Why do plants need water?

Plants need water to survive. Water is essential as all the chemical reactions that take place in the cells of plants take place in water. Water reacts with carbon dioxide in the presence of sunlight, to make sugars (photosynthesis) which are then used for energy and to make the structure of the plant.

The movement of water through the plant from roots to the leaves (transpiration) provides much needed cooling. When plant cells are fully inflated with water under pressure (turgid), they are much stronger and this provides plants with a degree of rigidity.

Watering plants is, therefore, the most important aspect of house plant care.  However, if it is carried out incorrectly, it can lead to a plant's early death and lead to houseplant pests and diseases.

Over-watering is a major cause of the early death of house plants

How much water do plants need?

The amount of water required to keep an indoor plant healthy is decided by a mix of factors including its species, size, environment, state of health and, in some cases, the time of year.

Plant Species

Water use varies considerably between plant species. Although differences in leaf area make some contribution, the physiological nature of the plant is more important.

Cacti and succulents, for example, are adapted to live under arid conditions where water must be conserved. Their water requirements are low, except during active growing and flowering periods.

Large-leaved jungle plants, such as the Peace Lily (Spathiphyllum) on the other hand, have their origins in the tropics where water is plentiful and its lush expansive foliage can indulge in massive transpiration.

Plant Size

Clearly the bigger the plant the more water it is likely to need for photosynthesis, transpiration and other metabolic processes. Larger plants will be in larger plant pots, with a greater volume of soil. This means that more water will be needed to keep the soil evenly moist around the roots. It is worth noting that a pot with a diameter of 20 cm has approximately twice as much soil as a pot that is 15 cm in diameter.

Plant Environment

The most important environmental factor affecting water usage is light. Generally speaking, plants in bright light (except cacti and succulents) will require more water than those houseplants in poor light because they are more physiologically active. Humidity and temperature will also affect the rate of water use, but in most indoor situations, these conditions will be fairly uniform.

Remember that evaporation from moist compost is similar to that from an expanse of open water and increases rapidly with temperature. A good top dressing will help conserve water as well as enhance the appearance of your plant display.

Watering consistently

Plants are quite adaptable and they will slowly get used to their conditions as they grow. However, once they have adapted, they cannot tolerate wide fluctuations in their conditions. This means that consistency is the key to successful watering.

The best way of ensuring consistency is to make sure that you know when the soil is dry enough to be watered. You should not water when the soil is too wet, or when it has dried out so much that the plant is wilting.

House plants in pots get all of their water from the compost. If the soil around the plant's roots is dry, then the plant cannot get any water.

Conversely, if the soil is too wet, adding extra will only cause problems - lack of oxygen at the roots, decay, fungi and gnats (sciarid flies) are just some of the problems that can occur in saturated soil.

Watering technique

Water the soil, not the plant!  The aim of watering is to get water to the plant’s roots without drowning them.  Many people water little and often, but this often means that the water often evaporates before it reaches the roots, and the soil surface is permanently moist - which can attract fungus gnats.

A better approach is to water more, but less often.  The soil will act as a reservoir and will dry out slowly as the plant uses the water.  This will allow air to get to the roots as well.

Certainly check your plants once a week (or more), but don’t be tempted to add a little water ‘just in case’.  Large plants can often last for three or four weeks without water.

The key to successful watering is an even spread of the water over the soil, which then soaks through to the roots. Avoid pouring the water around the stems of the plant - aim for the outer edges of the pot as this will encourage the roots to grow into the soil.

If your houseplant came with the foli8 coir hair pot, then watering around the edge will have extra benefits of the foli8 pot absorbs water and helps keep the soil evenly moist throughout.  Over time, the plant’s roots will find their way to foli8 pot and this will help strengthen the root system and provide a steady amount of water.

