Container planting: intuition vs. reality

I’m just starting to think about getting my containers planted for the summer and happened to get an email on the topic from a blog reader. John was frustrated with a local columnist’s advice on using gravel in the bottom of the containers for drainage. When challenged, the columnist refuted John’s accurate comments with “logical thinking.” (You can find the posting and comments here.)

Here’s part of the post: “I like to cover the hole with a layer of gravel to improve drainage. Plants need to have their roots exposed to air in the soil to survive and thrive. If the container has no holes for drainage, it will fill with water and drown the plants very quickly. It is better to keep your plants on the drier side than to keep them constantly moist or wet. The big danger in using pots is drowning plants.” Later, he goes on to explain “The potting soil plugs up the drain hole and the water is trapped behind the plug. The layer of gravel creates an area for the water to drain through to escape. The creation of drainage commonly involves a layer of gravel.” This reasoning is part of what he calls “Logical thinking 101.”

As my husband pointed out, this isn’t logical thinking: it’s intuitive. It’s what we think is going to happen in the absence of any evidence. And in this case, it’s wildly inaccurate.

Jeff and I have both discussed the phenomenon of perched water tables in containers as well as the landscape in previous posts and on our Facebook page. The fact is, when water moving through a soil reaches a horizontal or vertical interface between different soil types, it stops moving. Here’s a photo from a very old research paper on the topic:

A layer of silt loam sits above a layer of sand, and water from an Erlenmeyer flask drips in. Intuition says that when the water reaches the sand, it will move more quickly through the sand because the pore spaces are larger than those in the silt loam. But intuition is wrong, as this series of photographs clearly demonstrate. Water is finally forced into the sand layer by gravitational pressure, after, of course, saturating the silt loam.

Intuition has its uses (I am quite proud of my own intuitive powers), but it doesn’t trump reality.

46 thoughts on “Container planting: intuition vs. reality”

  1. I think this is apples and oranges. The author of the article in question is talking about holes plugging up in a container, not water flowing faster through the gravel.

    1. That’s exactly my thoughts reading this, too. The rock or rocks in bottom of a container (well, I only use one larger rock, not sand) is to keep the drainage whole from stopping up.

  2. The author says that gravel “improves drainage.” If that doesn’t meant water flows faster, what does it mean?

  3. I have seen this on HGTV and other “reputable” sources too. I explain this to my soils students in terms of water’s adhesion to soil particles, and not moving into large empty spaces until the soil reaches saturation. This condition is the opposite of good drainage!

  4. One of the first things I learned in Soil Science was about the perched water table. I’ve argued about it with people ever since; they just can’t give up the idea of gravel/sand improving drainage. Those photos are very persuasive. Thanks.

  5. This series of images clearly demonstrates that silt is more absorptive than sand.

    If you put a sponge on top of a layer of sand, of course it won’t make it through the sponge until it is over-saturated.

    If the sand were on top and the silt below, would the results be the same?

    This experiment doesn’t, in any way, simulate a ceramic container with a single drainage hole filled with potting soil over a layer of gravel (to keep the drainage hole open).

    Apples and oranges indeed.

  6. 1) Yes, you still have a perched water table if sand is on top of silt loam. Water doesn’t adhere to sand as well as it does to silt loam, so it moves through faster. It stops when it meets the silt loam.
    2) The drainage hole has nothing to do with the interface issue. In the photo, the water hasn’t reached the bottom of the system.
    3) Soil that gets into the drainage hole doesn’t turn into concrete and prevent water drainage, but it can make a mess as it dribbles out. A pot shard or a piece of wire mesh at the bottom will keep soil in and not interfere with water movement.

  7. “Water doesn’t adhere to sand as well as it does to silt loam, so it moves through faster.” But not through gravel?

    “The drainage hole has nothing to do with the interface issue” In your example but not in the case of a ceramic pot.

    “Soil that gets into the drainage hole doesn’t turn into concrete and prevent water drainage” True, it doesn’t turn into concrete but does impede (not prevent) water drainage.

    “A pot shard or a piece of wire mesh at the bottom will keep soil in” Why is even this necessay if “soil in the drainage hole doesn’t prevent drainage?

    I know you have a lot invested in your argument but it doesn’t hold water.

  8. 1) Sure, if you put gravel on top of soil, as you asked in your original post. That’s why I said “Yes, you still have a perched water table if sand is on top of silt loam.” Let’s not take this out of context.
    2)Soil in the drainage hole that is the same texture as that above it does not change drainage rates.
    3) Again, you are taking things out of context. I quite clearly said “but it can make a mess as it dribbles out.”

