It means there is already plenty of water in the soil when you're watering, and you're very likely over-watering, which is decidedly undesirable.

Water sticks to itself (cohesion - that's why water forms drops instead of every molecule going its own way) and to soil particles and pot walls (adhesion). The weight of water in the pot has to exert a force (gravity) greater than the sum of the force generated by water sticking to itself and to soil particles and to the pot. When there is enough water in the pot pushing down, the pot will drain until the the force of gravity equals the force of water sticking to itself.

You can see this in action by saturating a sponge and holding it until it stops draining. At that point, there is equilibrium - gravity vs the other 2 forces and it's a standoff. If you start spinning in a circle with your arm extended (the one with the sponge in it) the centrifugal force acts like artificial gravity, and the water remaining in the sponge will start to leak out of the sponge. Stop, and the water stops coming out of the sponge.

The first time you watered, there was little water in the soil, so you had to add more to make the force of gravity strong enough to overcome the forces holding the water in the soil, so it took longer for the water to start flowing from the drain.

Al

What Al said is the most likely scenario. A second, usually less likely, scenario is when the soil is utterly dry it becomes very hydrophobic (unable to absorb water) especially if it is largely peat based. It also shrinks away from the pot creating a clear channel down the sides of container. This is more pronounced for a plastic pot. In this case water may just run down the side almost immediately down the sides and out of the drainage hole without wetting anything. Under these circumstances it is best to soak the pot in a tub of warm water for half an hr or so.

Thank you guys for your comprehensive answers. Does this necessarily mean that I have to water less often, or could it also mean that I need to water a lower quantity everytime that I water the plants? I now usually try to give as much water too the point where the water drips out of the bottom. I thought this would be a good sign, meaning that the plants has fully received enough water.

How much do you guys water your plants every when you water them, and how do you make sure it is enough/not too much?

One thing that can be helpful to gauge when a plant needs water is to judge the weight of the pot. After using the tell, as Al described, lift the pot up to get a feel of the weight. Over time you will 'remember' this and you should be able to predict when it needs water. Do not select a specific interval between watering. Plants need for water changes with season, environment. Generally, in winter, plants slow down and so it may longer for the soil to dry out. In Summer or when plants are actively growing their water demand will be higher and you will have to check a bit more frequently.

Hello Al,

I have just read your post about good growing practices for beginners, and I have also read a couple of other posts from you embedded in that post. And I want to thank you so much for the extremely usefull and comprehensive information you have offered on gardenweb. I have been keeping plants for over a year now, and I actually own around 50 plants now, excluding my mosses and aquatic plants. During that time I have read quite a bit about keeping plants, and I also own a beginners guide book for houseplants. However I never really found the scientific basis for keeping container plants that I was looking for.

Now, while I was reading your posts on the subject I feel like I am finally closing in on the right trail towards keeping healthy, vital, sustainable plants.

I have several follow up questions:

If you have established that the soil is completely dry, using the dowel rod method (I used a wooden pencil, which I hope is also okay), is it then always a good idea to water the plant? It seems to me that in the case of some succulents (notably cacti, but also other dry spell loving succulents) it would be advisable to keep the soil dry for some time, how would you assess how long the soil needs to stay dry?

You talk a lot about the right potting soil in your posts. I often get my plants from the local super market, or my local nursery. How do I assess whether the potting soil they have used for their plants is correct?

How do you feel about submersion tactics for watering plants, since it seems to me that this would increase the amount of water the plant receives, while the PWT would stay on the same level?

I was also wondering whether there are certain books on scientific ways of caring for plants that you would recommend for people eager to learn, or whether you have maybe wrote some more comprehensive guides in other places?

Thank you so much for your advice, and I hope to hear from you,

Hidde Strikwerda

H - from the snippet you've shared about yourself, I can see you'll move forward quickly. If you skip the years of trial end error that keep most growers tide to an anchor, learn how to best proceed, then use your practical experience to validate what you know instead of finding out what you don't know, you'll be amazed at what's possible.

If you have established that the soil is completely dry, using the dowel
rod method (I used a wooden pencil, which I hope is also okay), is it
then always a good idea to water the plant? In a very high % of cases, it's best to water when moisture is barely detectable on the tell, but there are plenty of plants that need to be dry or almost completely dry during a portion of the growth cycle. No stress is good for plants, so if a plant should be actively growing, it's best to disallow them from going completely dry. There is sort of a built-in cush, however. When the soil feels dry to the touch, there is often still some residual moisture accessible to the plant. If you can see obvious signs of a plant being dry, it's too dry - again, unless you're adhering to a plan, the wisdom of which over-rides the rule of thumb. It seems to me that in the
case of some succulents (notably cacti, but also other dry spell loving
succulents) it would be advisable to keep the soil dry for some time,
how would you assess how long the soil needs to stay dry? I think I should have read your entire paragraph before I started to reply. I sort of gave you a reply in what I said immediately above. The rest of the answer is - researching RELIABLE sources for care instructions re individual species would always trump general rules or rules of thumb, but for the most part, for plants other than cacti and a middling measure of succulents, watering just before they go dry will keep you in good stead.

