Pocket BotanistFertilizer is the part of plant care people either forget entirely or overdo. Both are survivable, but the second is worse. A houseplant in a pot has a fixed supply of nutrients in its soil, and most potting mixes are exhausted within a few months. After that the plant lives on what you give it. The good news is that the right amount is small, and the rhythm is easy once it is tied to the season.
The rhythm: growing season only
Houseplants grow when the light is strong, roughly March to September in the northern hemisphere, and rest when it is not. Feeding follows the light. In the growing season, a balanced liquid feed every two to four weeks is plenty for most foliage plants. In autumn taper off, and from late October to February give nothing at all: a resting plant cannot use the nutrients, so they accumulate in the soil as salts and burn the roots.
If you keep plants under grow lights all year, they never really rest, and a light monthly feed through winter is fine. The plant tells you: if it is putting out new leaves, it can be fed.
Half strength, always
Fertilizer labels are written for optimum conditions and vigorous outdoor growth. Indoors, in lower light, plants use far less. Mixing at half the recommended concentration and feeding at the recommended interval gives everything a houseplant needs, with a wide margin against overdoing it. Some growers go further and feed at quarter strength every watering, which suits plants in very free-draining mixes.
Water first, then feed. Fertilizer on dry roots is the fastest way to burn them. Either water plainly the day before, or feed as the second half of a normal watering once the soil is already moist.
Which fertilizer
A balanced liquid feed, something like 10-10-10 or 20-20-20 on the label, covers almost every foliage houseplant. Flowering plants such as orchids and African violets do better with a formula slightly higher in phosphorus, the middle number, during their bloom cycle. Cacti and succulents want a low-nitrogen feed and far less of it, perhaps three times a year.
Slow-release granules mixed into the potting mix at repotting time work too, and remove the need to remember. They feed for three to six months, after which the plant is back on liquid feed or a new pot. Do not stack the two in the same season.
The signs of too much
Brown, crispy leaf tips and edges. A white crust on the soil surface or around the rim of the pot. Lower leaves yellowing and dropping while the plant is otherwise well watered. Wilting even though the soil is moist, because salt in the soil is pulling water out of the roots rather than letting it in. Any of these after a period of regular feeding means stop, and flush.
To flush, take the plant somewhere it can drain freely and run water through the pot slowly for several minutes, three or four times the pot's volume. That washes the accumulated salts out of the bottom. Then skip feeding for six to eight weeks. Doing this once or twice a year even without symptoms is a good habit for plants that have not been repotted in a while.
The signs of too little
Pale, small new leaves, slow or no growth in summer, and older leaves yellowing evenly from the bottom up. These arrive slowly, over months, and they are fixed just as gently: resume a half-strength feed every two weeks and expect the next flush of growth to look right. Do not try to catch up with a double dose.
Yellow leaves are more often overwatering than hunger, so check the soil before reaching for the feed. A plant that is both wet and hungry needs the watering fixed first.
The plants that want less
Snake plants, ZZ plants, cacti and most succulents are adapted to poor soil and grow slowly by design. Two or three feeds a year is enough. Carnivorous plants must never be fed through the soil. Air plants take a very dilute feed as a spray. And any plant that is sick, newly repotted, or freshly bought should be left alone for a month: fresh potting mix already carries nutrients, and damaged roots cannot handle more.
A reminder that follows the season
Pocket Botanist schedules feeding alongside watering for each plant you save, pauses it through the winter months, and tells you when to pick it up again in spring. Log a feed with one tap and the next one is set. Free on iPhone and Android.
Sources
- Royal Horticultural Society, "Houseplants: growing and care." rhs.org.uk