For most plants, the best time to fertilize is usually during spring and summer. The reason for this is that after they receive nutrients -phosphorus, nitrogen, and potassium- through the fertilizer, the plants need all the sunlight they can get to produce larger and healthier leaves, sturdier stems, healthy roots, and beautiful flowers.
If you live in a tropical or subtropical zone, you can fertilize all year long because the sun and warm weather are available all year round. But if you live in a zone that gets cold during fall and winter, you want to stick to fertilizing during spring and summer only. If you fertilize during the colder months, you will encourage the plants to produce new tissue that will be exposed to the worst environmental condition. The plant tissue production won’t be a healthy one.
When it comes to indoor plants, the rules are a bit different. The environment inside a home usually stays constant, compared to the environmental changes that occur outdoors during the season changes. That means you can keep fertilizing during fall and winter, depending on the natural light available in your zone.
Plants have different fertilizing needs depending on their species, so it is important to know their feeding needs before buying fertilizer. Most plants can benefit from a well-balanced fertilizer, but depending on your goals, you will want to buy a fertilizer that provides the specific nutrients that encourage flowering, leaf growth, roots, etc.