Thunderbird is right - the tall stalk with the flower has gone to seed and should be cut out at the base as it will sap the energy of the plant. It's best not to harvest too many stems from any one plant to conserve its ability to survive throughout the year. Try to stop harvesting around the end of June unless it's a really prolific plant. Incidentally, if you give them a good helping of old manure in the autumn when the stems have rotted down or before they start to sprout in spring, they'll reward you with a good crop.