Throughout most of the year, including spring, summer, and most of the fall, you should continue feeding your
Heartleaf philodendron regularly. The only exception to this is if you notice that your
Heartleaf philodendron has received too much fertilizer or if you have managed to fertilize your
Heartleaf philodendron incorrectly, causing fertilizer burn or some other issue that you'll need to remedy before returning to a regular feeding schedule.
The only time of year when you should not fertilize a
Heartleaf philodendron is during winter. If you grow this plant indoors in an area where the winters are cold, it will enter a dormant phase during the winter. Feeding this plant during its dormant phase is not only unnecessary and unhelpful to this plant's growth, but it is also very likely to cause fertilizer burn.