I recommend going for a raised bed if you're concerned about it. The soil would need good drainage and I'm not sure it'd be good if the soil only 2 inches deep - even adding soil might not do much. Most raised beds are about 11", allowing more water and less harmful pests for your plants. Don't forget that the depth also depends on the type of flowers you want to grow. Plants grow more or less roots than others, so you'd need deep soil to ensure that they can grow roots to their full potential. Also the deeper roots go, the more moisture as well as nutrients a plant can gain. Plus, you wouldn't want your flowers drooping to the ground; it's an invitation for pests, so I'd go with the raised bed. Climbers will need a tipi of some sort also, if you're planning to grow them.