Hi Mazie,
I had a good look at your photo link,that I wish I'd seen earlier, it's a lovely garden you have there and I can see that even grass would struggle to grow in the heavy shade of those trees
I wondered if the crowns of those trees could be thinned, to allow more light through, or maybe you could alter the shape of border to incorporate it in with the rest of it and introduce some woodland plants, which would thrive there, Primula's, I think would do well