Step 1: Understanding the Concept.
In pharmacognosy, different parts of a medicinal plant, leaves, flowers, roots, or bark, are collected at different points in the growth cycle because the concentration of active compounds shifts with the season.
Step 2: Key Approach.
For bark specifically, the general rule is to collect during dormancy, when sap flow is lowest and the plant is not directing resources into growth, flowering, or seed formation.
Step 3: Detailed Explanation.
Pre-flowering and post-flowering periods both pull the plant's resources toward reproductive structures, leaving less concentrated in the bark.
The period just before leaf fall still has active sap movement through the vascular tissue, keeping the bark moist and harder to process cleanly.
Once the leaves have dropped, the tree or shrub is dormant, sap flow drops off, and constituents that were being used for growth tend to be conserved and concentrated in storage tissue such as the bark.
This dormant, low-sap-flow state is also when bark can be stripped and dried most efficiently, with the lowest risk of microbial spoilage.
Step 4: Final Answer.
The best time to collect medicinal bark material is after the leaf falls.