Photo: Marcus McDonald
You’ve potted, watered, de-pested, and pruned your houseplant — but after several months, you notice it looks less vibrant. It may be time for fertilizer. “Plants get their food from photosynthesis, but they need some other micronutrients and macronutrients that are critical in creating enzymes, regulating water, and plant defense,” says Summer Rayne Oakes, author of How to Make a Plant Love You and host of the YouTube series Plant One on Me. For houseplants, those resources aren’t readily available once they’ve used up the nutrients in the soil. “In nature, a plant will exhaust all the nutrients in the area, so it keeps growing the roots farther and farther away,” says Christopher Satch, plant doctor at Horti and professor at the New York Botanical Garden. “Indoors, it’s just stuck in a pot. That’s its universe. Whenever it depletes the soil, you have to replace the nutrients in the soil by either repotting or fertilizing.”
Before working on this article, I was intimidated by fertilizer, which seemed like an impenetrable maze of chemical symbols, ratios, acronyms, and hyperspecific nutrient blends. I’d mostly cheered up my sad plants by repotting them. Fortunately, it’s simpler than it seems, and fertilizing plants instead of repotting them has given me more control and saved me money on soil. Experts say that there’s only one way to go seriously wrong: overfertilizing. Using too much fertilizer burns the plant’s roots, usually killing it; the best way to avoid this fate is to closely follow the directions for proper dilution on the package, erring on the side of using too little rather than too much. If you realize you’ve used too much fertilizer, it’s still possible to save your plant: “In the event that you haven’t overfertilized that badly, you can either repot it or give it a soil flush, which is running water through the soil for about a half hour until all the excess fertilizer salts leach out,” says Satch.
Fertilize conservatively when you’re starting out; other than that, most houseplants are happy with an all-purpose fertilizer, although if you want to optimize your orchid blooms or cactus growth, you can buy a specialized blend.
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Nutrient levels
The three main nutrients in fertilizer are nitrogen (N), phosphorus (P), and potassium (K). For general use, fertilizers with balanced NPK levels between 5 and 15 are “a pretty safe bet,” says Oakes. You can also choose a blend more closely tailored to your plants’ needs. “If you’re trying to fix a specific issue with your plants related to the lack of one of these elements, you’ll want a formula that’s rich in that particular one,” says Erinn Witz, a blogger at Seeds and Spades. Will Axelrod, a project manager at Brooklyn Grange, says, “Nitrogen helps with leaf growth and stem growth; phosphorus helps with root growth and flowering; and potassium helps the plant fight disease.” Flowering plants need more phosphorus; “stretchy” or “leggy” growth can be a sign of too much nitrogen. If you’re not sure, default to a fertilizer with an NPK ratio around 10-10-10. Even if it isn’t precisely what your plant needs at that moment, Satch says, the plant is “not gonna complain that much” if the soil contains “an excess of one nutrient over another.”
Fertilizers also supply nutrients that plants need in smaller quantities that get depleted from soil — for example, calcium, which two of the experts we spoke to mentioned as an important component in strengthening cell walls. You want a fertilizer with a good range of micronutrients, which are listed on the container under the heading “Guaranteed analysis.”
Medium
Fertilizers come in many forms: liquids, water-soluble granules, a dry powder, compost, and more. Many require dilution, in your watering can or mister or mixed into soil. Most houseplants are happy with any of these media; which you use mostly depends on your ease-of-use preference — just make sure to measure how much you’re using according to the directions on the label.
Organic vs. synthetic
Organic fertilizers derive nutrients from a “natural source of decay that releases nutrients,” says Satch. They are less likely to burn plants if you use too much, but tend to be less potent, pricier, and smellier than synthetics. (Like many gardeners, I have positive associations with organic-fertilizer smell, but indoors, it can be quite strong — like, don’t have a dinner party the day after dosing all your houseplants with an organic product.) Synthetic fertilizers are stronger, nutritionally complete — “like a Centrum multivitamin for humans,” says Satch — and hold a higher risk of burning plants. Your plants will be happy with either. According to Oakes, “Plants don’t really distinguish between organic and synthetic fertilizers,” although “organic fertilizers tend to build up the soil, helping promote healthy microbial activity.”
