Fertilizer isn’t usually the top concern for Calathea owners – it’s easily eclipsed by the struggle to find the right balance of water, humidity, and sunlight for these picky plants. But adequate nutrition is just as vital for the long-term health of your leafy companions. Let’s talk about how to properly fertilize your Calathea.
Calatheas don’t need a huge amount of nutrition; you can probably get away with a ¼ or ½ dose of well-balanced fertilizer, administered once a month throughout the growing season. You’re more likely to overfeed your plants than starve them, so start with low levels and work your way up. You can reduce the risk of root damage by planting your Calathea in soil with good drainage and occasionally flushing out its pot.
Slow-release nutrition is a gentler and safer alternative to liquid fertilizers, though it’s also slower to take effect. If you prefer to avoid the hassle of monthly feedings, you can mix fertilizer pellets or worm castings into your Calatheat’s pot once or twice per year. We’ll talk about all of these options in more detail below, and we’ll explain how to recognize when you’re overfeeding or underfeeding your Calathea.
What Nutrients Do Calatheas Need?
Like all plants, Calatheas get the most crucial elements for their survival – energy, carbon, oxygen, and hydrogen – from the air, the water, and the sun. If it gets enough light and water, your plant can stay alive for a long time without any added nutrients.
Of course, there’s a big difference between surviving and thriving. In order to build new cells and create new growth, plants require a more diverse blend of elements.
First up are the “big three,” which appear front and center on the package of any store-bought fertilizer: nitrogen, phosphorus, and potassium. The other important macronutrients are calcium, magnesium, and sulfur. Then there are the micronutrients, which are only necessary in minuscule amounts: iron, manganese, boron, molybdenum, chlorine, copper, zinc, and nickel.
If your Calathea can’t find these building blocks in the soil, it won’t be able to replace aging tissues or repair damage, and it definitely won’t be able to create healthy growth. Over time, it will wither away and die.
When you bring your Calathea home from the garden center, it should have some nutrients mixed into its potting soil already, but it will eventually work its way through that supply. Sooner or later, you’ll need to step in and provide some fertilizer.
When Should You Fertilize Your Calathea?
The simplest answer is that Calatheas need fertilizer when they’re growing. A plant that’s trying to put together new leaves and stems needs raw materials. If it doesn’t get them, any new growth that emerges will be stunted, fragile, and possibly deformed.
If you live in a temperate climate, your plant’s daily dose of sunlight will drop off sharply in the fall and winter, causing growth to slow or stop. That means you should only fertilize a Calathea during the spring and summer months (unless you’re using strong grow lights to make up for the shorter days).
Applying fertilizer when your Calathea has too little light to grow isn’t just useless – it’s often actively harmful. There’s a principle in agriculture called Liebig’s Law of the Minimum, which states that a plant’s growth rate is determined by the scarcest resource available.
In other words, when your Calathea lacks the energy to build a new leaf, it doesn’t matter how much nutrition you dump into its pot. The extra fertilizer will go unused, accumulating in the soil until it starts to poison your plant’s roots. So hold off on fertilizing until the warm season.
What Kind of Fertilizer is Best for Calatheas?
There are thousands of different plant fertilizers on the market, so how do you know which one is right for your Calathea? There are several important factors to consider. We’ll examine them in detail below.
The tiny concentrations of micronutrients don’t vary much between fertilizers. The real difference will be in the macronutrients – particularly the balance of nitrogen, phosphorus, and potassium. This trio of elements is often abbreviated as “NPK” based on their symbols in the periodic table.
An NPK ratio of 10:10:10 indicates that the “big three” are available in equal amounts, each one making up 10% of the solution. A 2:8:4 ratio, on the other hand, contains 2% nitrogen, 8% phosphorus, and 4% potassium.
There are a dizzying variety of different ratios out there, usually marketed for specialized purposes. For example, fertilizers with a high phosphorus ratio are often sold as “bloom boosters” since flowers require more phosphorus than leaves and stems do.
We’re going to let you in on a dirty little secret: those differences are basically just marketing gimmicks. Remember Liebig’s Law? As long as a plant has the minimum amount of phosphorus it needs to flower, adding extra won’t make any difference. And at standard dosage levels, most fertilizers provide substantially more than the minimum.
