There is a clearing in the Wildwood where the vines turn brittle and become covered in thorns. While impossible for me to visit, I've heard that the Hollows that root there are afflicted with a similar transformation.
Heartstone is effective on units that typically occupy the front line of a floor. Front line units are the ones that take most of the damage, leaving the role of dealing damage to the units safely tucked behind them. Heartstone is also good on the Thorned Hollow because it starts out with very low health. The extra Health provides a buffer, giving you the time to heal it before it dies.
Runestone is effective on units who have the Rejuvenate affix. Since most of the sources of healing you will be using come from spells, Runestone will also provide Armor as the unit is healed.