Best Fertilizer for Indoor Plants: How to Choose

Look for new leaves, flower buds or flowers before you feed, because those show the plant is actively growing and can use the nutrients. A plant that is not growing takes up little, and fertilizer added to a dormant plant can build up as harmful salts in the soil.

Most houseplants are fed between March and September, about once every one to three months, and in the short days of winter they rest and need little or no fertilizer. Plants that bloom in winter are the exception and are fed before their flowering period. A plant in a very bright south or west window, or under supplemental light, can keep growing through winter, and it is then fed like any other actively growing plant.

High-quality potting mixes usually contain slow-release fertilizer that supplies a plant's needs for a few months, and one common potting soil states that it feeds for up to six months. The bag on your potting mix says how long its feeding lasts, which tells you when more is due. After repotting, the fresh mix supplies nutrients of its own.

Pale leaves and slow growth do not always mean hunger. Too little light causes faded leaves and stunted growth, and a root-bound plant shows yellowing and stunted growth that call for repotting. Roots on the soil surface or coming out of the drainage holes point to a root-bound pot.

Start with a complete fertilizer labeled for indoor plants, one that lists nitrogen, phosphorus and potassium, and mix it as the label directs. Most houseplants grow in a soilless mix with no mineral soil, so they benefit from all three nutrients, and a general 20-20-20 is sufficient for many of them.

The three numbers on the package give the percentage by weight of nitrogen, phosphate and potash, so they show how concentrated the product is. Some labels offer a choice of concentrations depending on whether you feed once a month or at every watering, so the dilution you pick sets how much nutrient reaches the pot.

Extension advice on the ratio differs by state.

Micronutrients are deficient in many indoor plants, so Maryland Extension recommends replacing them once a year, either with a fertilizer labeled for indoor plants that contains micronutrients or with a small amount of well-composted, screened leaf mold or other compost. The same page notes that magnesium leaches from the soil at each watering and suggests replacing it with 1 teaspoon of Epsom salts per gallon of water, applied twice a year or sprayed on the leaves.

Indoor fertilizers come as liquids, water-soluble granules, tablets and slow-release granules, sticks or tablets. Water-soluble products are often preferred because dilute solutions reduce the potential for fertilizer burn.

Earthworm castings are also called an excellent houseplant fertilizer.

The longer comparison of plant-based and manufactured products is in organic vs synthetic fertilizer.

The goal indoors is just enough nutrients for new growth to replace the leaves the plant loses, so large amounts are unnecessary for most houseplants. Fast growth is often unwanted, because plants outgrow their locations and their pots too soon.

Published schedules differ. Minnesota starts plants at half strength every two to four weeks as spring light returns, Wisconsin feeds once every one to two months, and Illinois feeds no more than once every one to three months between March and September.

Plants in bright light may need feeding more often by late spring and low-light plants less, and too much fertilizer does the most damage when dim light slows growth. A plant in a dim spot therefore belongs at the long end of that range, and too much too soon can also cause weak, stretchy growth.

Mix the product as its label directs for dose, water volume and interval. Then water the soil thoroughly and let the excess run out of the drainage holes into a tray, which is emptied afterward.

Fertilizer dissolved in water becomes a soluble salt, and when the water evaporates the salts stay behind. As the salt concentration in the soil rises, roots find it harder to take up water even though the soil is moist, and at very high levels water moves out of the root tips, which dry out and die.

That explains the signs, which are reduced growth, brown leaf tips, dead root tips and wilting. A yellow to white crust on the soil, or a white ring around the rim of a clay pot or the drainage hole, shows where the salts collected, though the same crust can also point to overwatering. Salt-damaged roots also become more open to insects and diseases, and root rot is one of the most common problems with high salt levels.

Flushing the pot washes the salts out, and extension guides recommend doing it every 4 to 6 months as routine care.

A pot without a drainage hole cannot be flushed this way, so repotting into fresh soil is the other way to remove the salts. Hold fertilizer until the plant is growing again, since a wilted plant should not be fertilized.