Will the heat affect my plants/flowers?
Yes, if the plants or flowers are too close.
If you mount a heater in close proximity to a plant/s those plants are not going to like it.
If however the plants are in the vicinity of the people you’re heating, the plants will feel a heat comparable to the sun on them during a nice warm day.
Also consider that infrared heaters are heating you by sending a warming light at you, so you need to ensure that there are no plants between the area you’re heating and the heater… if the light is blocked, so is the heat.