Fertilizer Is the Key to Healthy, Strong Houseplants—Here's When (and How) to Apply It

If you have houseplants, you know that few things are more rewarding than watching them produce new, healthy growth. One way to ensure this happens is by applying fertilizer, which contains essential macro and micronutrients that encourage strong foliage. When your houseplants are healthy, they are more resistant to pests and disease and grow bigger, faster. While fertilizer is a great way to keep your houseplants healthy, overusing it can cause more harm than good. Here, we explain when and how to fertilize your houseplants to keep your collection healthy and happy.

Benefits of Fertilizing Houseplants

When done correctly, fertilizing your houseplants is good for their overall health and growth because it gives them the nutrients they need to thrive. "Outdoors in nature, many nutrients are being replenished through processes like the decomposition of organic matter," says Justin Hancock, horticulturist at Costa Farms. "But indoors, you don’t necessarily get this entire natural cycle, so fertilizer fills in these gaps." In turn, your houseplants grow faster and become more resilient against stressful conditions, pests, and disease, says Hancock.

Key Nutrients in Fertilizer

Fertilizer has three key nutrients—nitrogen, phosphorous, and potassium, also known as the NPK ratio. "Nitrogen is for leaf and stem growth, phosphorus is for root growth and helping with fruit and flower formation, and potassium is for overall plant health and plant function," says Patrick Hillman, owner of Buzz & Thrive. Generally, a ratio of 10-10-10 is suitable for indoor plants.

In addition to the three macronutrients in fertilizer, it also has many micronutrients that plants also need. "Iron may be the most well-known micronutrient, but others include copper, boron, magnesium, manganese, and zinc," says Hancock.

Types of Fertilizer

When shopping for fertilizer, there are a few common types you may encounter. No matter which type you buy, make sure it is labeled for use on houseplants.

• Slow-release fertilizer: This type of fertilizer typically comes as granules that slowly release nutrients to your plants over the course of weeks. "It provides a steady supply of nutrients to your plants (unlike some of the other types), and there’s less of a chance you'll accidentally hurt your plants through over-fertilization," says Hancock.
• Water-soluble fertilizer: Water-soluble fertilizer often comes in a form with a texture similar to salt or sugar. You mix it with water according to the instructions on the package and then apply that to your plant. "Unlike time-release fertilizers, you need to reapply water-soluble fertilizers frequently," says Hancock. "And you also need to make sure you don’t accidentally add too much fertilizer to your water."
• Liquid fertilizer: This type of fertilizer typically offers the convenience of a pre-mixed water-soluble product, so it’s ready to use when you buy it. But make sure you read the directions, as some liquid fertilizers need to be diluted in water. The directions will also tell you how to use the liquid fertilizer—some are applied to the soil, but some are best applied to the foliage, says Hancock.

When to Fertilize Houseplants

How often you fertilize your houseplants depends on the type of plant you're caring for, as some (like flowering plants) need more fertilizer than others (like succulents). As a general rule, it's best to fertilize houseplants during their growing season, which is usually during spring and summer. During this time, you can fertilize your houseplants every three to four weeks (depending on the type of fertilizer). "During late fall and early winter, we tell customers not to feed their plants and to wait until the days become longer, usually in March," says Hillman.

When fertilizing houseplants, the best time to do so is in the morning before the sun reaches its peak. "This helps to not burn the plants and gives the plant time to absorb the nutrients before it gets too bright or too hot," says Hillman.

How to Apply Fertilizer

To fertilize houseplants, start by reading the instructions on the product's packaging. "If you’re using a time-release fertilizer, there’s usually going to be a recommendation to use a certain amount per pot size," says Hancock. "Water-soluble fertilizers are similar—there’s a recommendation to mix a certain amount of fertilizer per certain volume of water." There’s no standardization between products, so knowing the recommended application rates is essential. Never use more fertilizer than what is listed on the packaging. Additionally, it's best to get the soil wet before fertilizing, which will help the plant absorb the nutrients quicker, says Hillman.

What Happens When You Over Fertilize Houseplants

Over-fertilizing your houseplants can lead to crispy, brown edges on the leaves, possible crown rot, dying roots, and an overall wilted appearance, says Adrienne Roethling, the garden director for Paul J. Ciener Botanical Garden. "If one thinks their plants are in this state, remove the soil, cut back the stems and leaves, and rinse with clean water," she says. "It could be beneficial to do this for a day, rinsing every couple of hours." Then, repot the plant in fresh soil and let it form new growth before resuming your fertilizer routine.