|What causes the tops of my tomatoes to remain green and hard while the rest of the tomato is fully ripe?|
|A couple of things may be at play here. Have you noticed circular or radial cracks in the tops of those tomatoes? The cracks often will interfere with even ripening. The best thing you can do is to keep the soil evenly moist - mulch will help in this respect. The mulch will also help keep the fruit off the ground should they droop or drop. Try to find tomato varieties that are crack tolerant or resistant.
There is a condition called "green shoulder" which results when there's not enough foliage to protect early fruit from summer sun. If you prune your tomatoes, leave more foliage around bud and fruit clusters, and this, along with the suggestions mentioned above, should alleviate the problem. Hope this helps!