Grass will not grow under the maple by the street and I've been trying to get something else to grow. Ferns and hosta have died when I couldn't keep watering. Right now I have a bed, about 16 square feet, abutting the sidewalk with nothing in it except mulch. I'm considering brunneria macrophylla and geranium oxanianum. Should I choose one or the other or combine them? What would look better? If I use both, should I alternate the plants or keep each kind together? I'm thinking about putting down cardboard and covering it with a mixture of topsoil and compost and plant in that. If I do that, how deep should it be? I'm hoping the cardboard would keep keep the tree roots from sucking up all the water.