Dryadella cristata, grown on a cork mount: no soil, no problem. Photo © In Situ Plants
Dryadella cristata, grown on a cork mount: no soil, no problem. Photo © In Situ Plants
Seemania purpurascens, one of the many fuzzy-leaved gesneriads that doesn't particularly appreciate cold water on their leaves. Photo © In Situ Plants
Seemania purpurascens, one of the many fuzzy-leaved gesneriads that doesn't particularly appreciate cold water on their leaves. Photo © In Situ Plants

Ways to Tell if a Plant Needs Water (Or Not)

We obviously would prefer not to greatly disturb a plant`s soil every time we water by performing a squeeze test to see how much water is in the soil (see here), but we definitely want a clear idea of how moist the soil is. Below are a few ways to find out.

One effective way is to stick your finger in it. For real: you will be able to feel if the soil is dry or moist, and the sense will become sharper with practice and experience, to the point where you will be able to tell whether it’s quite moist, just moist, etc. The only issue is that you can only measure the soil to the depth of your finger (unless you’re working in bigger containers, in which case go on and get your arm in there). You’ll also get dirt under your nails this way. There are soil moisture meters you can buy that likely do a better job than your own finger (they’re longer, for one thing), but I can’t really comment on them, having never used them myself.

Soils tend to dry from the top down, and even if it’s bone dry on the surface there may be an appreciable amount of water lower in the container, particularly if it is large or if the soil is of poor quality. Keep this in mind when you’re checking your plants. (This would probably be a good place to emphasize the value of good quality soil: the stuff you buy in the little bags at the hardware store or garden centre is not worth the bag you buy it in, in my own humble opinion. I will likely write another post on this, as it’s certainly worth one, but suffice it to say for now that you will have better luck with plants indoors grown in something described as a ‘growing medium’ instead of a ‘potting soil’. The differences in their properties make a large difference in how much water they hold and how available it is to the plants.)

Another way to gauge the amount of water in the soil is to lift the pot (or tip it, in the case of larger containers). Moist soil weighs considerably more than dry soil: fill one pot with moist soil and another with bone dry soil and try it for yourself. The difference is dramatic, and as you get into the habit of lifting your plants you will really get a feel for how much water there is in the soil. This technique works especially well with plants still in their plastic grower’s pots (and indeed, I first learned this technique in a greenhouse). Heavy ceramic containers make the method a little trickier, but not impossible.

Sometimes plants will let you know if they are thirsty by actually looking thirsty. This is called flagging in some circles, and it’s a fitting word: a flagging plant can stand out like a sore thumb to the trained eye amidst an acre of its counterparts. It can be a subtle paling of colour or slight drooping (or raising, in some cases- Calathea comes to mind) of the leaves. Though it`s probably best to water the plant before it gets to this point, at least you can (hopefully) take measures before there is extensive damage to the plant.

Which is the next point. There are two points beyond this: a temporary wilting point and a permanent wilting point, which are quite self-explanatory. There are quite a few plants that lack a temporary wilting point, though, and if they go too dry but once they’re done for. Other plants will recover but never be the same again, while yet others can completely collapse and look as dead as you like, but give them a drink and they bounce right back, without any apparent damage.Some don’t seem to even have a wilting point: they just soldier on, bone dry, as the weeks go by. (But even they get there, eventually.)

Plants damaged by too much or too little water can display symptoms such as yellowing/browning/loss of lower leaves, and wilting (though none of these are exclusive to water management alone- nutritional and other disorders can produce similar effects). As we’ve seen, the effect of too much or too little water is the same, and so the symptoms are similar. What will likely be different between an overwatered and underwatered plant is the soil moisture: if a plant looks dry but the soil is moist, you will know (unless you just watered it, of course) that the plant is too moist and needs to dry down. These symptoms above ground are a reflection of what’s happening in the root zone: if there has been extensive damage to the roots they may not be able to take up enough water for the plant to be able to support the top growth.

How To Water

I’d like to include a few things to think about when it comes time to actually water your plants. It isn’t, unfortunately (even after having waded through the preceding paragraphs), as simple as just dumping the right amount of water into the pot.

