From the Ground Up

by Derek Burch

How to Water a Houseplant

Throw away the calendars, you must check the soil before watering! This is why the interiorscape industry has worked so hard to find self-watering pots and other techniques that let them keep a regular interval on their routes. It isn't an easy job in which to satisfy the customers.


Some generalities first. People are often surprised to be told that a deep pot drains better than a shallow pot containing the same volume of soil.

To understand this, you have to think of the water in the soil spaces as making many tiny continuous columns from top to bottom. The longer columns in a tall pot weigh more than the short ones in a shallow container, and thus are pulled more strongly by gravity, and are more likely to flow out of the pot.

This may also help to explain why putting an inch or more of gravel or pieces of pottery in the bottom of a pot may actually make drainage worse by shortening the length of the columns of water in the pot, rather than helping drainage by giving the water some big open spaces into which to flow.

Well, assume that the pot size is set, the good quality potting medium has been used, and the plant looks good. Now, how do you water? It doesn't really matter whether the water gets in from above or below. What is important is that the whole of the soil ball becomes wet. Not just the top inch, and not just around the sides, but all the way through. Then the plant roots will find good moisture conditions wherever they go, and will not be unable to use part of the medium.

Would you like to test this?

You need a bowl of water,
a brick-shaped kitchen sponge and another container big enough to hold the sponge when it is laid flat.
Saturate the sponge in the water. Lift it carefully and put it flat in the empty container. Some water will run out. Now carefully turn the sponge onto its long edge. More water comes out. Finally turn the sponge onto its narrow end - and still more water comes out. The columns of water are the same as the up and down measurement of the sponge at any time, so the taller the sponge, the more they weigh and the more comes out.
(It is even more convincing to use three different sponges in three trays, but who keeps that many sponges around.)

So, we have a soil ball that has been made thoroughly wet. Now gravity comes into play, and the water starts to drain out, allowing air to come back into the spaces in the mix. The roots will still have liquid available to them from the film held around the soil particles, but the various gases making up the air will dissolve in this and also be available to the plant.

This ideal situation continues until the loosely-held water in the soil is drawn down more and more by gravity. Eventually the soil would become too dry, so before that can happen, it is time to reapply water to soak the whole ball again. How do you tell when this moment to reapply is? The usual rule of thumb is to wait until a finger pushed into the top of the soil for about an inch comes out clean. It is no good doing it by the clock or the calendar, since the temperature and the amount of light that the plant gets will affect how much it has used. And the plant itself will not look any different at that moment, so the rough and ready method is all you can do unless you have learned to use one of the gadgets that is stuck into the soil to give you the timing to use. It is also possible to judge by the weight of the pot since, obviously, a dry pot weighs less than one with wet soil. This may sound weird, but it is actually a very good way once you have observed the plant's growth for a while, testing the weight of the pot or basket as you go.

What can go wrong in setting up this cycle of water/dry/water? One thing is that it is important that the whole soil ball becomes wet - not the top two inches, and not the outside edge leaving the middle dry. These both lead to difficulties in getting the potting medium to take up water again, particularly if it is soilless as it probably will be. Total immersion for a while is the solution here if that is feasible.

The other problem, if the medium has become dry enough to shrink away from the edge of the pot, is that the water applied from the top takes the easy way down the space between medium and pot, and never wets the rootball at all. The approach here has to be a slow application that stays on the soil ball and gradually sinks in. repeated two or three times at 10-15 minutes apart to get the medium to expand. Once it fills the pot, normal applications can resume. A few drops of soap in the first water put on may help it to wet the soil better. Allowing ice cubes to melt on the soil surface (not touching the plant), is also a good way of adding the water slowly enough for it to sink in rather than run off.

Does it make a difference if the water is applied from the top of the pot or allowed to soak up? I said no to that question above, but it does matter in terms of what happens to the fertiliser that is put on, and where any excess accumulates in the soil. Whichever method of watering you choose, it is good occasionally to run several changes of water through the pot to flush out any extra fertiliser salts, and get a fresh start.

Not by any means a job for beginners: this watering business is the most important skill to learn in growing plants.

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