There are two schools of thought on many of the herbs potency.
One school believes that first sap flow is the best, most potent time of year, the other believes that right after sap stops flowing is the most potent time.
When you study the life of a plant, how the plant derives the energy and nutrients to thrive you can surmise that neither school of thought is incorrect.
When spring arrives and sap begins to flow, the stored nutrients from the roots is brought up through the stems to stimulate leaf buds to open and grow.
When fall arrives and the sap is pulled down into the roots for winter storage, the roots swell with nutrients and hold on to them, waiting for the next spring warmth.
Leaves loose their nutrients and thus the medicinal value as the sap leaves the leaf, in trees the leaf stem seals off not allowing any more nutrient flow to the leaf, as the leaf starves it changes color then turns brown and falls to the ground as the dried out leaf stem hardens and the seal becomes brittle.
When to harvest is actually more of a "what part of the medicinal uses are you needing right now?" question over a "when should I harvest for medicine?" question.
This is where knowledge of each herb is most useful and the more complete your knowledge the better you can make the all important determinations of time to pick, use it fresh or use if dried, decoction, tincture, essential oil, or some other method should be used for the particular need at the time.
This is why it takes so many years to become a true Herbalist, there are literally hundreds of medicinal herbs and not surprisingly, the culinary herbs are also medicinal herbs, even the needles of evergreens are medicinal herbs.
One of my Herbal books lists 450 different herbs, what each one effects, how it effects, the known side effects, what type of affects to expect and all the methods of extraction along with how to use those extractions to best effect on each malady.
You did your search correctly by looking for ephedrine content, since that is the drug you seek for your treatment. One other way to search would be "plants that contain ephedrine"
You could also search by "herbs for treating asthma" which might give some additional plants to research.
You are starting a long and very interesting journey, should you decide to continue herbology.