Your Calathea should be just fine with a balanced mixture like a 10:10:10 fertilizer. If you want to get really scientific, you could use a 3:1:2 formula, which mimics the actual ratio at which plants utilize N, P, and K.
Liquid vs. Pellets
Some fertilizers are sold as liquids or powders that you can add to your plant’s water, while others come in the form of granules or pellets which you mix into the soil. Granular fertilizer decays slowly, releasing its nutrients a little at a time, while liquid fertilizer gives your plant the full dose all at once.
There are advantages and drawbacks to each type. If you use pellet fertilizer, it’ll be months before you need to add more – which comes in handy if you’re busy or forgetful. The slow release of nutrition is also less likely to damage your plant’s roots. However, if your Calathea is suffering from an urgent nutrient shortfall, granular fertilizer may not act quickly enough.
It’s also easier to control the dosage of liquid fertilizer. That’s useful if you want to experiment to find the balance that’s exactly right for your Calathea, or if you like to adjust the concentration of fertilizer throughout the growing season.
Organic vs. Synthetic Fertilizer
Lots of indoor gardeners feel uncomfortable giving their Calatheas a synthetic formula whipped up in a lab. Luckily for them, there are plenty of fertilizers derived from all-natural organic materials.
For example, many Calathea growers swear by fish emulsion. This is a fertilizer made from leftover scraps of fish – so when you use it, you’re helping turn a waste product into healthy green foliage. Points for eco-friendliness! Other organic fertilizers use a more diverse blend of ingredients, avoiding the pungent smell that often accompanies fish emulsion.
Compost – the dark, fluffy, nutrient-rich substance that results when soil organisms break down organic matter – is another excellent source of organic nutrition. It’s not as nutrient-dense as processed fertilizers, but on the plus side, it pulls double duty by helping to enrich the structure of your soil.
You use compost much like pellet fertilizer, stirring it into your Calathea’s potting mix once every six months or so. If you’re going this route, we recommend vermicompost, the technical name for the compost produced by earthworms. Vermicompost is great at creating tiny air pockets in your potting mix that help your Calathea’s roots breathe. For maximum earth-friendliness, you can even create your own compost at home by feeding your kitchen scraps to a bin of red wigglers.
Our Recommendations for Fertilizing Calatheas
If you like the control and reliability of synthetic fertilizers, try Jack’s Classic All Purpose formula. It’s got a 20:20:20 ratio, which is on the high side, so we recommend diluting it to at least ⅛ strength when you first use it on your Calathea. You can gradually increase the dosage if you find that the weaker blend doesn’t cut it.
For those who prefer organic options, this indoor plant fertilizer from Eon Natural has great reviews and provides the same 3:1:2 ratio that plants absorb in the wild. It’s not as intense as Jack’s Classic, but you should still start with a ½ or ¼-strength dose at first.
How Often Should You Fertilize Your Calathea?
The ideal fertilizing schedule for Calatheas depends on the product you’re using. Liquid fertilizer – organic or synthetic – should be applied roughly once per month through the spring and summer. For best results, taper the concentration up at the beginning of the growing season and down at the end rather than starting and stopping with the full dose.
For granular fertilizer, gently mix ½ of the recommended amount into the soil every six months or so. Again, you can increase the amount little by little if your Calathea still seems to be craving nutrition.
Compost is best used as a soil amendment, blending it into the potting mix when you repot your Calathea. It should make up roughly 10% of the total volume in the container. Then you can stir in a little more every few months.
Avoiding Fertilizer Burn in Calatheas
You may be rolling your eyes at our repeated warnings to dilute the fertilizer you give to your Calathea. But it’s important to remember that being too generous with your plant’s nutrition is much more dangerous than being stingy.
Fertilizers – even organic ones – release ionic compounds called mineral salts as they break down in the soil. In small doses, that’s a good thing because those ions dissolve in water and nourish your Calathea’s roots. Unfortunately, they have the opposite effect at high concentrations, leaching water away from the plant. The result is dehydrated, damaged roots, a condition known as fertilizer burn.
In theory, you can avoid this problem by giving your plant exactly the amount of nutrition it needs. In practice, it’s impossible to be this precise. A standard fertilizer regimen usually provides at least a little bit more than your Calathea can use.
The best way to get around the issue of fertilizer burn is to use a potting mix with excellent drainage. If moisture flows quickly through your Calathea’s container, most of the excess mineral salts will be flushed out when you water the plant, rather than hanging around in the soil.
