Lunaria annua. Lovely pinkish-purple large heads of flowers in April that provide nectar for some of our early butterflies - especially Orange Tips, and can be used as their larval foodplant. Inspite of it's latin name, it is a biennial, and easily germinated if the seed are sown in damp compost in Autumn (or in the ground where they are to grow) and left outside through the winter as they seem to need a cold spell to break dormancy.
The seed pods form the lovely 'silver penny' heads beloved of winter flower arrangers, also available from us - see separate category in this section.