I would plant a dwarf evergreen, perhaps Juniperus procumbens 'Nana' which doesn't grow very tall but spreads out, and can cover some of the "rust" that sunflowers eventually succumb to at the bottom of the stems. Perennials that go well with sunflowers include Agastache cana (Hummingbird Mint), Lavandula spp. (Lavender) and Hollyhocks. As to deciduous trees, you don't want something that grows too high and will shade your sunflowers. A compatible deciduous shrub might be Chenault Barberry (Berberis x chenaultii) or Creeping Cotoneaster (Cotoneaster adpressus).