The USDA does not offer organic certifications for fertilizers as it does with organic foods. This means the term is used less strictly, but the lack of a USDA organic certification isn’t a demerit. Independent groups fill that role, including the nonprofit Organic Materials Review Institute (OMRI), which Kristin Monji, founder of landscaping firm Birch and Basil Design, describes as “the gold standard for products in the horticulture industry.”
In general, synthetic fertilizers represent the simplest, cheapest path to complete plant nutrition, but there are also plenty of options for gardeners who prefer to spend a little extra money and time on organic practices. Organic fertilizer is also good for gardeners who are more concerned about avoiding damage to their plants than getting optimized results quickly — you may get slower growth, but organic products won’t wreak as much havoc if you accidentally use too much.
Nutrient levels: 7-9-5 | Medium: Liquid | Organic or synthetic: Synthetic
Photo: Author
Dyna-Gro’s fertilizer was recommended by both Satch and Susannah Strazzera, a horticulturist at Wave Hill public garden — it became Satch’s favorite after doing “a lot of research” on plant nutrient acquisition in grad school. Strazzera likes the blend’s slightly lower nitrogen level. “If you use too much nitrogen, you get stretchy growth, and that attracts aphids and other insects,” she says. “You want to have a tight, full plant; you don’t don’t want to have a long, stretchy plant.” Satch also appreciates that Dyna-Gro contains calcium, which is “a super-important component of plant cell walls” and reduces the risk of infection.
I’ve been using DynaGro in my smart garden for about six months and can confirm that it keeps my plants happy, vibrant, and rapidly growing — everyone looks perkier and happier after I use it. I was away from home for three days and when I got back, a Thai basil plant had gotten so tall and bushy that I had to prune it back immediately and make pesto. I dilute a capful (about a teaspoon) in about two quarts of water, which follows the label’s guidance of two to three teaspoons of fertilizer per gallon of water for circulating hydroponic systems.
Nutrient levels: 6-4-5; 4-9-3 | Medium: Dry blend | Organic or synthetic: Organic, OMRI-listed
California company FoxFarm was mentioned by many of our experts as a great source for organic fertilizers; it’s also OMRI listed and thus held to a stricter standard for organic material. Its all-purpose fertilizer is a perennial favorite, and the brand’s Fruit & Flower fertilizer has been my go-to for over a year. (When I bought it, my plants were looking a bit stretchy, so I chose it for the proportionally lower nitrogen levels.) I’ve been shocked by how well it’s worked, especially for a spider plant that has produced so many babies that I’ve given them away in deli containers at dinner parties and on my neighborhood sub-Reddit. Plants that I don’t expect to flower have flowered: I’ve gotten little pink blooms from an angel wing begonia and delicate white flowers from my spider plant.
It does smell insane the first few days after using, but that’s easy enough to manage by aerating the soil or staggering your fertilizing schedule so not every plant is diffusing the aroma of fresh decay at the same time. When I recently used it, I tried burying the fertilizer under a layer of soil instead of simply top-dressing (adding a fertilizer to the surface, then watering), and it has dramatically reduced the funky smell. Although it is more expensive than DynaGro, a little goes a long way — in a year and a half of tending to about 20 houseplants, I’ve used up about half of a four-pound bag.
Jack’s Classic Blossom Booster Fertilizer 10-30-20
$18
$18
Nutrient levels: 10-30-20 | Medium: Water-soluble granules | Organic or synthetic: Synthetic
Jack’s Classic was also a favorite brand among our experts, and Strazzera especially likes its Blossom Booster, which has high phosphorus levels to support flowering. She likes that “the nitrogen is low. Nitrogen is to make green stuff like stems and leaves. So now that your stems and leaves are made, it’s mature … and it’s ready to make flowers.”
