If your fern looks a little pale or slow, the best homemade fertilizer for ferns is finished compost or worm castings, used as a thin top-dressing in spring and summer, because both feed gently and are unlikely to burn shallow fern roots. A weak compost tea or a heavily diluted fish emulsion also works when you want a liquid option.
Most other kitchen remedies do much less than people hope. Coffee grounds, eggshells, banana peels, and Epsom salt each supply a sliver of nutrition at best, and some cause problems in pots.
Ferns are light feeders, so a small dose of the right natural fertilizer goes further than a heavy dose of the wrong one.
Do Ferns Need Homemade Feed?
Ferns need some feeding, but far less than most houseplants, and many go years on good soil alone.
Potted ferns lean on you for nutrients because watering slowly washes them out, while ferns in rich garden beds pull what they need from decaying leaves and organic matter.
Which Nutrients Matter Most for Fern Growth?
Nitrogen is the nutrient ferns use most, since their whole job is making green fronds. A shortage shows up as pale, light green growth and slow new fronds.
Phosphorus supports roots, and potassium helps with water movement and overall toughness. Ferns need both, just in smaller amounts than flowering plants do.
The micronutrients count too, even in tiny doses:
- Iron keeps new fronds green. Low iron causes yellowing between the veins on young growth.
- Manganese helps with chlorophyll and shows similar yellowing when short.
- Magnesium sits at the center of chlorophyll, so older fronds yellow first when it runs low.
A balanced, mild feed with a slight nitrogen lean covers these essential nutrients for ferns. Compost and worm castings carry small amounts of most of them, which is why I favor them over single-ingredient remedies.
When Is Fertilizer Less Important Than Basic Care?
Most fern problems come from water, light, or humidity, and fertilizer can’t fix any of those. A fern in dry air or soggy soil will look sick no matter what you feed it.
Before you add anything, check these basics of fern care:
- Soil stays evenly moist but never soggy. Water when the top inch feels dry.
- Light is bright and indirect. Direct sun scorches fronds.
- Humidity sits around 50% or higher for tropical types like Boston fern.
- Temperatures stay between 60 and 75°F for indoor ferns.
If those are off, start there. My guide on how to care for ferns walks through each one, and fertilizing ferns only makes sense once the plant is otherwise comfortable.
How Should You Use Compost and Compost Tea?
Compost is my top pick for DIY fern fertilizer because it feeds slowly and improves the soil at the same time.
You can use it dry on the surface or steep it into a mild liquid, and both methods suit ferns well when you keep the amounts small.
Use Finished Compost as a Light Top-Dressing
Finished compost should look dark and crumbly and smell like a forest floor. If you can still see food scraps or it smells sour, give it more time.
For potted ferns, spread about a 1/4- to 1/2-inch layer over the soil in spring. Keep it an inch away from the crown, where fronds emerge, so moisture doesn’t sit against the base.
For outdoor ferns in beds, a 1- to 2-inch layer of compost or shredded leaf mulch each spring copies what happens in a woodland. That one step often handles all of a native fern’s yearly feeding.
Make a Mild Tea and Apply It to the Soil
Compost tea gives you a gentle liquid option for homemade fern fertilizer. Here’s the simple version I recommend:
- Fill a bucket with water and let it sit out for several hours so chlorine can fade. Well water or rainwater skips this step.
- Add about 1 part finished compost to 4 or 5 parts water.
- Stir well and let it steep for 24 to 48 hours, stirring a few times a day.
- Strain it through cheesecloth or an old pillowcase.
- Dilute until it looks like weak iced tea.
Use it as a soil drench, pouring it around the base the way you’d water. Many ferns dislike foliar feeding, and wet fronds indoors invite fungal spots.
Use the tea within a day or two. If it smells rotten, it went stale without air, so pour it on the compost pile and start fresh.
Are Worm Castings a Good Choice for Potted Ferns?
Worm castings are my favorite option for potted ferns, especially indoor ferns. They’re mild, low in odor, and release nutrients slowly, so they’re very hard to overdo in sensible amounts.
How Much Should You Add to a Pot?
Go by pot size and apply every 4 to 6 weeks during the growing season:
| Pot diameter | Worm castings per application |
|---|---|
| 4 inches | 1/2 tablespoon |
| 6 inches | 1 to 2 tablespoons |
| 8 inches | 2 to 3 tablespoons |
| 10 inches or larger | About 1/4 cup |
Scratch the castings lightly into the top inch of soil and water as usual. When you repot, you can mix castings into fresh potting mix at about 1 part castings to 4 or 5 parts mix.
