I have wondered for years why nurseries don’t have gravel or the like in the bottom of their pots. This means that, when looking for what to fill a large planter with, you can use any of the following: Putting pebbles on top of potted plants can help to prevent evaporation and avoid soil washing away when you water your plants. I like to teach from first principles, as I believe this way we can really come to a deeper level of understanding, but then again, I’ve got qualifications in the biological sciences, so I’m biased! Gardening Questions #002 – How Many Trays to Use When Starting a Worm Farm? Using a pebble tray also provides space for drainage, given it will be located directly below your pot. There’s also the added benefit that it can look really beautiful, giving your indoor plant a tidy, “finished” look. We’ll show you exactly why putting rocks in potted plants isn’t a good idea – as well as some alternate ways to improve your houseplants’ drainage and to incorporate stones around plants to help them thrive. 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…. This weekend I took out of the pot a lemon tree which was planted in 70% expanded clay balls mixed with 15% coco peat and 15% compost all the way through the pot. I should have explained that the perched water table will always be the same height in the potting medium, no matter what you sit under it. In Memory of Bill Mollison, the “Father of Permaculture”, 3. Because they have large air spaces within them, they are used to increase the drainage and aeration in potting mixes. 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. This would make a great classroom experiment. But otherwise, this is a super easy way to make sure the humidity is just right for your houseplants. One of the problems gardeners encounter often is unknowingly planting a tiny plant into an overly large pot. Give plants as much space to spread their roots out, relative to what they can use or need. The forces of attraction between water molecules and another material above the water’s surface which doesn’t already have water clinging to it already (adhesion), causing the water molecules to climb upwards a little. To learn how to improve drainage in pots, please see my article – How to Improve Drainage in Plant Pots, The Proper Way to Do It! ( Log Out /  To be very fair, not having drainage holes in a pot used for indoor plants isn’t quite as big a problem as it is for outdoor plants. ( Log Out /  Capillary action by definition is the tendency of a fluid to be raised (or suppressed in the case of mercury) in a narrow tube (capillary tube) due to the relative strength of cohesive and adhesive forces. Water each with the same volume of water, wait till they drain, and then observe the perched water table. It seems this mix could also be useful as it retained 10-15% of it’s weight in water. How to Keep Cats from Urinating in House Plants. Hope it’s not too cheeky of me to say that my explanation is the only one from first principles and goes to a greater depth scientifically though. If you have pots without drainage holes and they’re made from a material that means you can’t add any, you have to be very careful about not over-watering. If you pour water from the top, the medium will act like a sponge and prevent a certain amount draining out of the bottom, that is the perched water table. Excellent article, well written! The problem arises when you do not remove the rock wool from the roots. Click on this article for tips on getting rocks glued to soil off without harming the plant. It’s important to understand that the perched water table does not drain, the water stays there unless plant roots draw the water up, or it evaporates away when the potting mix dries out, in which case the plant won’t survive! That may not seem possible, but rocks and alpine plants accustomed to growing on a mountain can make a fine container … Potting mediums, being absorbent materials, behave much like any other when wet. But rocks serve no useful purpose in planters, and as you have seen, only put them at risk of root-rot. Great article. I have some succulents in pots ( small, medium, sized) and 3 dwarf citrus in large pots. In the first pot, place only potting mix, and in the rest, put increasing amounts of gravel beneath the potting mix. Change ). A good potting medium (potting mix) has to strike the perfect balance between sufficient moisture retention and good drainage for plants to thrive. With these pots, it was a traditional practice (and still is) to sit a very loosely fitting stone over the hole to stop the potting mix falling out. That’s how people find common ground for understanding! By putting rocks in the bottom of plant pots, you’re effectively making the soil area of the pot smaller, meaning the “sponge” holding the water in is also now smaller. The capillary action can only wick the weight of the water upwards to a limited height against gravity, and no higher. Final thoughts on putting rocks in the bottom of plant pots, natural-colored decorative gravel for plant pots. 