Bird's foot trefoil is native to poor grasslands around Europe and is considered invasive in some areas. It's supposed to be hardy, but I have not had luck with it. It grows best in sun, in well drained, poor, lime-rich soil. It dislikes wet, acidic soil with a high nitrogen content and disappears from grasslands if fertilizer is applied. It has been tried in land restoration projects here and has proved effective and hardy in those situations. Maybe the soil in my garden was too rich.