Why Does a Gentle Top-Dressing Work Well Indoors?
Fern roots are fine and shallow, and they react fast to excess salts. Worm castings hold most of their nitrogen in organic form, which breaks down slowly as soil microbes work on it.
That slow pace suits a Boston fern hanging in a living room. It also keeps smells down, which matters in a small apartment.
Keep the layer thin, under 1/4 inch. A thick crust can shed water, and unfinished castings with visible scraps attract fungus gnats. If you grow a Boston fern indoors, my notes on how to keep a Boston fern alive pair well with this feeding routine.
Should You Use Fish Scraps or Diluted Fish Emulsion?
Diluted fish emulsion is a solid nitrogen source for ferns, but raw fish scraps are a different story. The emulsion is already broken down and easy to measure, while scraps need weeks to rot and bring plenty of trouble along.
Why Raw Fish Scraps Are a Poor Choice for Pots
Burying fish heads or guts in a pot is a bad idea. The scraps rot slowly in a small space, smell strongly, and draw flies, raccoons, cats, and dogs.
Homemade fermented fish brews are possible, but they take weeks, smell awful, and vary a lot in strength. I’d leave fish scraps for a hot outdoor compost pile, buried deep, where they break down safely before reaching any plant.
How to Use a Diluted Emulsion Without Overfeeding
Store-bought fish emulsion runs high in nitrogen, often around 5-1-1, so dilute it well. For ferns, I’d use about half of the label’s recommended strength.
A common label rate is 1 tablespoon per gallon, which means about 1 1/2 teaspoons per gallon for ferns. Apply it as a soil drench on damp soil, never dry soil, so it can’t scorch roots.
Once a month in spring and summer is plenty. It suits outdoor ferns and patio pots best, since the fishy smell lingers indoors for a day or two.
Will Coffee Grounds Help or Hurt Ferns?
Coffee grounds help ferns only a little, and a thick layer can hurt them. Think of used grounds as a minor soil amendment, best added to compost first.
What Used Grounds Can and Cannot Supply
Used grounds contain around 2% nitrogen plus small amounts of potassium and magnesium. That nitrogen releases slowly as the grounds decompose, so they won’t green up a pale fern quickly.
A lot of coffee ground fertilizer advice claims grounds make soil acidic. Fresh, unbrewed coffee is acidic, but brewing pulls most of that acid out, and used grounds sit close to neutral.
They won’t meaningfully shift soil pH, so skip them if you’re trying to acidify soil.
How to Avoid a Thick, Moldy Layer
Fine grounds pack down into a mat. In a damp fern pot, that mat holds too much water, blocks air, and grows mold quickly.
If you want to use them, follow these limits:
- Compost them first. This is the safest route.
- For direct use, sprinkle no more than a thin dusting, mixed into the top inch of soil.
- Never spread a visible layer on top of an indoor pot.
- Outdoors, blend grounds with shredded leaves or bark mulch so they can’t crust.
What Can Eggshells and Banana Peels Actually Do?
Eggshells and banana peels do very little for ferns on any useful timeline. They break down slowly, carry only one or two nutrients each, and the popular “water” and “tea” versions pull out tiny amounts.
Why Eggshell Water Is Not a Quick Calcium Fix
Eggshells are mostly calcium carbonate, which barely dissolves in plain water. Soaking shells overnight produces water with almost no calcium in it.
Ferns rarely run short on calcium anyway. Most potting mixes and tap water supply enough. If you like recycling shells, crush them finely and add them to compost, where they break down over months or years.
Why Banana Peel Tea Is Not a Complete Fertilizer
Banana peels hold potassium, but almost no nitrogen, and nitrogen is what ferns want most. A banana peel fertilizer can’t fix pale fronds for that reason.
Banana peel tea pulls out only a small share of the potassium, and the sugars in it can attract fungus gnats indoors. Burying whole peels in a pot draws fruit flies and rots unevenly.
The peel belongs in the compost bin, where it becomes part of a balanced amendment.
When Does Epsom Salt Make Sense for Ferns?
Epsom salt fertilizer only makes sense if your fern is short on magnesium, which is uncommon. Epsom salt is magnesium sulfate, so it supplies magnesium and sulfur and nothing else.