It helps to have at least a rough idea of how much water you’re applying to give you a good frame of reference for whether that amount is too much/not enough/just right. I don’t mean breaking out the graduated cylinder, but knowing how much your watering can can hold, for example, will help give you a good enough idea of how much you’re actually giving the plant, so you can adjust or keep steady the amount that’s best for it.

And I know I spent a whole long while talking about the value of a schedule, but if you check your plant on the given day and it doesn’t seem like it needs any water, by all means skip it. You may need to keep an eye as you get closer to next week’s watering to make sure it hasn’t dried out too much, but if the plant doesn’t need a drink then it shouldn’t get one. Fairly common sense, but this is the danger everyone warns of when they tell you not to get on a schedule in the first place.

It’s not always possible, but using a watering can with a breaker (the round bit that attaches to the spout with the little holes in it) is best, in my books: it slows the flow of water so that you can be more accurate in the amount you’re applying, and it waters across the entire surface of the soil, which is helpful if the surface has dried faster than it has further down in the pot. A good soil will move moisture throughout itself to a point, but soil will be more easily and evenly moistened if water is applied across the entire surface, so even without a breaker I try and apply the water somewhat evenly around the pot.

Use (at least) room temperature water: this is really important if you want your plants to actually grow. I haven’t actually experienced damage to plants through irrigating with cold water (maybe because I don’t do it), but I have heard and would be inclined to believe that it is a risk, particularly with cold-sensitive plants like Aglaonema. (I looked around online for papers or some credible source, but couldn’t find anything). What I do know for a fact is that water temperature is used in commercial greenhouses to control certain crops, with applications of cool or cold water controlling height and rate of growth in order to produce a crop that is finished on time, and at the standard size (think Easter lilies).

Watering from overhead is great if you can swing it: it helps to clean the leaves, and I feel like that’s how the plant would typically receive water in nature. Without the type of air movement experienced out of doors, though (to say nothing of the impracticality of it indoors, too), overhead watering can cause water to collect and remain in the crowns or axils of plants without evaporating away, where pathogens can take up residence and cause problems; crown rot of Phalaenopsis orchids is a big one that comes up a lot. Again, use warmer water, and water early in the day so that the foliage has a chance to dry before it cools down in the evening in order to keep these diseases from establishing themselves.

Well, there you have it. I guess in closing I should probably apologize for making such a simple thing so complicated, but as I mentioned at the beginning, watering is probably the thing that most people have a tough time getting right. I hope that the info I’ve included here is of use in your growing endeavors, and please let me know in the comments if there’s anything you feel I missed or that needs clarification. And I’d like to thank Albert Grimm and Bill MacDonald for first complicating the subject for me: many (if not most) of the concepts you’ve read here came from them.

Utricularia australis, which prefers to grow on the wetter side of wet. Image © Josef Hlasek; retrieved from his website
Utricularia australis, which prefers to grow on the wetter side of wet. Image © Josef Hlasek; retrieved from his website

How Much Water?

Once a schedule (see the last post) is in place that allows you to maintain better constancy in your soil moisture, you can now tailor the amount of water you apply to different plant species. There is no plant I know of that does not appreciate water, unless it is in dormancy. The issue is with the amount of water, and thus the amount of air, that is in the soil at any given time. Most cacti, for example, are evolved for long periods of drought, but this doesn’t mean that this is their preference. So often certain plants are described as ‘needing to dry out between watering’, but the truth is that this is not the case: rather, the plant needs to be prevented from ever becoming too moist for too long. Maintaining a very light but consistent level of soil moisture will allow xerophytic plants like cacti to maximize their growth without causing damage to the roots. This requires something of a steady hand in order to not give your plants too much water, and adapting your soil for different species can help a lot to get things perfect (in the case of cacti and other succulent species, I recommend adding something with a large particle size that improves the aeration porosity of the soil). It is worth noting, though, that with careful watering alone, you should be able to keep widely different species in a standard peat/perlite mix.

