Garlic Mustard (Alliaria Petiolata)
This is a plant I have always known as Jack-by -the-Hedge and it almost exclusively grows along hedgerows and woodland edges, mainly on chalk soils so it is not uncommon down here in Dorset.
It comes in to flower in April and can hang on in to July. In May it can line a hedgerow with these white clusters of small, four petalled flowers.
Although easily passed by without a second glance, this plant is a good place to look for early insects. It is an important food plant for several species, especially the Orange Tip butterfly which emerges in to its flight stage to coincide with Garlic Mustard coming in to flower.