Overwatering vs underwatering: how to tell

More houseplants die from too much water than from too little, and the two are easy to confuse. A plant short of water goes limp and dull. A plant whose roots have rotted in wet compost also goes limp, because dead roots cannot draw water up to the leaves. Both look thirsty, so it is easy to water them again. If the compost is already wet, that can kill the remaining roots.

The two problems need opposite treatments, so the first job is always to work out which one you have.

Check the soil before you water

Push a finger into the compost as far as the second knuckle, or lift the pot and feel its weight, before you add any water at all. Dry, light and crumbling means the plant is thirsty. Wet, heavy and cold, sometimes with a sour smell, means it is not, whatever the leaves are doing.

That check takes a few seconds and shows what to do next. A wilting plant in wet soil will drown if you water it. A wilting plant in bone-dry soil needs a thorough soak. You cannot tell which from the top of the plant, so check the soil instead of guessing.

What overwatering does

The name overwatering is a little misleading. What harms the plant is a lack of air around the roots, and constant watering is the commonest way to cause it. Roots need oxygen as much as leaves need light, and they draw it from the spaces between the soil particles. Every time you water, those spaces fill and then drain, letting air back in. That wetting and draining cycle is normal and healthy. It is also why hydroponics has to keep air bubbling through its tanks, since roots sitting in water rot unless the water stays full of oxygen.

When soil stays wet all the time, the spaces never refill with air. The roots suffocate and begin to rot, and fungi that thrive in wet soil, such as phytophthora, move in on the dying tissue. Rotten roots cannot take up water, so the plant wilts and yellows while standing in plenty of it. This is why overwatering is easy to make worse. The plant looks thirsty, so it gets watered again, and the last living roots drown too. Root rot is one of the commonest ways a houseplant dies.

You confirm it from the soil and the roots. Compost still wet a fortnight after watering, a sour smell, and lower leaves yellowing and dropping all point to overwatering. Tip the pot out and check the roots. A healthy root is firm and pale; a rotting one is brown and soft, and on a bad case the outer layer slides off between finger and thumb and leaves a bare thread behind. Constantly damp compost also breeds fungus gnats, whose grubs live in the top layer of wet soil, so a cloud of small flies round a pot is often a sign the plant is being kept too wet.

What underwatering does

Underwatering is simpler, and a plant recovers from it more readily, at least at first. A plant holds its shape with water pressure inside its cells. When the roots cannot supply enough, the cells lose pressure, the leaves and stems go limp, and the pores on the leaves close to save what water is left. A plant that wilts on a hot afternoon and stands back up by morning usually recovers without much harm.

Repeated or severe drying does lasting harm. Air bubbles form in the fine tubes that carry water up the stem and break the columns, so those channels stop working for good, and fine roots die back. A plant dried to the point of collapse several times stops recovering the way it did the first time.

The damage shows at the leaf edges and tips first, because they sit furthest from the water supply. They turn brown and crisp, with a sharp line between the brown and the green. Older leaves lower down yellow and drop as the plant gives them up to keep its growing tip alive. Outdoors, a tree or shrub that colours and sheds its leaves in the middle of summer has been short of water for weeks. The full picture is on the drought stress page.

Telling the two apart

Both problems end in a wilting, yellowing plant, so the leaves alone will not tell you which you have. The soil will.

• Wet soil under a wilting plant means the roots are failing. Do not add water; let it dry out.
• Dry, light soil under a wilting plant means simple thirst. Give it a thorough soak.
• Brown, crisp leaf edges, with a sharp line between the brown and the green, usually mean drought.
• Soft, yellowing lower leaves over wet, sour-smelling compost usually mean root rot.
• Firm, pale roots when you tip the pot out are healthy; brown, soft, stringy ones are rotting.

When a plant matters to you and the signs are mixed, tip it out of its pot and look at the roots directly. Firm and white is fine. Brown, soft and smelling of drains is root rot, and the sooner you cut the dead roots away and repot into fresh, free-draining compost, the better its chances.

Water thoroughly, then leave it alone

When a plant does need water, give it a proper soak rather than a splash. Water until it runs from the drainage holes of a pot, or until the ground is wet to a spade’s depth in a border. Then wait, and do not water again until the plant has used most of it. A deep soak followed by a dry spell encourages roots to grow downwards and outwards, to where the moisture lasts. This helps the plant cope for longer between waterings.

Frequent light watering keeps roots near the surface. It wets only the top couple of centimetres of soil, so the plant grows more dependent on the next watering rather than less. Outdoors this is the difference between a plant that survives a fortnight away in high summer and one that does not.

Water the soil rather than the leaves, and water in the early morning where you can. Wet foliage overnight invites disease, and midday watering loses more to evaporation.

Drainage, pots and compost

A plant can only be overwatered if the water has nowhere to go, so drainage matters as much as how often you water. A pot needs holes in the bottom, and it needs to drain freely rather than stand in a full saucer or a decorative cover that holds water. Empty the saucer half an hour after watering, and never leave a plant sitting in it.

The compost matters as much as the pot. A dense, water-retentive mix suits a thirsty fern, but can leave cactus roots wet for too long. Succulents, a snake plant, a ZZ plant and other dry-loving plants need a gritty, open mix that drains fast. If a plant rots again and again however carefully you water it, the mix is usually too heavy for it, and working in a handful of grit or perlite does more good than any change to your routine.

In the garden the same rule applies on a larger scale. Heavy, compacted or waterlogged ground drowns roots over winter, and you need better structure and drainage rather than fewer waterings. Raising a bed, forking in organic matter and keeping off the soil when it is wet all help air get back in.

Weather, season and the plant change watering

A fixed schedule does not account for weather or the plant itself. Both change how fast the soil dries from one week to the next. Wind and warmth pull water from a plant faster than sun alone, so a pot on an exposed balcony can need twice the water of the same plant in a sheltered corner.

Grouping plants by what they need makes the whole job easier. A shelf holding a maidenhair fern, a snake plant and a peace lily cannot be watered on one routine. The fern needs steady moisture, the snake plant needs to dry out fully between waterings, and the peace lily wilts quickly when it runs short. Put the thirsty ones together, keep the succulents apart and away from a radiator, and water each group separately.

Growth changes the demand too. A plant in active growth through spring and summer drinks far more than the same plant resting in winter, so ease the watering off as growth slows and the days shorten, and pick it up again in spring.

When you cannot decide

When the signs are mixed and you cannot tell, err on the side of not watering. A plant recovers from a short drought far more often than from drowned roots, and a dry plant often perks up within a day of a soak. An overwatered one takes weeks to show whether it will pull through.

For a pot that has dried so hard the water runs straight through without wetting anything, stand the whole pot in a bucket of water up to the rim until the bubbles stop, then lift it out and let it drain. Water poured on top runs straight down the gap between the shrunken compost and the side of the pot, so a soak is usually the best way to rewet the root ball.

Watering works best when you respond to the plant rather than the calendar. Check the soil before you add any, soak the plant thoroughly when it is dry, let the pot drain, and water less as growth slows in winter. Those few habits prevent most of the problems on this page.