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The simple answer is “whenever they need it.”
Unfortunately, an answer like “once a week,” or some such, is more or less meaningless, because so many factors influence how much water a given plant needs or uses.
The most important factor is light, because the more light it has, the more water a plant will use.
Other factors are the kind of plant it is, its size, the make up of the soil, even the way you water it.
So the real question is not “How often should I water my indoor plants?,”
but “how do I know when the plant needs water “
and , “how do you figure that out?“
Let’s start by talking a bit about soil moisture.
Moisture in the soil is a dynamic and constantly changing process, not an event.
From the moment you pour water into a potted plant, the soil moisture is going through a process of draining, evaporating, and being absorbed.
If the soil is too wet today, wait a few days, it will be drier.
And being drier – or being more aerated, which is probably a better word – is super important because the roots need access to air around them if they’re going to be healthy.
Over-watering doesn’t happen because too much water is poured on, or even because the plant is sitting in a dish of water – overwatering happens because the soil isn’t allowed to aerate enough before more water is added.
Plant species also differ in the amount of aeration the roots like, so start by knowing the name of the plant, and doing some research* to find out what level of soil dryness/moisture is best.
I call that the target moisture level, or TML. The next thing is to test the soil moisture to find out what the moisture level is like, all the way to the bottom of the pot. More about that a little later.
* Unfortunately, research often doesn’t yield consistent information, so your own observation is really important. If your plant isn’t doing well, try a different moisture level, light level, etc.
So You have determined that it is time to water…
The next question becomes “how much should I water?”
The answer is quite simple – water enough to get a runoff from the drainage holes.
If the plant is sitting in a drainage saucer, for a 6″ or 8″ pot, a runoff of 1/4 – 1/2″ is good; for a 10″, 12″, or 14″ pot, you want a runoff of an inch or so.
The reason is that it’s very important to get moisture to all the roots, not just those at the top. Watering till you see a runoff helps to make sure the whole root mass gets water.
You don’t have to empty the water from the saucer!
It will either evaporate or be absorbed by the soil in a couple of days. If it’s still there after a week, toss it out so it doesn’t attract bugs.
Next time you know you watered too a bit too much.
When you first water after starting to test soil moisture, keep a record of the moisture level of the soil before watering, the amount of water you use – that is, the amount you need to pour on to get the nice run off from the drainage holes – and the date you watered.
Test the soil every few days until the next watering time, then record all the information again.
By adjusting the amount of water, you can adjust the time between waterings, and get your plant/s on a regular schedule – whatever is convenient for you, like once a week, or once every other week.
Then you’ll be able to answer your own question of “how often should I water my plants,” as well as “how much should I water.”
So now, let me go on a little bit about how you actually test the soil moisture, and what it means.
1. Make sure you test from the bottom of the pot.
You want to check the moisture all the way to the bottom of the pot, not just on the surface or the top couple of inches. The reason? – moisture on the surface can evaporate so the surface feels dry, even though down deep where the roots are, the soil is staying wet.
2. There are several tools you can use – use a wooden probe, an electronic moisture meter, a pinch test, or lift pot to judge weight.
- wooden probe – a thin wooden dowel, a bamboo kebob skewer, even a pencil will work. Stick into the soil as if testing a cake, then pull up – if soil is dry the probe will be clean, if soil is damp some bits will be sticking to the probe. Also, when run between your fingers, the probe will feel dry, slightly damp, damp, or wet.
- moisture meter – this is an inexpensive electronic device sold in most plant stores and garden depts., or online. Stick this all the way down into the soil, and the meter* will give you a reading from “dry” to “wet.” Here is a great low cost moisture meter on amazon.
- Moisture meters are cheap, and notoriously apt to break down. That’s why it’s a good idea to RUN THE PROBE BETWEEN YOUR FINGERS when you pull it out of the soil, then compare what you feel with the meter. If the meter is saying “moist” but the probe feels dry, or vice versa, the thing isn’t working.
- A moisture meter that feels dry when the dial is reading moist can be a WARNING OF ELEVATED SALT LEVELS IN THE SOIL. Try washing the salts out of the soil by running copious quantities of water through the pot (about 5X’s the pot volume,) let the soil dry for a week or two, then try the meter again.
- pinch test – reach down into the pot with a spoon, pull up some soil, and pinch it between your fingers. This can tell you a lot about the soil moisture, although you should also use a probe, since the soil you pull up with a spoon will be closer to the soil surface.
- Sometimes the soil has good capillary action, so the moisture content is mostly the same throughout. If the cap. action is not good, however, that’s when the soil will be drier on top than at the bottom of the pot. A quick double-check with a probe will tell you if you can judge the bottom of the soil mass by how the top is feeling.
- weight – the less water in the soil, the lighter the plant will feel. Best to use this method only after you have determined the moisture level by actually feeling it with a probe or meter, and understand the relationship between the moisture level and the weight of your particular plant. After you understand this, you can effectively judge moisture by simply lifting the plant.
3. What does it look like, how does it feel, who wants what?
Start by knowing what kind of plant you have, and what it prefers in terms of soil moisture, especially how much moisture should be out of the soil before watering again – the target moisture level or TML, as I mentioned earlier. The soil should reach this level before you add more water.
- In general, for cacti and other succulents (and some foliage plants in low light) the TML is almost dry. The probe will feel dry or almost dry; the meter will read just slightly to the right of “dry”; the pinch will feel a dry but not scratchy* and will not stick together; the weight will feel relatively light.
- If the soil is so dry that it feels scratchy, it’s usually too dry, and you should either water more than you did the previous time, or water more frequently. However it’s true that succulents and cacti in low light may need completely dry soil.
- Most indoor foliage plants have a TML of very slightly damp soil. The probe will feel just barely damp; the meter will read 1/4 to 1/2 the way from “dry” to “moist”; the pinch will feel soft and cool, will stick together, but fall apart easily if you touch it; the weight will still be relatively light.
- larger foliage plants in higher light need to reach the damp to slightly moist TML The probe will feel damp but not wet; the meter will read 1/2 to 3/4 of the way from “dry” to “moist”; the pinch will feel a bit damp and will separate into 2 or 3 bits if you touch it; the weight will be somewhat heavier.
- some few plants – those that are usually listed as “liking moisture” may have a “moist” TML . The probe will feel definitely damp but not wet; the meter will read “moist” or just a bit to the left of it; the pinch will feel damp* and will stick together if you touch it; the weight will be a bit heavy, but still much lighter than just after you water.
- If you see water actually lying on the surface of the pinch, or if water drops emerge when you pinch, the soil is already wet, DON’T ADD MORE WATER YET.
It may seem too long and complicated, but once you read through the whole thing, I think you’ll see that it’s really pretty simple. And once you figure out what’s going on, you might even find that your thumb is starting to turn green.