  9. Thank you Linda! I have the hardest time convincing folks that putting rocks in the bottom of containers is a no-no. I tell people to imagine that the potting soil is like a kitchen sponge. When it is soaked, if you hold it vertically upright most of the water gathers at the bottom. By adding gravel, you are moving the bottom of the sponge closer to the roots and thus displacing more air in the root zone. It is more an issue of where the water is in the soil than an issue of drainage.

  10. I quit putting anything in the bottom of my pots years ago, and I haven’t had any problem with soil getting out through the hole. (I moisten the potting soil thoroughly before putting it in; dry potting soil probably would leak.)

  11. I love these old photos – and the distinction you make between logical and intuitive thinking, something I think we’re all guilty of from time to time.

    We run up against the incorrect assumption you examine here all the time as it pertains to trees and planting media. Would you be willing to let me reprint this post on our blog? Of course we would credit and link back to the original post.

  12. Leda, please feel free to use on your blog. We’re all about spreading good information wherever we can!

  13. what if PB said, “Put some gravel in the bottom of your pot…it’ll keep the soil in place!” Then would it be ok to put gravel in the bottom of your pot?

  14. Thanks for the link, Gary, although your suggestions don’t show up for some reason. In any case, what Peter has reported is a good example of anecdotal information. He hasn’t conducted a controlled, replicated scientific experiment and it hasn’t been peer-reviewed and published in the appropriate journal. That may seem like overkill, but that’s what experimental scientific evidence is.
    One of us (Jeff or myself) will do a followup post on this later, and we’ll try to include some information on how to set up and conduct this experiment properly.

  15. I agree 100%. I stopped using anything but pure potting soil in my containers years ago. But I do have a question. I know someone who puts small to medium size fieldstone in her large containers as they are made of plastic as she doesn’t want anyone to steal them so she makes them very heavy. But this is creating the same issue then with the water table?

  16. Yes, the fieldstone at the bottom would create the same problem, unless the gaps between the stones were filled with the same soil as above (you don’t mention if there’s a barrier separating the soil from the stones.)

  17. Ahhhhh, now this could get interesting . . . . . . scientists versus a widely read/viewed (and respected) regional mass media horticulturalist that is interested in debunking myths, too (try this link, Linda, http://blog.timesunion.com/gardening/). This blog interaction could – if well documented – provide very a very powerful and publishable argument for the value of direct online interaction of scientists and the consuming public (practically worth the $300.00+ I just shelled out for my annual ASHS membership)! I’ll be very interested to see the experimental design created by the Garden Professors to debunk that already published online (and possibly in print) by Peter Bowden!

  18. A couple of additional thoughts/comments to my first post . . . . . . at the end of the day, any experiment to prove/refute the value of gravel in the bottom of containers should address the
    question, “which results in a knock your socks off display of flowers/foliage that will make your neighbors green with envy.” After all, isn’t that our real goal? Also, I don’t check this blog out more frequently because the conversations often make me forget to shower, shave, eat, go to work, etc.;-) Keep up the great work!

  19. Linda, if you’re getting a lot of traffic from Florida on your informed gardener website, I may be adding to that volume. You are part of every presentation I do, and since I’ve been on book tour there have been quite a few. I talk about old gardeners’ tales, especially this one with the gravel in the containers impeding drainage.
    Thanks for all your hard work!

  20. What is the real question?

    I assume that people believe in perched water tables.

    The pictures Are very interesting, but they don’t mimic the real life of watering some potted plants.

    One question is; will a pot of just soil, and one drainage hole, plug up. Easy to prove. Since most plants sold today don’t have drainage gravel it is sort of proved already, except that these pots usually have multiple holes.

    Most people will add lots of water to their pots. Point the hose on it, and water until water comes out the bottom. So water slowing down at the interface between soil and drainage rocks is not important. My understanding is that the soil above the gravel will hold more water than the same soil without gravel, in the case where lots of water is added.

    One question is; is this last statement true? Or is it only true with smaller amounts of water? ie once the water passes the interface, will the soil above the interface still hold more water.

    another question is; is this extra water detrimental to plants in a pot? A key side question might be – how much extra water is there, and how long does it last – in a pot under normal growing conditions.

    My ‘intuition’ says that in a pot of flowers sitting on the porch the effect would be negligible; a relatively small amount of excess water that evaporates fairly quickly. But I will believe the science.