$18
at Amazon$23
at Macy's
FoxFarm Big Bloom Liquid Concentrate
$16
$16
Nutrient levels: 0-0.5-0.7 | Medium: Liquid | Organic or synthetic: Organic, OMRI-listed
Dimitri Gatanas, general manager at Urban Garden Center, says FoxFarm’s flower concentrate is a “high-demand fertilizer” at the store, and they “have trouble keeping this product on the shelves.” The blend contains bat guano and earthworm castings, which Gatanas says are excellent sources of nutrients.
$16
at Amazon$16
at Amazon
Nutrient levels: 16-16-16; 3-20-20 | Medium: Water-soluble granules | Organic or synthetic: Synthetic
Benoit and Gordon also like Maxsea fertilizer, which is derived in part from seaweed and contains a good blend of micronutrients. They recommend the all-purpose formulation for common leafy houseplants (like ZZ plants and philodendron), which need plenty of nitrogen to promote leaf growth; carnivorous plant growers also swear by the all-purpose blend, diluted to a weaker solution than is listed on the container. The brand also makes a flowering plant formula that’s especially beloved by rose gardeners.
Down to Earth Organic Vegan Fertilizer Mix
$18
$18
Nutrient levels: 3-2-2 | Medium: Granules | Organic or synthetic: Organic, OMRI-listed
There aren’t many vegan organic fertilizers on the market — organic options often contain animal-derived ingredients like bone meal, hydrolyzed fish, and blood. This mix, which has an animal-product-free ingredient list that includes soybean, canola, alfalfa, and kelp meal, was mentioned by Dimitri Gatanas, Witz, and Gabi Stone, garden manager on the design-and-build team at Brooklyn Grange. “I am sure there are gardeners out there that believe the animal byproducts are more effective, but I think the jury is still out on this,” says Gatanas, who describes it as a “great alternative” to organic fertilizers that contain animal products.
$18
at Arbico Organics$21
at Amazon
Malibu Compost Bu’s Blend Biodynamic Compost
$25
Nutrient levels: Not applicable | Medium: Compost | Organic or synthetic: Organic
“Compost and compost tea” — the concentrated liquid that leaches from compost as it decomposes — are “great and often overlooked additives for indoor plants,” says Axelrod. Compost doesn’t list specific nutrient ratios, and although Witz warns that compost “typically contains a lot of nitrogen, and you might still need a high-phosphorus or high-potassium fertilizer if your soil is deficient in these minerals,” the diverse organic matter in compost will enrich your soil holistically. You can make your own compost in an outdoor space (or even indoors), and there are also plenty of good options on the market. Gordon and Benoit especially like Malibu’s biodynamic compost (which they mention is surprisingly “non-stinky”) because the lower concentration of nutrients makes it “nearly impossible to overfertilize” and “provides a natural time release of nutrients with every watering.”
$25
at Malibu Compost$32
at AmazonOur experts
• Will Axelrod, project manager at Brooklyn Grange
• Ryan Benoit, co-founder of The Horticult and co-author of How to Window Box
• Dimitri Gatanas, general manager at Urban Garden Center
• Sara Gatanas, special events and public relations at Urban Garden Center
• Chantal Aida Gordon, co-founder The Horticult and co-author of How to Window Box
• Kristin Monji, founder of landscaping firm Birch and Basil Design
• Summer Rayne Oakes, author of How to Make a Plant Love You and host of the YouTube series Plant One on Me
• Christopher Satch, plant doctor at Horti and professor at the New York Botanical Garden
• Gabi Stone, garden manager on the design-and-build team at Brooklyn Grange
• Susannah Strazzera, horticulturist at Wave Hill
• Erinn Witz, blogger at Seeds and Spade
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If you have any questions on NPK 19-19-19 Water Soluble Fertilizer, NPK 10-52-10 Water Soluble Fertilizer. We will give the professional answers to your questions.