You might try watering them once with a solution of 1 Tbs Epsom salts to a gallon of water at the root zone, not foliage. I've seen this green them up before, incredible.

Yep, epsom salts is worth a shot since the green you see is largely the working of good available nitrogen and magnesium (found in empsom salts).

One thing you said did catch my attention though:
"I prepared the bed last fall with aged horse manure and ground up oak leaves."

Assuming that the leaves were not composted, they are actually taking nitrogen from the soil until they are broken down. They are a good soil amendment for adding organic matter and improving soil structure; however, until they break-down completely, they will take away available nitrogen from your plants. Usually when people add things like leaves to the soil, they add extra nitrogen to compensate.

I wouldn't assume that the yellowing is from root damage, I think that would have more of a wilting effect on them - not a yellowing. What is more likely is that the nutrient / micro-nutrient levels are off - I'd look there first.