Plant Watering Tips

In general, water plants more if:

  • Light levels are higher than average
  • Temperatures are warmer than average
  • Humidity is low
  • Plant vigour is good (the plant is healthy, developing new growth and readily using water)

In general, water plants less if:

  • Light levels are lower than average
  • Temperatures are lower than average
  • Humidity is high
  • Plant vigour is low or non-existent (the plant is weak and developing little or no new growth)

Remember! It is always easier to cure under watering than over watering
Avoid watering with very cold or very hot water. Cold water shocks plants, and plants like Aglaonemas are especially sensitive to cold water. Most indoor plants are native to tropical environments where cold water is not present. Ideally water should be tepid, or close to room temperature

Plant Watering FAQs

How do I know a plant needs watering?

Ideally, if you follow our tips, your plant should never look as if it needs watering.  However, if it does, do not worry - most plants will recover well if they are wilting, or if the soil is very dry.  If this is the case, then give your plant a decent drink and things will improve.

If your plant is wilting and the soil is wet, then you have over watered your plant.  In that case, it might be best to re-pot your plant in fresh compost and hope it will recover.

How often should I water my plants?

Less often than you think.  Little and often is not usually a good idea.  You would be better off giving your plants a larger amount of water (don’t saturate the soil) and leave a much longer interval between watering

Should I set a routine, or check every day?

That is purely a matter of preference, but remember, don’t be tempted to add water every time you check.

How much water should I give my plant?

Ideally, you should give enough water to make the soil evenly moist at the roots - so this might be a little more than you might expect.  However, you won’t have to do it too often

Do I need drainage in my houseplant pot?

No.  There are many myths about this, but it is not necessary to provide drainage unless you are prone to over watering your plant.  If you water carefully (and follow our guidelines), there will always be enough oxygen around the roots.  The foli8 pot is also porous (not plastic), so air will be able to get to the roots quite easily.

How do I measure the right amount of water for a plant?

This comes with practice.  Some people feel the weight of the plant pot and then add enough water to make it feel right.  Others count as they water - if they water at a steady rate, they might find that a count of 10 is good for small plants, and a count of 20 is best for big plants.

So much depends on the type of plant and its environment that it is difficult to give a precise measure for the right amount of water for a plant.

What happens if I give a plant too much water?

The roots and soil of an overwatered plant often smell foul.  The roots are damaged, frail, thin and dark in colour. Over-watering causes roots to fail, and when roots fail, plants can’t absorb water and nutrients properly, causing stress or death.

Remember that when watering, you’re watering the soil, not the plant. Plants will only take up what they need. All extra water accumulates in the soil, taking up air space. When air is removed and replaced with water, the roots fail because they need air to function properly.  Roots need air to breath.  Over-watering literally chokes the roots of plants, which can also rot when sitting in water for too long.

What happens if I don’t give my plant enough water?

The first thing is not to worry.  You can always cure underwatering (unless you have really left it too long).  If you have under watered your plant, just give it enough to get the soil moist right down to the roots.  If the soil has dried out a lot, it might be tricky to get it wet again.  However, with the foli8 coir pot, it is easier to manage as that will also soak up water and help get it back into the soil.

Should I add fertilizer?

A small dose of a dilute plant feed is beneficial at every watering.  Some people may think that fertilizing at every watering is too much for the plants, but if you add fertilizer to houseplants at very low rates every time you water, you won’t forget and you will make sure that the plant gets the nutrients it needs. You might prefer to fertilize less frequently but at higher rates, but you might forget.

What happens if I forget to water my plants?

Don’t worry - just add the right amount of water when you remember

What about when I go on holiday?

It is always a good idea to give your plants some water before you go away, especially if you are going to be away for a while.  If it is going to be hot whilst you are away, you might want to ask a friend to check for you, but in most cases, this won’t be necessary.  However, you might want to move any plants in windows that get direct sunshine, as that might dry them out quickly.

What are the signs that I have got the watering wrong?

The main signs of incorrect watering are yellow leaves and wilting.  These symptoms can indicate over watering and under watering, and this is because the roots might be suffering.


by Kenneth Freeman
4 February 2021

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