Watering, and why plants die of it

Watering is the aspect of plant care most people get wrong, largely because the symptoms of the two errors resemble each other.

Why the symptoms confuse

A plant that is underwatered wilts because there is insufficient water reaching the leaves.

A plant that is overwatered also wilts, because the roots have rotted and can no longer take up water even though it is present.

Which means the natural response to wilting — watering — is correct in one case and fatal in the other.

The distinguishing check is the compost: dry means underwatered; wet means overwatered.

A sour smell from the compost, and dark mushy roots rather than firm pale ones, confirm root rot.

How roots work

The mechanism that explains everything.

Roots require oxygen as well as water. Compost holds both — water in the smaller spaces, air in the larger ones.

Permanently saturated compost has no air spaces, the roots cannot respire, and they die and then rot.

Which is why drainage matters more than watering frequency, and why a pot without drainage holes will kill nearly anything.

The method

Check before watering rather than watering on a schedule.

A finger pushed into the compost a couple of centimetres is sufficient for most pots; for large containers, a longer probe or lifting the pot to judge weight works better.

Water thoroughly when you do, until water runs from the base.

Partial watering wets only the surface, which encourages shallow roots and leaves the lower compost dry.

Let it drain completely and empty any saucer.

Water the compost, not the leaves, particularly for plants prone to fungal problems.

Water in the morning outdoors, which reduces evaporation and allows foliage to dry before evening.

What changes the requirement

Frequency varies enormously with conditions, which is why schedules fail.

Light: more light means more growth and more water use.

Temperature and humidity.

Season: most plants need substantially less in winter, when growth slows.

Pot size and material: terracotta breathes and dries faster than plastic or glazed ceramic.

Plant size relative to pot: a large plant in a small pot dries within a day; a small plant in a large pot stays wet for a week.

And the plant itself — succulents want to dry out completely; ferns want consistent moisture.

The techniques worth knowing

Bottom watering, standing the pot in water for twenty minutes and allowing it to draw up.

This wets the compost evenly and encourages deeper roots, and is particularly useful for plants that resist wetting from above.

Rehydrating dried-out compost, which repels water when completely dry.

Water runs down the sides and out without wetting the middle, giving a false impression of watering.

The fix is a long soak from below.

Grouping plants, which raises local humidity and reduces water loss.

Mulching containers outdoors, which substantially reduces evaporation.

Outdoor watering

Where the principles differ slightly.

Established plants in the ground rarely need watering except in drought, and frequent light watering encourages shallow rooting that makes them less resilient.

A thorough soak occasionally is far better than a daily sprinkle.

Containers outdoors need substantially more than plants in the ground, and in hot weather daily or twice daily.

New plantings need regular watering through their first season while roots establish.

Holidays

The practical solutions.

Move plants out of direct sun and group them together, which extends how long they last.

Water thoroughly before leaving.

Capillary matting or wicks running from a reservoir work for a week or more.

Self-watering containers, for anything that will be left regularly.

And for a longer absence, someone with instructions, since the common alternative — a very heavy watering before departure — generally causes root rot rather than preventing drought.

Water quality and temperature

A secondary factor that matters for some plants.

Tap water in hard water areas is alkaline and leaves mineral deposits, which affects lime-hating plants and shows as white crust on compost and pots.

Rainwater is preferable for those species and is free where a butt can be fitted.

Very cold water straight from the tap shocks the roots of tropical houseplants, and standing it for an hour resolves this.

Softened water from an ion-exchange softener is generally unsuitable for plants because of its sodium content, which is worth knowing in a household that has one.

Reading the plant rather than the calendar

The habit that replaces every watering schedule.

Wilting means either too little water or too much, since roots destroyed by waterlogging cannot take water up either — which is why checking the compost before responding to a wilt matters so much.

Yellowing lower leaves that feel soft generally indicate overwatering; crisp brown edges generally indicate underwatering or dry air.

A pot that feels light for its size is dry; one that stays heavy for days is not draining.

And a plant that recovers within an hour of watering was thirsty, while one that does not has a root problem that more water will worsen.