How often should house plants be fertilized?

As a rough rule, every three weeks if they are growing well and are plants that require a very rich diet; every four weeks otherwise. It is better to under-fertilize than to over-fertilize. Do not feel that you must stick to a schedule. Be guided by the condition of the plant. And don’t fertilize when a plant is in a period of rest. Follow the fertilizer manufacturer’s advice for amounts and frequency when you are using a commercial product.