Creo que se refiere al momento en el que se ha cortado el bamboo, no a la forma.
Mira esto:

http://www.inbar.int/publication/txt/INBAR_Technical_Report_...

http://www.panplanet.com/garden/plantmoon.html:

As a general rule of thumb, when the Moon is waxing, plants develop leaves and 'above ground' systems, when it is waning plants develop their root systems. Planting leafy crops that grow above ground are best sown at waxing moon and those that will require strong root systems or grow below ground should be sown after full moon, in the waning phase. You will have noticed there are 8 phases listed. The Moon's Quarters and Intermediate phases are important as each represents a turning point at which human beings can act or intervene with nature to promote appropriate results.

http://www.haydenplanetarium.org/hp/vo/ava/avapages/P0409moo...

After Full Moon, the Moon is waning, the illuminated portion grows smaller and smaller.

http://www.survivalschool.com/articles/weather/waxing_waning...
Another way to identify a Waxing or Waning Moon is to look at which side is "incomplete". When the Moon is illuminated in such a way that the side facing West is "incomplete" the Moon is Waning (getting smaller). It the side to the East is "incomplete" the Moon is Waxing (getting bigger)

remember "crescent" in English is just the shape it has nothing to do with whether the moon is getting bigger or not. So Menguante = waning, and there are three stages, half moon, waning crescent moon and new moon, then New moon, waxing cresecent moon and (gibbous moon (nearly full)) and full moon.