  21. There are really several discussions going on here…. The original argument is about rocks improving drainage… they don’t.
    BUT there are lots of reasons to include rocks… preventing spills (as suggested), adding weight so the pot doesn’t tip, decoration (if it’s clear container (such as terrarium), or to impede drainage (this is key)… The biggest enemies to your flower planters looking good are 1) Keeping them watered and 2) washing the nutrients away. Adding rocks to the bottom of a well-drained (and maybe too well-drained) potting mix might actually help your flowers look better as the much needed water is held rather than drained due to gravity and pore size.
    Also keep in mind that most potting mixes are developed for smaller containers than a patio planter… so at the end of the day, adding rocks could perpetuate a myth if the plants look better “it must be because of improved drainage” when it’s probably more along the lines of “must be because of decreased drainage”

  22. Two things come to mind when I think “Perched Water Table”: 1- (this might sound silly, but).. when I drain pasta in my colander, I can hold it until it stops dripping, however, if I tilt it diagonally, it will begin to drip again. That is because what I have done is to change the position of the water table. Just because you move (or alter) the position of the contents of the container (with gravel or sand, at the bottom of the pot, presumably), does not mean you’ve gotten rid of the perched water table; you’ve still got it, all you’ve done is move it! If you’re trying to keep your soil from spilling out from the pot while filling it, moisten the soil first, like one poster above mentioned. If you’re trying to keep your roots from drowning, however, try an over-all larger particle size growing medium. Once upon a time, I read an excellent article on perched water tables here: http://davesgarden.com/community/forums/t/527353/ , and found the information to be a treasure trove of information I now cannot live without. It’s nice to see here that the intelligence lives on.

  23. Well, at least we all agree containers need to drain, so what the best way to achieve that? If I fill the bottom of my container will drainage holes, and fill the container with silt loam, won’t the water STILL prefer to saturate the silt loam before running out of the drainage holes?

    I’m interested in a formula for the number and size of holes needed related to soil volume/ density and surface area on the bottom of the pot.

    Thank you for all of this!!!

    1. Paige, the difference is that water also flows down the inside of the container so you have a hydrated surface at the bottom. You won’t get the backup problem, though the more holes you have the better. What the best formula is, I don’t know. I’ve asked Jeff Gillman to weigh in as well.

  24. Thank you Linda. I seem to remember reading in one of your publications/ posts that more small holes are better than fewer large holes? I feel that as long as excess water doesn’t have to travel more than 12-15″ to exit the container, it’s fine, but this is based on nothing but intuition. Look forward to Jeff’s response. Also, any thoughts on pitching the bottom of a container towards one large drainage hole? Thx again.

  25. The thing that would interest me (currently) is the longer term moisture distribution in large outdoor containers (tomatoes, eggplants, peppers).

    With a big plant in hot weather would a PWT last?

    1. You won’t have roots develop into or under a PWT. So your plant will stay more shallowly rooted than it would if the soil was uniform throughout the container. Sure, it might temporarily dry out, but by that time your plant will be under stress.

  26. Thanks for the speedy reply!

    As I understand it, most common potting soils have some kind of PWT.

    I think a strategy of “10 gallon pots for 5 gallon plants” acknowledges that, perhaps in a different way than trying to eliminate PMT in that 5 gallon pot.

    So in terms of those cross-sections, I am thinking of a large pot (specifically a 10 gallon nursery pot) and what happens as I run my small patio drip system on a hot day. My intuition is that it isn’t very static, and that the 24 hr cycle would be pretty dynamic.

    Ah well, I guess commercial greenhouses wouldn’t sweat it, and would just use 5 gal poly bags, a conventional mix, and drip. Perhaps they think a perfected watering schedule moots the issue.

    1. Hi John –

      A potting mix won’t have a PWT unless it’s layered onto or into another type of soil. That’s what creates the problem. So potting mixes are perfectly fine, as long as it’s understood that much of the volume will disappear as the OM decomposes.

  27. I’ve been experimenting with various types of container media for about 20 years. The most effective media I’ve used consists of somewhere around equal parts of screened calcined clay (Turface), screened crushed granite or cherrystone, and screened pine or fir bark. The media are adjustable for water retention by varying the ratio of porous: nonporous ingredients.

    Perched water disappears in media made of particles larger than about .100″, so screening the particles to between .100-.187 (1/10 – 3/16″) leaves me with a medium that doesn’t hold perched water, yet holds ample amounts of water to satisfy plant needs inside of the porous particles, on particle surfaces, and at the interface where particles contact each other, leaving all the inter-particular spaces filled with air from top of the medium to the bottom, regardless of container depth.

    I’ve not found that the number or size of holes in container bottoms makes a difference in how much water is held at container capacity. The weight of the water column will force water from the drain hole(s) until the combined forces of adhesion/cohesion equal the gravitational flow potential of the water column. At that point, the water perches in the container (if the particles are small enough to support perched water). If there is a medium substrata in the container, water will perch above the substrata only if the particles in the substrata are at least 2.1X larger than the particles in the upper strata and the upper strata will support a perched water table.

    If a container medium that supports perched water is at container capacity, additional water will exit the pot if a wick is employed, if the container is tipped, or if the container is lifted smartly up and down, which employs inertia and a little muscle power to rid containers of unwanted perched water (no help to the nurseryman, but a good tip for the hobbyist).

