You know your houseplants need sunlight and water, but what about fertilizer? Fertilizing houseplants during the growing season can provide them with essential nutrients they need to thrive: nitrogen (N), phosphorus (P), and potassium (K).
Fertilizing your houseplants can seem daunting, but similar to learning how to repot a plant, once you have the basics of fertilizing down, you’ll wonder how your plants ever got along without it. Below we dive into what fertilizer is (hint: fertilizer is not plant food), the different types of fertilizer to choose from, and how & when to fertilize houseplants.
What is fertilizer?
First and foremost, fertilizer is not food for plants. Plants make their food using sunlight in a process called photosynthesis. Fertilizer is more like a multi-vitamin that encourages new, healthy growth. It can also be used to replace essential nutrients our plants' potting mix loses as plants grow over time. Fertilizers can contain quite a few nutrients, but the core nutrients, or macronutrients, are nitrogen (N), phosphorus (P), and potassium (K), which we will talk more about later.
When should I fertilize my houseplants?
Like too much light or too much water—too much fertilizer can damage your plants. We recommend fertilizing houseplants sparingly during the growing season: early spring to later summer. During this time, when plants are actively growing, is when they will benefit from fresh nutrients the most. Generally, you can fertilize your plants on a bi-weekly (every other week) or a monthly basis depending on the fertilizer you’re using. Make sure to read the label provided, as the recommendations on dilution and timeline can vary by brand.
Recently potted or repotted plants will not benefit from fertilizer. Their fresh potting mix is packed with nutrients they have yet to use! To avoid potentially damaging recently repotted plants, wait 2–3 months after freshly repotting before fertilizing actively growing plants during the growing season. If you’ve repotted during the fall and winter months—you can forgo fertilizer until the next growing season.
Note: Fertilizer isn’t necessary for plants in low-light environments because their metabolic activity is slower. They are not using nutrients as much as those in brighter light.
What do the numbers on fertilizer mean?
There are about 17 essential plant nutrients, but 3 of them are prominently displayed on the front of most fertilizers. The three numbers you see are the N-P-K ratio: N for nitrogen, P for phosphorus, and K for potassium. They are the primary macronutrients your plant needs.
An N-P-K ratio on a fertilizer label may look something like 10-5-8. Fertilizers with higher ratios are more concentrated than fertilizers with lower ratios. Keep in mind if a fertilizer has higher numbers, it does not necessarily mean it is a better fertilizer than one with lower numbers—it is simply more concentrated and will require more dilution with water before using.
Your fertilizer may also have micronutrients, making it a complete fertilizer. These can include iron, manganese, zinc, copper, boron, molybdenum, chlorine, etc. Each micronutrient serves a role in plant enzymatic, cellular, and developmental processes—but is not needed in large amounts like the NPK macronutrients. The micronutrients in your fertilizer will most likely be mentioned on the back of the container.
You can use any all-balanced fertilizers (example: 5-5-5) for your houseplants, or you can choose one with a ratio that corresponds to what you are trying to facilitate. For example, a fertilizer with a higher nitrogen ratio is best to increase leaf production, while a higher phosphorous ratio helps to promote fruiting and flowering.
Is solid or liquid fertilizer better?
Fertilizers come in different forms. Choosing what form of fertilizer to use is more of a preference. There are pros and cons to each, and they both supply the essential nutrients plants need.
Liquid fertilizers and powder fertilizers are the most common for indoor plants. Depending on how concentrated they are, i.e., how high their N-P-K ratio is, they can also be the most cost-effective. Liquid and powder fertilizers are also easy to use and dilute with water. Many powder fertilizers can also be added to the potting mix directly.
Solid fertilizers, also called dry or granular fertilizers, are arguably less common for houseplants. They aren’t as easy to dilute, and—because some granular fertilizers are time-release (fertilizer pellets)—you can run the risk of over-fertilization or fertilization when the plant is dormant or slow-growing due to low light.
Is chemical or organic fertilizer better?
Choosing between organic and chemical fertilizers, also called synthetic fertilizers, is also a preference. Synthetic fertilizers tend to be more concentrated and formulated with a near-perfect amount of each macro and micro nutrient. They can be more cost-effective, especially if you purchase them in liquid form to dilute. You can use less and get the same amount of nutrients.
Organic fertilizers are made from all-natural ingredients, like recycled food waste, so they tend to be less concentrated. But mild is a pro, not a con, for houseplant fertilizer. Organic fertilizer can also be a little more costly than other types, but it’s a safe, chemical-free alternative. If you share your space with pets, going with an organic fertilizer might be best.
Organic and chemical fertilizers do the same thing in different ways: delivering nitrates, potassium ions, and phosphates to plants. It’s a balance of personal preference—both yours and your plants’.
6 Quick Tips for Fertilizing Houseplants
Tip 1: Spring is the best time to start fertilizing plants because that’s when they are starting to actively grow. Plants that grow faster should be fertilized more often than plants that grow slowly, like a cactus, or are dormant (i.e., most plants in winter.)
Tip 2: Dilute your fertilizer. It’s best to under-fertilize than over-fertilize. If there is a nutrient deficiency in the potting mix and you have not fertilized in a year or so, you can increase the potency by adding less water when you’re diluting the fertilizer.
Tip 3: Plants that give us fruits or flowers will require more fertilizer in their lifetimes. When we pick off fruits or flowers, we are taking away those nutrients and should restore them.
Tip 4: Know your N-P-K values. That’s the ratio of macronutrients your plant needs and what should be in your fertilizer. It looks something like 10-8-10. If you don’t see this on the package, find another fertilizer.
Tip 5: Micronutrients are just as important as macronutrients, plants just need less of them. Most fertilizers include micronutrients—they’re just not prominently displayed on the front of the fertilizer container.
Tip 6: Organic or chemical fertilizer? It’s your call.
Words By The Sill
Empowering all people to be plant people—a collection of articles from The Sill’s team of plant experts across a variety of plant care topics to inspire confidence in the next generation of plant parents. Welcome to Plant Parenthood™.