Ma Huang Harvesting and growing in pots

How do I know that my ma huang herbs (ephedra sinica) are ready for harvesting? They seemed to have stopped growing at about 3 inches and are getting very frail and yellow looking. Does that mean they are getting too much water? I am trying to grow them in shallow pots in my sun room.
Also, how do I harvest these herbs and prepare them for use?

Ephedra sinica is a perennial and perennials often do not grow to full size the first year. However it is a bit early for the plant to turn yellow in early summer. The reason could be over watering, since this species is adapted to dry conditions with good drainage. Too much water after the spring could kill the roots.
The shallow pot might also be the problem, because the soil may be depleted of nitrogen already. The soil may be the wrong pH (it should be just below neutral) and thus make nutrients unavailable for this plant. I would suggest gentle repotting into fresh soil.
The plant is a shrub and the branches are cut for harvest at any time. Never harvest more than one third of the plant at a time and wait till this has regrown before harvesting from this plant again. You dry the twigs and then powder them or use them for concoctions, tinctures or liquid extracts. May I warn you that the plant has a powerful effect on the heart and should never be used by an amateur. Consult an experienced, licensed herbalist. It is combined with various other herbs for the specific uses it is renowned for.