Most indoor plants don't die of neglect. They die of kindness — the generous glug from the watering can every time we walk past, because the surface of the compost looked a bit dry. Overwatering and underwatering produce very similar symptoms at first: drooping leaves, yellowing, brown tips. The only reliable way to tell them apart is to look below the surface, at the compost and the roots.

Check the compost, not the calendar

Watering on a fixed schedule — every Sunday, whatever the weather — is the most common mistake. A plant in a bright, warm room in July drinks far faster than the same plant in a cool north-facing hallway in January. So stop asking "is it Tuesday?" and start asking "is the compost dry?"

Push a finger into the compost up to the second knuckle, roughly 3–4cm. If it feels cool and damp at that depth, leave it alone. If it's dry, water. For large pots your finger won't reach, use a bamboo skewer: push it deep, leave it for a minute, and see whether it comes out clean or speckled with damp compost. An inexpensive moisture meter does the same job.

Signs you're overwatering

Roots need air as well as water. When compost stays saturated for days, roots suffocate and start to rot, and the plant loses the ability to take up water at all — which is why an overwatered plant so often looks like a thirsty one.

• Yellowing lower leaves that feel soft, limp or mushy
• Leaves that droop even though the compost is wet
• Brown or black patches, particularly where stems meet the compost
• A sour, musty smell coming from the pot
• Fungus gnats hovering over the surface
• Mould or green algae on the top of the compost
• New growth that is pale, weak and floppy

Overwatering looks a lot like underwatering. The compost, not the leaves, tells you which one you're dealing with.

If you suspect root rot, take the plant out of its pot and look. Healthy roots are firm and pale; rotten ones are brown, soft and break apart. Trim the damaged roots with clean scissors, repot into fresh, free-draining compost, and hold off watering for a few days.

Signs you're underwatering

• Crispy brown tips, especially on spider plants, ferns and calatheas
• Leaves curling inwards or folding along the central vein
• Compost shrinking away from the sides of the pot
• A pot that feels unexpectedly light when you lift it
• Whole leaves dropping, usually from the bottom upwards
• Pale, slow growth and leaves that look dull rather than glossy

Bone-dry compost can become water-repellent, so water runs straight down the sides and out of the drainage holes without reaching the roots. If that happens, stand the pot in a bowl of water for 20–30 minutes so it can soak upwards, then let it drain thoroughly.

Four watering rules worth keeping

1. Water thoroughly, until it runs out of the drainage holes, then empty the saucer after 15 minutes. Little and often is how rot starts.
2. Never leave a plant standing in water for hours. Very few indoor plants enjoy wet feet.
3. Use room-temperature water. Cold tap water can shock tropical roots.
4. Water in the morning where you can, so any splashes on the leaves dry off during the day.

Adjusting for the seasons

Spring and summer

Longer days and active growth mean most plants drink more. A pothos or peace lily that needed water weekly in winter may want it every three or four days in June. Check more often, and don't be surprised if the compost dries faster than you expect.

Autumn and winter

Light levels drop, growth slows, and most plants need far less — often half or a third of their summer amount. Snake plants and ZZ plants may go a month or more between waterings. Cooler rooms slow drying further, so a plant sitting near a cold window may need very little at all.

Heating and dry air

Central heating dries the air rather than the compost, so you end up with crisp brown tips on a plant sitting in damp soil. Group plants together to raise humidity, keep them clear of radiators, and treat the problem as a humidity one rather than reaching for the watering can.

Match the watering to the plant

There is no single correct answer for everything on the windowsill.

• Pothos, spider plants, philodendrons: water when the top 3–4cm is dry.
• Peace lilies: dramatic droopers that will tell you when they're thirsty, but try not to let them flop repeatedly.
• Snake plants, ZZ plants, most succulents: let the compost dry almost completely, then water well.
• Calatheas, marantas, ferns: keep lightly moist — never soggy, never bone dry.
• Phalaenopsis orchids: grown in bark, so water when the roots shift from green to silvery grey.

A five-minute weekly routine that prevents most problems

Once a week, walk round your plants with a finger rather than a watering can. Lift each pot to feel its weight, test the compost, tip away any water sitting in the saucers, and pick off dead leaves. Rotate pots a quarter turn so they grow evenly, and glance under the leaves for pests while you're there.

Then water only the plants that need it. If you're unsure, wait a day — most houseplants cope with being slightly dry far better than being constantly wet, and overwatering is much harder to undo than forgetting to water. More indoor plants are lost to a full watering can than to a forgotten one.

Photo: analogicus / Pixabay