For truly HARD to extract species, I recommend trying the protocols found in the following two publications. The Hughes and Galau procedure is much faster than the Baker et al protocol. However, the Baker et al protocol has been successfully used with grape, a plant very high in tannins. You might want to look at Gambino et al paper as well.
Article A Rapid and effective method for RNA extraction from differe...
Article RNA and DNA isolation from recalcitrant plant tissues
Article Preparation of RNA from cotton leaves and pollen