Your indoor greenery can probably survive without fertilizer (as long as it gets adequate water and sunlight), but adding it can help plants really thrive. “In general, nitrogen is the be-all and end-all of nutrients,” Jonathan Russell-Anelli, Ph.D., a senior lecturer in Cornell’s School of Integrative Plant Science, tells mbg. “It is generally the most limiting, and it is needed in abundance and is often needed quickly. P and K also are often required beyond the soil’s ability to meet immediate needs.” Beyond that, plants use sulfur (S), magnesium (Mg), carbon (C), oxygen (O), hydrogen (H), and very small amounts of iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), and molybdenum (Mo). “Plants combine these nutrients with sunlight, air, and water to make their own food through the process of photosynthesis,” Leslie F. Halleck, M.S., a certified professional horticulturist and author, explains. You’ll typically see three numbers on your fertilizer label, which speak to its ratio of nitrogen, phosphorous, and potassium. If yours reads 24-8-16, for example, it has 24% N, 8% P, and 16% K by weight, and potentially some of those micronutrients. Water and/or fillers make up the rest of the weight. Houseplants that have fewer leaves, like succulents, require less nitrogen, which is why you might find a succulent-specific fertilizer at a 5-10-10 ratio, for example. Flowering houseplants, on the other hand, might appreciate a higher P value. All in all, though, most houseplants will be happy with a fertilizer that has an even ratio of all three, making this a good starter pick. The most common fertilizer types are liquid fertilizers, which you can use directly on a plant, and granular ones that need to be dissolved in water. You can also find slow-release fertilizers that need to be applied only once every few months, but this option gives you less control over how much fertilizer your plants are actually getting. Synthetic fertilizers tend to have higher percentages of nutrients than natural ones and should therefore be applied more judiciously. If you add too much of them to a plant, its leaves can burn and its soil can develop salt buildups due to all those extra nutrients. That’s why Halleck opts for organic fertilizer whenever possible: “That includes liquid humus, composted manures, seaweed, etc., for my houseplants,” she says. These fertilizers more closely mimic the nutrients a plant would receive in its natural habitat and tend to be gentler—though they are usually more expensive. You might have heard of people sprinkling coffee grounds, eggshells, salt, or banana peels onto their plant pals. However, Russell-Anelli warns that most of these kitchen DIYs are “generally based on garden folklore and can quickly damage gardens if not understood and used correctly.” “Uncomposted kitchen scraps in your potting soil can also cause fungal or bacterial problems in your soil, as well as fungus gnat problems,” warns Halleck. That’s not to say that your kitchen scraps can’t be repurposed as plant food; you just might want to compost or vermicompost them first to make their nutrients more readily available. “Essentially, any organic matter can be composted down into ingredients for natural plant fertilizers,” says Halleck. Learn all about what can and can’t be composted at home here. Russell-Anelli adds that green veggie scraps and grass clippings are particularly good for houseplant-bound compost since they’re high in nitrogen. (Remember, nitrogen corresponds with leaves and all things green!). Once your compost is good to go, you can add it directly to soil until it makes up around 10 to 20% of your plant’s pot by volume. (You won’t want to go higher than that since it can attract pests and diseases, Halleck explains.) You can also soak it in water to make your own “compost tea”—essentially a more nutrient-dense, liquid form of compost. Your plants might actually enjoy some actual tea every now and again, too. “Twice-brewed green tea can offer a small amount of soluble nutrients and possibly a slight acidifying effect (if used repeatedly), when used to directly water your plants,” says Halleck. “But do not expect your plants to green up quickly or significantly from green tea. I typically only use leftover green tea for my plants. Be sure to compost the used tea leaves after you have brewed them.” “If you do use synthetic fertilizers, I recommend cutting the application rate in half,” says Halleck. Every type of fertilizer will come with its own application instructions, so look out for those on the box or bottle. “For tiny houseplants, cut the application rate down to ¼ of the recommended rate, even with natural fertilizers.” As for when to use fertilizer, your plants won’t need it for the first two months or so in new soil. After that, most plant experts say to fertilize once every two weeks to one month during your plant’s active growing season, while you’re watering your plant. You might want to use even less fertilizer at the very beginning and end of growing season, as your plant won’t be actively taking in as many nutrients. Again, higher concentrations of N-P-K mean that a fertilizer will be more intense and should therefore be diluted in more water or used less frequently. “When it comes to fertilizers, unless you know for certain, go with the less-is-more strategy and then wait at least a couple of days before adding more,” Russell-Anelli recommends. Signs that you’ve overdone it include yellow leaves, salt buildup in soil, and leaf loss. Emma received her B.A. in Environmental Science & Policy with a specialty in environmental communications from Duke University. In addition to penning over 1,000 mbg articles on topics from the water crisis in California to the rise of urban beekeeping, her work has appeared on Grist, Bloomberg News, Bustle, and Forbes. She’s spoken about the intersection of self-care and sustainability on podcasts and live events alongside environmental thought leaders like Marci Zaroff, Gay Browne, and Summer Rayne Oakes.