You talk a lot about the right potting soil in your posts. I often get
my plants from the local super market, or my local nursery. How do I
assess whether the potting soil they have used for their plants is
correct? For greenhouses and nurseries, the soil that's correct for them is the soil that allows them to get their plants to their POS (point of sale) for a minimal outlay. Generally speaking hobby growers are unconsciously seeking the gratification we get from nurturing. We're willing to spend more time, effort, and $ to ensure our plants are healthy/happy so we get that gratification/personal satisfaction. A few days ago, I mentioned the relationship of greenhouse to the plants they sell is closer to the relationship between puppy mill and puppies than it would be to you or I and the plants we tend.

Potting media run the gamut of suitability from virtually unusable to almost perfect. It's easy to make the case that a "good" soil would necessarily be one you don't have to battle for control of your plants' vitality. IOW, unless you can water correctly w/o having to worry your plants will suffer from extended periods of soil saturation, it's hard to say you're using a good soil. Things take a very sharp turn in favor of the grower when they understand the concept that allows them to control how much water their soil will or can hold. Where fertilizing containerized plants over the long term is quite difficult to get right when you can't water correctly, it's monkey-easy when you can. Getting past that whole 'water-retention/good or bad soil thing' is undoubtedly the biggest obstacle container gardeners will find in their path, no matter whether they ever realize it or not.

How do you feel about submersion tactics for watering plants, since it
seems to me that this would increase the amount of water the plant
receives, while the PWT would stay on the same level? Submersion has it's place - mainly for plantings that have gone dry, and contain media that turn hydrophobic (water repellent) once the % of moisture in the soil drops below a certain point. I like watering from the top because it's more effective at flushing accumulating salts out of the soil.

For any given soil that supports perched water, the maximum ht of the perched water table is a constant. No matter how large a container is or what shape it is, if the soil supports 4" of perched water, it will do that in every pot it's used in. It follows then, that no matter how you water, as long as you water to beyond the point of complete soil saturation, the ht of the PWT will be exactly the same at the point the pot has stopped draining (for a given soil).

I was also wondering whether there are certain books on scientific ways
of caring for plants that you would recommend for people eager to learn,
or whether you have maybe wrote some more comprehensive guides in other
places? I learned a lot from the Ball Publishing book "Water, Media, and Nutrition for Greenhouse Crops", from "Plant Production in Containers II" by C.E. Whitcomb, Ph,D, and from Growing Media for Ornamental Plants and Turf by Kevin Handreck & Neil Black.

I don't know if you've read what I wrote about How Water Behaves in Soils, but it's a collection of information designed specifically to unburden growers of the limitations associated with overly water-retentive soils. There are also threads that describe work-arounds for those who can't or are unwilling to modify water-retentive soils to make them less limiting.

Al

Al,

What an amazing post and refresher to other long form threads you have written, thank you!

Regarding PWT - Do we wait until they are nearly dry using a tell, or should we be watering once the soil is nearly dry to the height of the PWT? I have been doing the former, but have meant to ask this question to you for a while now.

Depends on the plant. From above: If you have established that the soil is completely dry, using the dowel rod method (I used a wooden pencil, which I hope is also okay), is it
then always a good idea to water the plant? In a very high % of cases, it's best to water when moisture is barely detectable on the tell, but there are plenty of plants that need to be dry or almost completely dry during a portion of the growth cycle. No stress is good for plants, so if a plant should be actively growing, it's best to disallow them from going completely dry. There is sort of a built-in cush, however. When the soil feels dry to the touch, there is often still some residual moisture accessible to the plant. If you can see obvious signs of a plant being dry, it's too dry - again, unless you're adhering to a plan, the wisdom of which over-rides the rule of thumb.

I could have been clearer by writing "moisture levels of soil deep in the pot".

Plants are able to absorb water in vapor form from air pockets in soil; so, if there is moisture in the lower reaches of the pot, vapor will be diffusing into air pockets above the water reservoir deeper in the pot. So, when moisture levels are getting low in soil deep in the pot is when you should be watering most plants. Of course, cacti and a fair number of succulents (especially during periods of quiescence) will want to go completely dry, often for long periods to avoid root complications associated with too much water in the rootzone. Most trees and houseplants, and a fair share of succulents, especially during the growing season, will want water before the plant goes dry. Generally speaking, even though no plant responds favorably to stress (from the plant's perspective), it's better that the plant should suffer some slight drought stress than the stress issues associated with over-watering.

I'm not 100% sure I fully understood the question, but w/regard to the PWT: The maximum height of a perched water table in any given soil will be a constant, not withstanding the effects of altitude (gravitational flow potential decreases with increasing altitude) and temperature (warmer water is less viscous than cold water, so the max PWT height in a given soil is inversely related to temp). However, the ht of the PWT steadily decreases with time, as long as no additional water is added to the soil.

Al

Got it. I understood you correctly, so my reply above is appropriate. Adding to what I said - any PWT remaining in soils that can/do support PW should have been entirely or almost entirely used by the plant and/or evaporated by the time you next water.

Al

Hello Al,

Thank you so much for once again a very comprehensive answering of my questions. I have read the information in the embedded link in your comment. And I am planning on buying the plant production in containers II, when I have raised the necessary funds (college student :).

Kind regards,

Hidde