How to feed indoor plants without burning them

Feed second, and get the light right first

A pale, leggy, sulking houseplant is usually short of light rather than short of food, and the reason that matters is that feeding a plant which cannot photosynthesise properly does not help it and can actively hurt it, because the fertiliser salts build up in a pot the plant is not drawing hard on. So before you buy anything, move the plant somewhere brighter for a few weeks and see what happens. If it perks up, that was the problem. If the light is genuinely fine and the growth is still slow and the leaves are pale, then yes, it is almost certainly hungry, because the nutrients in a bag of potting mix are gone within about 6 months and every watering washes a bit more out the bottom.

What the numbers on the bottle mean

Every fertiliser carries an N-P-K ratio and it is simpler than it looks. Nitrogen drives leaf and stem growth, so a foliage plant wants plenty of it. Phosphorus goes into roots and flowers. Potassium is the general toughness one, and it is what helps a plant hold up to stress and disease. For most indoor foliage plants a balanced liquid feed is all you need, and a flowering plant does better on something with more phosphorus in it. Specialist mixes exist for the fussy ones, and orchids and African violets genuinely do want their own rather than a general purpose bottle.

How often, and the honest version

The calendar advice you see everywhere is to feed fortnightly in spring and summer and cut back to every 6 to 8 weeks over autumn and winter, and that is a reasonable starting point. What is actually driving it is light rather than the date, so a plant sitting in a bright north-facing room in July may well still be growing and still want feeding, while one in a dim corner in January is not going to use what you give it. Watch the plant and feed when it is putting out new leaves, not when the calendar says so.

Half strength, twice as often, is a better habit than full strength occasionally. The instructions on the bottle are written for a plant in the ground with somewhere for the excess to go, and a pot has nowhere.

Never feed a dry pot

This is the one that kills plants. Fertiliser in dry potting mix goes straight onto dry roots at full concentration and scorches them, and the damage shows up a week later as brown crispy leaf edges that everybody blames on the air being too dry. Water the plant with plain water first, let it drain, and feed after that. Do not feed a plant that is sick, wilting, or has just been repotted either, because fresh potting mix already has several months of slow-release fertiliser in it and the plant has enough going on.

Flush the pot out twice a year

A white or yellowish crust on the top of the mix or around the drainage holes is fertiliser salt, and once it builds up it burns roots and blocks the plant taking up water properly. Twice a year, take the pot to the sink or outside and run plain water through it slowly for a few minutes, about four times the volume of the pot, and let it drain right out. It is a two-minute job and it undoes a year of gradual salt build-up.

When feeding is the wrong answer entirely

If the mix has shrunk away from the sides of the pot, water is running straight down the gap and out the bottom without wetting anything, and no amount of liquid feed fixes that. If roots are coming out the drainage holes and the plant dries out within a day of watering, it has outgrown the pot. Both of those are repotting jobs, and a plant repotted into fresh mix will not need feeding again for a good few months.

Shop related products

• PowerFeed 600ml. A balanced liquid feed, easy to dilute down to half strength.
• Seasol Seaweed Concentrate 600ml. A tonic rather than a fertiliser, good after a repot.
• Brunnings Easy Wetta Potting Mix 70L. For the plants that need repotting rather than feeding.
• Brunnings White Oil RTU 1L. For the scale and mealybug that find indoor plants sooner or later.

Did you know we also stock indoor plant food concentrate, indoor plant drippers, orchid food, African violet food and orchid mix in the garden centre, and most of the small indoor lines are not on the website, so give us a ring on (08) 8536 4333 to check what is on the shelf.

More from the garden centre

The feeds and the potting mixes are all in the garden centre, and if you bring a photo of the plant and where it sits in the house, we will tell you honestly whether it needs food or just a better window.

Keep reading

• Tired of dead plants? Master indoor gardening today
• How to grow vegetables indoors on a small budget
• How to grow potatoes in containers without breaking a sweat

Nine times out of ten the answer is a better window rather than another bottle, and I still reach for the bottle first.

Candeece