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.