My guess is that your soil has too much clay content in it for rhubarb. It does this in heavy soil. I tried to grow it in the ground here, I have very rich, dark soil but it borders on high clay content. My plant suffered until I removed it and potted it up in large terracotta pot with light potting mix. Now it's doing brilliantly. The terracotta keeps the roots cool and stops the crown from rotting. But it has to be watered daily potted up like that as it wilts very easily. I ended up growing my rhubarb and strawberries in pots because they just didn't like the heavy soil. They are producing wonderfully now and make a nice display together. An alternative could be to add a lot of organic material to your soil and plant it into a built up mound for free draining situation.

- Clara