beginning of Fall month.
I'd say go ahead and skip retting; nettle fibre is the quick fix option when you need yarn.
In terms of numbers:
- 50 is the maximum number of nettle patches generated on the local map at a given 64x64 sector of suitable terrain
- 10 is the maximum number of plants per tile that make up a patch
- 15 nettle plants are required to make up 1 bundle of fibre (according to the changelog for URW version 3.72)
- a single local map sector may yield at most 500 plants, which is enough to make 33 bundles of nettle fibre
In contrast, the fibre crops require high investment and have limited availability.
- hemp and flax seeds are obtainable only from settlements (whether through theft or barter)
- growing your own requires a shovel and an axe: you must clear tiles and chop wood in order to prepare soil
- flax seeds must be planted before Hay month at the latest in order to have enough time to harvest the crop before it withers in Dirt month
- flowering flax will not yield seeds on threshing (as of URW version 3.72)
plan accordingly if you wish to re-seed your agricultural fields - re-seeding before Dirt month allows the crops to sprout as soon as possible in next year's growing season
hemp - Soil month, ~3 weeks before summer season
flax - Swidden month, ~12 weeks before mid-summer - retting can take anywhere between 5-25 days to prepare depending on water temperature when this action is available
the earlier you bring in the harvest, the shorter the soaking period - drying plants takes about 1 week
After harvesting, the soonest you'll end up waiting could be nearly 2 weeks before you can process plants into fibre.
Although you can work on other projects in that period, certain activities can be time-sensitive such as drying meat or fish (which becomes possible in Dirt month).
In order to craft 33 bundles of fibre to compete with a highly productive local map sector of natural nettle...
- At 5 plants per bundle of flax fibre, you'll need 165 flax plants.
If your Agriculture skill is high, 1 fistful of flax seed is more likely to grow the maximum of 20 flax plants in a patch.
In this case, a minimum of 9 tiles blessed by farmer-RNGesus will match the nettle output of a sector blessed by forest-RNGesus. - At 8 plants per bundle of hemp fibre, you'll need 264 hemp plants.
1 fistful of hemp seed may grow up to 15 plants in a patch, so the equivalent is 18 high-productivity tiles.
But if you're neither lucky nor skilled, your crop patches will look a lot thinner and these best-case scenarios will be further removed from UnReality.