Miscanthus – or “elephant grass” – is a perennial that can be harvested annually after about 3 years and requires no nitrogen fertiliser. It has woody stalks up 4 metres, leaves that rot back into the soil, grows anywhere, improves the soil, and may even be flood resistant. Each hectare produces about 50MWh of heat per annum, the equivalent of 5,000 litres of heating oil. You need a biomass boiler that can cope with a slightly lower combustion temperature than wood, but Miscanthus outperforms short rotation Willow coppicing (SRC) as a sustainable biomass crop.
You can see a presentation on the topic here.