Try planting your Calathea in the following blend:
- 25% coarse-grade perlite. These large chunks of volcanic glass are great for spacing out the soil.
- 25% orchid bark. Derived from conifer trees, this rough bark further enhances drainage, as well as pushing the pH balance in the direction Calatheas favor.
- 40% coconut coir. An eco-friendly replacement for peat moss that’s light and airy yet retains enough moisture to keep the roots mildly damp.
- 10% vermicompost. A little bit of extra nutrition in a gentle form, which also helps to aerate the mix.
It’s best to use distilled or filtered water on Calatheas because the chemicals and minerals in tap water can contribute to salt buildup in the soil. And it should go without saying that you need to use a pot with drainage holes, or all the effort you put into making a nice airy soil blend is wasted.
You can also give the soil a more thorough flush every 1-2 months as a preventative measure. Slowly pour a large quantity of water – 3-4 times the volume of your Calathea’s container – into the pot, letting it permeate the soil and drain out the bottom.
Signs of Overfertilizing in Calatheas
How can you tell if your Calathea is suffering from an excess of fertilizer in the soil?
Although the problem starts at the roots, the leaves usually show the first signs of trouble. The damaged root system can’t take in enough water to hydrate the plant, so the foliage begins to dry out, turning brown and brittle. This shriveling typically begins at the very tips of the uppermost leaves – the parts of the plant that are furthest away from the water supply.
Several other issues can cause dehydrated leaves, though. The most common one is underwatering. If the plant looks wilted and droopy, and the soil is dry and caked, you’ve probably gone too long without watering your Calathea. You can verify this by inserting a plain wooden chopstick into the pot’s base to see if the soil by the roots has completely dried out.
Overwatering can also produce browning leaf tips because it cuts off the root system’s oxygen supply and promotes bacterial infections. When the upper layers of the soil remain damp for more than 5 days after watering, it’s a warning sign that the pot isn’t draining fast enough.
Other symptoms of overwatering include yellowing leaves, mushy stems, and a foul odor coming from the soil. Those last two are especially concerning, as they strongly suggest root rot. See our article on root rot in Calatheas for advice on tackling this problem.
Humidity levels below 55% or so can also cause Calathea leaves to shrivel. You can use a simple hygrometer to check whether the air is moist enough for your plant.
If you’ve ruled out humidity and watering issues, you probably have a case of fertilizer burn on your hands. A whitish crust on top of the soil is also a clear indicator – it means the mineral deposits have built up enough to be visible on the surface.
Fixing Fertilizer Burn in Calatheas
Although the parts of your plant that have withered can’t be revived, you can stop the damage from getting worse by performing an emergency soil flush. Use the same procedure we described in the section on preventative measures. Make sure to use filtered or distilled water for the best results.
You can speed your Calathea’s recovery by trimming away any leaves that have gone completely dead. Use a pair of pruning scissors, and before you cut, disinfect the blades with a 10% bleach solution or some ordinary rubbing alcohol. This will help keep the trimmed ends from getting infected.
Signs of Nutrient Deficiency in Calatheas
We’ve covered how to recognize overfertilizing in your Calathea, but what about malnutrition? Unfortunately, this can be a lot harder to diagnose.
The clearest sign is stunted growth. If your Calathea produces little to no new foliage even though you’re smack in the middle of the growing season, it may need more fertilizer. An undernourished plant may also put out leaves that are small, deformed, or pale.
However, you should check for other potential problems before you start adding fertilizer. Stunted or discolored foliage can be caused by inadequate lighting, dehydration, or a root system that’s outgrown its pot.
Don’t add fertilizer until you’ve eliminated all of these other issues – otherwise, you may worsen the problem. For more on diagnosing leaf problems, take a look at our post on the subject.
Every home-grown Calathea needs to be fertilized at some point, but it’s wise to take a cautious approach. The best way to learn the ropes is to start with a low dose of balanced, liquid fertilizer and increase it little by little. However, if you prefer a low-maintenance option, a slow-acting soil amendment like vermicompost can also work well.
Whatever method you choose, good soil drainage and an occasional container flush will go a long way toward preventing fertilizer burn. Proceed with care, watching closely for any changes to your plant’s health, and you’ll soon learn how to keep your Calathea healthy and strong.