Knowing which species you are caring for is the first part of knowing how much water a given plant will need. There are countless resources online and elsewhere that recommend care for the majority of species in circulation. If you’re more inclined to nerdiness like me, you’ll also probably look into where the plant originates in order to learn more about its natural habitat, and take cues from this towards the plant’s care. (I will write more on this at some point; I promise.) All this, though, needs to be tempered with your own experience of the plant and how it grows in your environment, and this requires that you pay attention to what your plants are doing, and how their environment changes throughout the year. Some species are known to rest a little through the winter, for instance, and typically these do not require much moisture at that time. Conversely, the air generally becomes much drier indoors during the winter, when we run our furnaces, which may contribute to the plants’ increased respiration and thus need for additional water. Your growing situation is unique, and it is up to you to find what works best.

Some plants prefer a moist soil, and yet do not take up too much of the water, so after obtaining the correct level of moisture, one can apply small amounts to compensate for the plant’s uptake and any loss through evaporation. One the other hand, some plants which prefer moist soil use a tremendous amount of the available water, and so will need to be watered heavily on the regular. Plants receiving more light will use more water than those in shadier spots (and will require more feeding, but this is best left for another time). Larger plants will obviously use more water than smaller ones, though larger plants are often more drought-tolerant than smaller ones.

A quick note on watering until water flows from the bottom of the pot: unless your water is of very poor quality or you are feeding large amounts of fertilizer, this practice is not really necessary. It is a good way to ensure that your soil becomes evenly moist (read saturated), and it is definitely helpful to have drainage in the event that you do overwater, but it is also a waste of water. In our industry we keep plants for many, many years in containers which do not drain, and they fare quite well. Another aspect to this is that if soil is completely dry, often the applied water will travel straight through it and out the bottom of the pot without wetting anything along the way, which can give a false sense of accomplishment if all we`re looking for is water pouring out from the bottom. If your soil is already a tiny bit moist, simply water carefully (read slowly) until you bring the soil to where you want it, moisture-wise, and you won’t need to waste any water.

Quantifying (sort of) Soil Moisture

So I just spent a whole bunch of paragraphs explaining that different plants prefer different levels of soil moisture, often at different times. But what does barely moist or nearly wet actually look and feel like?

Enter the squeeze test. In most quality peat-based media (not the off-the-shelf stuff), the soil will dry to a conspicuous pale tan colour. If the soil is completely dry, take a handful and squeeze it and it will not hold together. A slightly moist soil will be slightly darker and will mostly hold its shape if you squeeze it as above. Moderately moist will be darker still, and should hold together and even release a tiny bit of moisture if squeezed. Moist and saturated soil will be as dark as the soil can get, with some and quite a bit of water, respectively, released upon squeezing.

This squeeze test is a good way to familiarize yourself with how much water is in the soil in relation to its appearance. Once you get a feel for it (ha ha) you will be better able to eyeball the moisture level of the soil more accurately, though relying on appearance alone can sometimes be deceiving (more on this in the third part).

If a plant wants to stay moist but not wet (and many do, but please don`t accept the care instructions that come with purchased plants as gospel, for there are quite a few variables I’ve already mentioned [and more that I haven`t] that can affect how much water a plant uses), keeping it between moderately moist and moist will likely keep it happy. Keeping a soil barely moist, as I mentioned above, is a great way to optimize the soil environment for succulents or similar plants. And then there’s all of the in-between.

You will need to make your best judgement as you work with soil moisture levels based on your growing environment and the species you’re working with. Don’t be afraid to experiment, but make your changes incremental as you pay close attention to the plant in order to draw useful conclusions and avoid issues.

One more thing I think I should note before I leave off: you’ll notice that I keep (almost exhaustingly, probably) using the word soil, and I should clarify that when I say this I am referring to whatever you’ve got your plants growing in (which I hope is a high-quality peat-based growing medium), and not that heavy mineral stuff from the back yard. These high-quality growing media are referred to as soilless mixes for the fact that they do not actually contain proper soil, but for our purposes here the term soil will be used (in part because it’s shorter and I seem to be typing it an awful lot).

I’m going to follow this post with one on Ways to Tell if a Plant Needs Water next week.