    The medium I mentioned is something I’ve been using for all my houseplants, cacti, succulents, bonsai trees, and all woody material I grow on as candidates for future bonsai. Because it does not have inherent perched water limitations, it offers plants a much better opportunity to realize the their genetic potential when compared to media based on fine particulates that support several inches of perched water.

    Al Fassezke

  28. Long ago, when I first potted up some plants, I followed the “place a pot shard over the hole” advice. (I had to look up the definition of “shard.”) Potting soil dribbled out and made a mess, or somehow plugged the hole entirely. And then I tried gravel. Same result, and the best I remember, when I did any re-potting, the gravel layer didn’t have any roots in it, or rotting roots. Anyway, it didn’t look good to me.
    So, I thought, “Cloth. Cloth will let the water through, but not the dirt.” What kind of cloth won’t rot? Maybe some synthetic?” So, I put a 2 or three layers of old panty hose in the bottom. I’ve done it ever since, though now I don’t just line the bottom, I let the cloth run up the inside of the pot a little, too, thinking “wick.” I’ve also gotten so I’ll use just about any cloth, including old cotton terry towels. Plants seem to like it. At least they don’t die on me any more. I have plants that have survived for decades, with being re-potted every now and then when their roots get crowded.
    Recently, I’ve seen a few how-to posts that say, “cover the hole with a coffee filter” instead of a shard, or chips, or gravel. And, I’ve read some articles about perched water tables. How likely is it that running the cloth part way up the inside of the pot is actually helpful? Is it wicking?

    1. I don’t know whether there would be any significant wicking action. I don’t think it hurts anything, but it would be a nuisance if you were trying to repot the plant. The roots would probably have colonized the cloth, and removing it would damage some of the roots. I think just have the layer at the bottom to keep soil in is a great idea.

      1. Linda, you have often mentioned that landscape fabric is a no-no because it plugs with soil and inhibits drainage. Wouldn’t the same thing happen with cloth or a similar material placed on the bottom of a pot?

  29. I am new to this site, there has been much debate about PWT in containers on other sites. I have mostly propagating native plants in Conetainers ( RL SC10 and Deepots). The medium used is either Peat/Perlite or Peat/Vermiculite 50/50mix. Most forestry nurseries are using this medium. Is the PWT really that much of a problem in this situation. The trees are growing great, roots look excellent. I use gravimetric Irrigation Scheduling at 80% to 70 % container Capacity. Also what particle sizes for medium are optimum for containers 6” to 12” depth. Thanks

      1. Consider: Uniformity of particle size alone doesn’t guarantee you won’t have to deal with a PWT, and stratification alone isn’t enough to create a PWT.

        PWTs exist when the gravitational flow potential of the water column is less than the combined attraction of adhesion/cohesion (capillarity) acting on water in the medium. As particle size decreases, the sum of adhesion and cohesion increases. PWTs begin to appear in spaces between soil particles as particle size becomes smaller than about .100″, and the PWT increases in ht as particle size becomes smaller.

        In order for water to perch in the soil column above another layer, particles in the lower layer must be at least 2.1X the size of the particles in the upper layer, and the upper layer must consist of particles small enough to support perched water. IOW, a layer of .177″ BBs above a layer of .5″ marbles won’t support perched water because the BBs won’t support perched water; but a layer of fine sand (.01″) above a layer of BBs will support perched water because the BBs are more than 2.1X the size of the sand and the sand is small enough to support perched water.

        Al Fassezke

  30. Okay I could possibly be committing some serious Horticultural crime according to this article…
    .At my nursery we regularly pot up really large (over two and a half feet tall by 3 feet wide) pots with seasonal annuals. We place upside down empty 1 gallon or 4 inch size plastic pots to fill about the bottom third of the pot, then we sift in the potting soil and its shifts down around all the crevices and between the voids.
    We do this so that the pot isn’t so heavy and truly there is no need for all that extra potting soil down there. Our containers are lovely and do very well through the long, hot, dry summers here in Central Oregon.

  31. You are essentially using the overturned pot as ballast. From the plant’s perspective, the most value your habit offers is likely in the fact that the overturned pot displaces soil that would potentially support perched water at container capacity. Think of a large pot filled with a soil that supports 4″ of perched water. Add a brick or empty soda bottle to the bottom of the pot and you displace a volume of soil equal to the volume of the brick or soda bottle that might otherwise hold perched water. Add several bricks/other objects and you can displace nearly ALL the soil that would hold perched water.

    However, if you added 4″ of gravel to the bottom of the pot instead, you end up with 4″ of well-aerated gravel with 4″ of ‘perched water’ residing in the soil above it – far from an ideal situation.

    Al Fassezke

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