I've got a few water-circulating wall radiators that are mostly useless space, and I'd like to put some shelving over them and use the surfaces for houseplants, but I'm concerned that the elevated temperature or else lower humidity right near the radiators would be bad for plants. Should I worry about this?
The dry and warm air flow could create a very low humidity micro climate there, lower than your average humidity would be in the apartment.
This would definitely kill anything that has thin foliage and in general requires moderate humidity, around 40-50%. (I have killed a Parlor palm this way, because I have forgot to move it in time.)
Your choices could be plants that has thick foliage and more accustomed to drier areas. Such plants could be cacti or succulent type of plants.
What I could suggest based on my personal experience are:
- Peperomia polybotrya. It has succulent like leaves. I have it right on the windowsill, just above the heater. I have to water it a little bit more often than usual, which means 9-11 days in my case.
- Zamioculcas zamiifolia. Very hardy plant with waxy leaves, so it does not seem to be much affected by the dryer air. Again as it receives a bit more light and dryer air, I have to water it a little bit more often. Maybe every two weeks.
In any case monitor your plants often in the early days, weeks to see if they really work there. If you see any issue move them.
On the whole, above the radiator would be a bad place to put a plant due to the drying nature of sitting above a heater.
There are several plants that like it warm, so the higher temperature is not necessarily a major problem, depending on the plant.
Perhaps, perhaps, you could find a very dry loving plant and just give it a try to see how it goes. I lean toward experimentation and thus I'm a little harder on my plants than others like to be. If the plant starts to suffer then you could move it to another location if you have one. The shelf will protect the plant at least from direct drying airflow.