Last notable comment in the article mentioned a stone on the drainage hole to prevent soil leaching from the pot. Take a rock garden and put it in a pot. Sorry, your blog cannot share posts by email. Why Is My Aloe Vera Plant Turning Yellow and Brown? . Some plants require extremely well draining potting mixes in containers. Wrap as breathable a material as possible around the plants — like burlap, linen, newspaper, or old bed sheets — then secure it against the wind with heavy rocks or clothespins. If you are working with a liquid nutrient medium (hydroponics, essentially), then no, stones or marbles are not an impediment to growth. I grow aquatic edible plants that way, water chestnuts and taro, and about half a dozen others. Should I line my planter box with plastic? If not, then see for yourself. You have a nice blog, read a few articles till now and looking forward to your next ones! Thanks for your question, to help make my article clearer I’ve updated it to address your excellent points and included two diagrams to illustrate the point, one from the North Willamette Research and Extension Center, Oregon State University and another from University of California, Agriculture and Natural Resources, Master Gardeners of Monterey Bay. And it’s not even clear where this idea came from. Seed Saving – How Long Can You Keep Seeds? A lot of orchids for example are epiphytes (plants don’t grow in soil but obtain moisture and nutrients from the air and rain and usually grow on the surface of another plant), and many grow in trees. The way to increase drainage of the perched water table is to add materials throughout all of the potting medium to increasing the air spaces in the mix and reduce capillary action. In Memory of Bill Mollison, the “Father of Permaculture”, 3. If you want to put gravel to work with your potted plants, use it outside the pot. From the University of Illinois Extension, Urban Programs Resource Network – Successful Container Gardens, Choosing a Container for Planting – Drainage Is Critical to Plant Health: From the University of Tennessee, Institute of Agriculture, Agricultural Extension Service – PB1618, Growing Media for Greenhouse Production, we see that reducing the depth of the pot actually increases water retention and. Add enough water so that it comes about halfway up the stones. The Perched Water Table The interaction of these forces and gravity creates something called a water table, which forms whenever you put water in a container. Sand will improve soil, as it has larger particles than clay or silt, but it’s way smaller than potting medium particles and will clog it up instead, filling the air spaces. So what effect will adding gravel at the bottom of a pot below the growing medium have? I also tried to mix sifted1-2mm sand particles with 3-4mm sand particles and I measured the water retained by the mix. Different growing media will have different perched table heights, the more absorbent materials will have higher perched water table, and the less absorbent ones will have lower levels. The gravel will hold water and increase humidity, while keeping Would it make any difference if we placed a wet sponge upright in the sink, or on a layer of gravel in the same sink? Too much water and plant roots rot, not enough water and plants dry out. I did some research and put together this article so it can help you as well. Unfortunately, however, the concept that you need to put rocks in the bottom of your planter is a complete myth. Each Important Function is Supported by Many Elements, 4. Why take the plants out of their pots Most plants that are sold in stores come in these little plastic pots we are talking about. Welcome to Deep Green Permaculture – Connecting Pe…. You can actually buy mesh mats for the bottom of your pot that do exactly this. Wonderful informative and well written article-thanks‼️, Thank you for this enlightening article. There are only two forces at play on water in a pot of growing medium. Well, I have a super simple little trick to help you water those potted plants less frequently. It also helps to keep the soil from getting on any low leaves while also stopping soil from blowing away if your pot plant is caught in a draft of wind. Clearly, the answer is “no.” Not sure if I needed to read an exhaustive treatise to get the answer, but, well, it was fascinating. Change ), You are commenting using your Facebook account. Pots, planters, tubs and containers designed to hold plants always have drainage holes in their bases to allow any excess water to drain out freely, preventing water accumulating at the bottom of the pot. Do you need to put rocks in the bottom of plant pots? Cohesive forces are forces of attraction between molecules of the same type. This is a proper controlled experiment, so please don’t go changing the experiment design parameters on whim! With the exception of the rock (or similar item) we’d recommend putting over each of these, stick to keeping the stones either on top or underneath each indoor plant, depending on what you’re trying to achieve. You’ve preempted my next article on which explains the proper way to increase drainage in pots, which I’ll publish soon, but your solution is half-way there. The more absorbent the potting medium, the taller the perched water table will be. That is, when people ask why put rocks in potted plants, they usually have some idea that this will