Why not let nature do the work and let plants decide where they want to grow. If seeds germinate and grow they are generally going to be happy like this Cerinthe. If they pop up in the wrong place you can always pull them up.
Rupert Bannister is a fully qualified horticulturist, RHS tutor and assessor, with over 30 years of gardening & horticulture experience in the UK and overseas. Working with businesses and homeowners in Devon and across the south-west, he helps people get the best from their plants and gardens.