Watering 102: A Guide to Watering Interior Plants, Part One

Watering is perhaps the greatest challenge people face when keeping plants indoors: getting it wrong can prove disastrous for many species, and it can be frustrating to lose a plant. As professionals, we are often asked how much water a plant needs, how often it needs it, and how to tell if it’s too much or too little.

I can’t give you all the answers: every plant is different, as is every interior environment, and both plants and their environment can change quite often. I can, however, outline (in a long-winded, but hopefully informative way) a few principles and techniques to help you answer these questions for yourself so that you can have greater success keeping plants indoors. This is Watering 102 because there are some more complex ideas here than are covered by other watering articles online for those that really want to nerd out about it.

After having finished this first bit, I've decided to release this guide in three parts, as it’s become almost absurdly long and you’ll probably want a break in between. I apologize in advance for not having kept it all more concise; please let me know in the comments if anything needs clarification. Today's tome of a post covers irrigation frequencies and the moisture release curve of growing media. Don't be scared. 

How Often Should I Water?

Irrigation frequency is a something of a complicated subject, as there are many factors at play. Some plants need more or less water than others, and these needs can change throughout the year. What follows are a few considerations to keep in mind as you decide for yourself how often to water your plants.

From an industry standpoint, many professional companies, in the interest of reducing labour spent on maintenance, visit their plants every two or even three weeks. (There are some that go even longer, but they either do a horrible job or else make use of sub-irrigation technologies to ensure plants have access to water in the interim.) At In Situ, we typically visit our plants once a week. (Just sayin'). 

The reasons for doing this are many, but an important one is to ensure that our plants make the best use of the water that is applied. We are typically maintaining larger plants than those often found in homes, but unless your plants are very small (4” pots or smaller, say), your space is especially warm or dry or the plants are in direct sun, weekly is probably a safe bet, or at least a good place to start when creating a schedule. Pick a day where you know you'll have the time, and stick with it. 

Note that when I said above that we water once a week (typically) in order to aid the plants in making the best use of the water we apply, I did not say that we necessarily apply more water than if we were to water every two weeks. In fact, the chances are that we apply less water over a two week period in two doses than we would do in a single dose. This is because soil that is still slightly moist can reabsorb water better than one that is bone dry. Peat moss (the main ingredient in most growing media), when dry, is hydrophobic, and actually repels water: we've probably all tried to water a plant and had the water glance off the soil and fly out of the pot and onto the floor, right? Never allowing the soil to completely dry allows one to apply only a minimum of water in order to bring the soil to the desired level of moisture, without having to rewet all the peat every time, which usually leaves it saturated, which can be dangerous.
 

A photo of a sopping wet rainforest which I've put here instead of a boring stock image of someone watering a houseplant. Bosque Protector Los Cedros, Ecuador. Image © In Situ Plants.

A photo of a sopping wet rainforest which I've put here instead of a boring stock image of someone watering a houseplant. Bosque Protector Los Cedros, Ecuador. Image © In Situ Plants.

Other sources may tell you that watering on a schedule can be bad for plants, and that the best thing is to 'water when the plant needs it'. Watering exactly when the plant needs it is great if we have little else to do but hover over our plants with a watering can all day. Watering on a schedule can be bad if you are just blindly applying the same amount of water every week, say.

Watering at regular intervals, though, can allow one to monitor the plants and to see what the effect of the last watering has been. For instance, if a week has passed and you check your soil to find that it is still very moist, chances are that you probably applied a bit too much water the last week, and you can ease up a little going forward. Likewise, if you discover a very dry soil (and hopefully not a wilted plant!), you can probably begin applying a little more water every week. Plants’ needs can change from week to week as well, due to the weather, whether the plant is flowering, etc., and so we need to take this into account when we do water.

Most potted plants (as opposed to those in hydroponics or the like), cannot take up water from saturated soils, because the extra water has displaced all the gases in the soil, primarily oxygen. This interferes with the normal growth of the plant (part of which involves taking up the water from the soil) and also causes roots to die, having been, in effect, suffocated.

Plants that are watered infrequently typically need a large amount of water applied to ensure that they do not go too dry before they are watered again. The consequence of this is that there is a period immediately after the plant is watered where not only is it not using the water (because it is unable), but it is also not receiving any oxygen to its roots, which can damage the root system.

When a plant is allowed to go too dry (again, another risk inherent in long irrigation frequencies), roots can also be damaged: the obvious way is through desiccation, when root tissues dry out, collapse and die. But there is another danger: when a plant`s soil is dry, roots, in effect, suck harder at the soil to try and take up as much water as they are able (this isn't technically exactly how it works, of course, but I'm not getting into capillary pressure, vapor pressure deficit, etc. here) to compensate for the water loss from the leaves through transpiration. If a plant is watered heavily at this point (and we`re all guilty of really soaking a plant that has gone too dry, mostly out of guilt), the plant takes up too much water too quickly, and the cells of the roots can rupture and, you guessed it, die. Proper restoration of a plant that has gone too dry involves gradually bringing soil moisture back up to a level where the plant can take it up without damage, which can take several hours’ worth of applying tiny increments of water to be successful. (Best, perhaps, to never let it get to that point, although as an aside I have heard that some plants can respond to drought stress by flowering, Spathiphyllum and Nematanthus among them.) It is perhaps a lot to ask someone to spend half their day watering one plant every half hour, and there are, to be fair, at least a few species which seem not to mind going from bone dry to wet, in my experience. Just be aware that this can be a concern.
 

The Moisture Release Curve, showing how water is available (or not) to plants. Image ©  Dr. Heiner Lieth; Retrieved from his page on the University of Caifornia website.

The diagram to the right shows an example of a moisture release curve, which illustrates the availability (and unavailability) of soil moisture to plants. In this, case, anything above about 75% and under 20% is pretty much unavailable to plants, and furthermore those extremes can be dangerous to plant root health. So in order to optimize water uptake and keep our root zone healthy, we need to keep the water between (again, just in this hypothetical instance) 20-75%. How do we do this? Water more often! 

This is where the weekly watering, as opposed to bi- or tri-weekly, can be a very useful tool. If we apply smaller amounts of water more frequently, we can maintain a more balanced soil moisture level, without the wild swings of ‘feast and famine’ watering. In this way we can keep the plant as close to its preferred level of moisture as possible for as often as possible, allowing the plant to grow its best without periods of inactivity or even stress. 

Stay tuned for the next installment; How Much Water, wherein I don't tell you how much water to give your plants.