help with drainage. The only thing that forces the water down is gravity, and the downward force of gravity always remains the same no matter what sits underneath. The water will move downward, some of it will drip away, and some of it will be retained. The gravitational force can only exert a limited downward pull on the water against the upward pull of the capillary action, and no more. Also, be aware that all pots filled with any kind of potting mix, potting medium or growing  medium, call it what you will, have a perched water table. Cactus and succulent mix contains a little organic matter but plenty of coarse gritty material which acts like a sandy soil and lets water pass almost straight through. Excellent Article, in one article you have addressed many phenomenons. That said, it’s still important to keep an eye on your pots without drainage to make sure they’re not getting too much water. Mixing either of these amendment materials right though a potting mix will increase aeration, improve drainage and reduce the height of the perched water table. Roots constantly in contact with the wettest soil will lead to root. Larger retailers of common plants often have stock with stones glued on top of soil. The more moisture retentive growing medium/potting mix available, the less often a plant will need to be watered, as long as the pot is not too big. If pots drain because they have holes in them, then why the need to increase drainage? What do you put in the bottom of a planter? From the North Willamette Research and Extension Center, Oregon State University, Physical Properties of Container Media. Make the same number of decent sized holes (around 6mm or 1/4″) in the bottom of each clear pot. Most people will place a stone or pebble over drainage holes in pots, especially the large central ones at the base of terracotta pots, to prevent the potting mix falling out and making a mess. Hi Paul, thanks for your comment, I had a look at the links you suggested and only included the Oregon State University one, it was the only one which provides any real scientific explanation, albeit a very limited and simplified one. As a final thought worth pondering, it’s curious how gardening has as its foundations the applied sciences of horticulture and agriculture, yet it’s filled with so much dogma and myths, very strange indeed…. Adding gravel into the bottom of  terracotta pots creates a small water holding area for the excess water that would normally drain out on its own  if the gravel wasn’t there to collect into if the single drainage hole become blocked. The simple way to test this is to make some clear plastic pots by cutting the tops of clear plastic soft drink bottles and making the same number of holes in each, so the perched water table can be viewed. A better solution is perlite, a heat-expanded mineral used in hydroponic systems, from experience, it works really well. The upward-acting wicking force of capillary action and the downward-acting force of gravity. Potting Mix acts like a sponge and will not release water until it is saturated. I'm very lazy about watering and tend to underwater, so I guess that's my secret. It's best to avoid using plastic or vinyl sheets alone to protect plants from frost, since these materials can trap condensation inside, which can freeze and damage the plant. Small plants don’t have enough roots to take up huge quantities of water, and in large pots the potting mix stays too wet for too long, causing root rot once again. However, make sure you add holes to the plastic to allow water to drain out. I really liked that you went into detail, it becomes very clear. To get into some basic physics, two opposing natural forces are at play within a wet potting medium in a pot. What about putting rocks on top of potted plants? Water (H2O) is considered a polar molecule because it has a negative charge on one side of the molecule and the positive charge on the other. This post may contain references to products from one or more of our partners. Because they have large air spaces within them, they are used to increase the drainage and aeration in potting mixes. The pwt will NOT be “higher up in the pot”. Otherwise, you should improve the drainability of the soil rather than looking to add something to the bottom of your plant pots. That is not right. Root Rot in Potted Plants. Most plants don’t like water-logged soil as the roots are susceptible to rot, fungus and bacteria, so it’s vital that you provide decent drainage. Now that we understand how the forces of adhesion and cohesion within liquids create capillary action, leading to the formation of a perched water table at the bottom of an absorbent medium, we can see that it won’t have any effect on these forces in any way at all. There’s an old gardening myth that it’s best to put a layer of gravel or rocks at the bottom of a plant pot to improve drainage, but how true is it? Hi Johnny, thanks for your question! The problem arises when you do not remove the rock wool from the roots. The plants end up sitting in too wet soil, increasing the chances of plant root rots and you’ve wasted valuable pot space with gravel that’s doing no good. And as this shows, it’s the soil itself that determines how fast the water eventually drains, not what’s put underneath it. This means that we may receive compensation when you click on links to those products. Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. Adhesive forces are forces of attraction between molecules of different types. My nest article “How to Improve Drainage in Plant Pots, The Proper Way to Do It” is coming soon! Gardening Questions #001 – Grow Nasturtiums from Cuttings? Some people say you don’t need drainage holes; just put rocks or packing peanuts at the bottom of your container. It depends upon your growing medium. When the positive side on one water molecule comes near the negative side of another water molecule, they attract each other and form a hydrogen bond, and this creates the strong cohesive forces between water molecules, and this explains why water clings to itself. This would make a great classroom experiment. The main reason for wanting to improve drainage in pots is because most plants don’t like having ‘wet feet’, otherwise known as waterlogged roots, because this leads to root rot, which can kill a plant. But assuming you have that under control, there are some times when putting rocks in the bottom of plant pots or something else besides soil in the bottom of a planter can help. You may also be looking for how to weigh down plastic planters if you feel that yours are too top-heavy and could tip over if they’re just filled with soil. http://oregonstate.edu/dept/nursery-weeds/feature_articles/physical_properties/physical_properties.html. One option is to simply make sure you always soil that’s excellent for drainage. Thanks for your comment Will, the article is in fact correct, you can try it out by making some clear pots out of soft drink bottles and testing it out, it’s been documented many times by others and I’ve explained the science of it and how it works. However, if you already have a plastic sheet at home, the DIY option is also fine. It prompted me to look around. The absorbency of the potting medium pulls the water upward and stops the water draining out, no matter what is underneath. Hydroponic systems use potting media such as perlite, which is an expanded mineral filled with air, or ‘clay balls’ which are in fact clay coated pumice balls which are very porous, to drain extremely well but hold a slight amount of moisture. This is speculation on my behalf, but I suspect that the reason gardeners traditionally used gravel in the bottom of pots is probably because pots were traditionally made of terracotta clay rather than plastic, and these pots only have a large single drainage hole in the base of the pot. If you want, I will leave you my contact and I will explain it to you with a practical video. You’re welcome to disagree! The drainage and aeration should also be okay as there are no particles smaller than 1mm in the mix. Capillary action is created by the cohesive and adhesive forces of liquids. Well, it’s because the potting medium in which the plant grows is designed to retain moisture, to a certain degree at least…. What can I put in my pots without drainage? That’s my educated guess, and like most traditions, people eventually forget the reason why something was done in the first place, and just keep doing the same thing simply out of habit. Change ), You are commenting using your Twitter account. Put rocks on your soil! Thanks for sharing that, I use a mixture of the clay balls and potting medium in my hydroponic setup, but the reverse, with all clay balls at the top which reduces evaporation, but more potting medium mixed lower down. That may not seem possible, but rocks and alpine plants accustomed to growing on a mountain can make a fine container … The roots look beautiful, I’ve never seen better roots before. I was wondering recently why, two of my plants had different results, although they were the same size, planted in identical pots, using the same compost and watering almost equally. So if you have a pot, like in your illustration, and the pwt is at the bottom 3 inches of the pot (for example), if you replace those 3 inches of soil with a very course medium like clay pebbles (which has much lower capillary action, then more water at that level will drain out of the drain holes in the pot. Now, your article makes me curious to see how perched water table differs from let’s say compost versus sand. If the physics is true, then the perched water table, the wet bottom layer of the potting medium will be the same thickness in every pot, and the gravel will simply push it up higher in the pot because it’s pushing all the potting mix up higher in the pot. Could you comment on the use of a gravel layer (probably clay pebbles) in combination with a fabric wick reaching from the very bottom of the pot, through the pebbles and into the solid. Post was not sent - check your email addresses! Water each pot with the same volume of water. Adding gravel a the the bottom of a pot will create two potentially serious problem: There is no benefit to be gained by adding a layer of gravel or rocks to a pot when we examine the matter from scientific first principles! One rock to cover the drainage hole is enough – just enough so that the soil doesn’t leach out of the bottom but water can flow freely through the pot. So how can we turn the problems created by adding gravel at the bottom of pots into solutions? Hydroponic systems also use perlite as a potting medium, or ‘clay balls’ which are in fact clay coated pumice balls which are very porous and weigh almost nothing. 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