Before you touch the watering, look at the whole leaf
The most common reaction to a plant that isn’t growing is to change how you water it. Almost always the problem isn’t where you’re looking. Before you water more or less, work out which part of the plant the symptom is on, because that says a lot:
- Yellow, dropping lower leaves: too much water or not enough light.
- Dry, brown edges: not enough water or very low ambient humidity.
- Brown tips, the rest green: very hard water or over-fertilising.
- Brown spots with a wet halo: fungus or rot.
- Small, pale leaves and weak stems: a lack of nutrients or light.
- A soft stem that folds over: root or stem rot.
The diagnosis starts with the roots. Taking the plant out of the pot and looking is the fastest method there is.
The roots tell you what’s happening
- White or cream roots that feel firm: the root is healthy.
- Brown, soft roots with a bad smell: rot. Soaked substrate, no drainage or standing water.
- White, dry roots: the plant dried out between waterings.
- Lots of thin, dense roots: the pot is full of root and needs repotting.
- Roots out of the pot and growing downwards: it’s looking for water; check the drainage.
The difference between “it dried out” and “it rotted” is what decides whether the problem is watering more or watering less, which is why it’s worth checking before you change anything.
Cause 1: too much water (the most common one)
The symptoms are yellow leaves falling from the bottom, soil that’s still wet days after watering and, if you wait, a smell of rot.
What to do:
- Take the plant out and remove the substrate around the roots.
- Trim the brown roots with clean scissors.
- Leave the healthy roots in the air for an hour or two.
- Repot in fresh, airy substrate, with drainage holes and no saucer underneath.
- Wait a few days before watering again.
Prevention: this is a problem of substrate and drainage, not of watering frequency.
Cause 2: not enough water and dehydration
Symptoms: wilted leaves, gummosis on the stems, soil that’s very dry and crusty on the surface, or a pot that weighs less than it should.
What to do:
- Water thoroughly until it comes out of the holes and let it drain well.
- Repeat after ten minutes: if the substrate dries out immediately, the problem is the mix, not the amount.
- If the substrate has compacted, loosen the top layer and consider a fabric pot or replacing the substrate.
The pot weight method is the tool that stops you guessing.
Cause 3: the plant isn’t getting enough light
It isn’t a watering problem, but it’s often mistaken for one. A plant that gets two hours of direct light a day survives but doesn’t grow: it stretches towards the window, new leaves are small and stems are thin and bleached.
What to do:
- Measure the real hours of direct sun the spot gets, not the afternoon light coming through the window.
- Use a light sensor if you already have the weather station set up, or a cheap lux meter.
- Clean the glass: in winter a dirty window can noticeably reduce the light.
Cause 4: exhausted or compacted substrate
The symptoms are slow: weak growth, pale leaves and substrate that no longer holds water. In pots with gardenia, fern or any demanding plant, over time the medium degrades and has to be replaced.
What to do each season:
- Remove the top 3-5 cm and replace it with fresh substrate.
- Remove the old drainage layer if it’s dark and compacted.
- Add slow-release fertiliser or feed with the watering.
Cause 5: salts and hard water
With very hard water, brown tips and edges appear while the centre of the leaf stays green. It isn’t a lack of water: it’s salt build-up, because the water that evaporates leaves the calcium behind.
What to do:
- Water more often and in smaller amounts to keep the substrate better flushed.
- Use rainwater if you can.
- Flush the substrate every two or three waterings.
- Supply calcium through the leaves if the damage persists.
The full detail is in hard water and limescale.
Cause 6: temperature and draughts
A tropical plant next to a radiator or in the direct draught from an air conditioner loses water faster than it can absorb it, even when the substrate is moist. The symptoms are brown leaves on the side facing the cold air or the radiator.
The diagnostic order that works
- Take the plant out and look at the roots. It’s the step that gives the most information.
- Check the weight of the pot against the dry pot.
- Check the drainage and the drainage holes.
- Look at the light it gets and the hours of direct sun.
- Check the temperature around the plant.
- Only then, change the watering or the substrate.
And if an automatic system is part of all this, start with automatic irrigation not working: if the problem started when you installed the irrigation, that’s where the cause is.
Sources and next step
The effect of overly saline water or excess salts is covered in FAO-29; for the volume of water each plant needs, see the water use guide.
External sources
- FAO: Water quality for agriculture (Ayers and Westcot, FAO-29) — how salinity, sodium and bicarbonate in the water affect the plant.