The Time of Leaves

You can harvest the green parts of plants at any time of the year, but spring and early summer are when most things are at their best; before flowering and setting seed, when the energy of the plant is drawn out of the leaves to be used elsewhere. If you are gathering herbs, be selective; choose the leaves that are bright and vigorous looking, from plants that are not yet in full flower.
There is a paradox here, though. A plant that is under stress – growing in poor soil, in dry or shady or windy conditions, for example – is often richer in active constituents than one growing in a lush, rich environment. In other words, the biggest and juiciest-looking leaves may not be the most useful medicinally. This is why farmed herbs are sometimes seen as inferior to wildcrafted ones. Ultimately, of course, you would have to get them analysed to know for certain. A good rule of thumb is to gather a variety of plant material from various locations, and that way you’ll be likely to end up with good quality herbal remedies.