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. They are put in the plant for several reasons: 1) aesthetic- they look nice 2) to keep your cats from pooping in your plants- … 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. Using a … I am the Technical Director of Dümmen Orange in Spain and Portugal and I have 20 years of experience in pot cultivation with various drainage systems and gravel is one of the most recommended for fans of ornamental plants. If someone tells you to put striker matches in potted plants to help them grow, you might think it’s a superstition—but there’s science behind this old wives’ tale. The gravel will hold water and increase humidity, while keeping 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. By pushing a few match heads into the soil around potted plants, you can grow outstanding, healthy plants … We’ve all heard the advice to put gravel in the bottom of a plant pot, and some of us may even have done it. The pwt will NOT be “higher up in the pot”. Ignore it at your own risk. The problem would be even worse in glazed terracotta pots, which don’t seep moisture from their sides and stay wet longer. Citrus Problems – Why Is My Citrus Tree Dying? We can see that the common practice in horticulture to increase drainage in pots and containers is to alter the composition of the potting medium to increase the air spaces within it, and not by making changes to the the space beneath the pot. The same thing happens with people who have been told about adding gravel to soil for drainage, putting wood chips in the bottom of a planter or adding charcoal in the bottom of pots. Zones and Sectors – Efficient Energy Planning, No Dig Gardening, Sustainable Gardening With Less Effort, Chop and Drop Gardening (Sheet Composting), Wicking Bed Construction, How to Build a Self-Watering Wicking Bed, Home Made Plant Rooting Hormone – Willow Water, How to Kill a Tree Stump Without Poisonous Chemicals, How to Make Your Own DIY Homemade Garden Sprays, DIY Self-Watering Pots and Mini Wicking Beds, Bokashi Composting – How to process waste that can’t go in your compost or worm farm, Bokashi Soil Generator– Turn food waste into nutrient-rich soil, Starting Annual Vegetable Seedlings Indoors, How to Build a Self Watering Seedling Tray, How to Build a Self-Cleaning Water Tank from a Wheelie Bin, The Complete Guide to Worm Farming, Vermicomposting Made Easy, How to Build a Worm Farm with Polystyrene Foam Broccoli Boxes, How To Build a Worm Tunnel Vermicomposting System, Plant Labels & Watering Trays from Recycled Containers, Download Gardening Calendars (Australian Temperate Climate), Crop Rotation Systems for Annual Vegetables. If you want, I will leave you my contact and I will explain it to you with a practical video. With the exception of putting something over each drainage hole, you should only put things in the bottom of your planter if you’re sure that the soil area is large enough and drains well enough to avoid problems from too much moisture. Why should I not put rocks in the bottom of my planter? Enter your email address to follow this blog and receive notifications of new posts by email. While this will obviously cover the drainage hole a bit, there’ll still be some small spaces for the water to get out. I faced this problem recently and wanted to know how to prevent this problem. Even if you balance it on some gravel for house plants, these won’t help the sponge dry out any faster. gravel in bottom of plant pots for drainage, how did the tradition of putting gravel at the bottom of pots originate, how to increase drainage in pots and containers, the correct way to increase drainage in pots and containers, http://oregonstate.edu/dept/nursery-weeds/feature_articles/physical_properties/physical_properties.html, Gardening Calendar (Australian Temperate Climate) – December, Gardening Calendar (Australian Temperate Climate) – November. It adds extra weight which keep top-heavy plants from falling over. Tap the sides gently to settle the potting mix slightly, don’t compress it down. I'm very lazy about watering and tend to underwater, so I guess that's my secret. 8 Simple Ways to Make Rooting Hormone at Home. Reducing the volume of growing medium available to the plant roots will reduce root growth space and overall root volume, as well as available moisture, thereby decreasing the plant’s drought tolerance and potential maximum growth size. Instead, your plant’s soil is the sponge. 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. Water each pot with the same volume of water. If you use the same potting medium, irrespective of the size or shape of the pot, the perched water table always stays the same height because it is determined by the wicking ability of the potting medium, as gravity doesn’t change. In this section we’ll go a bit deeper into the science if you’re interested, if not, please skip to the next section. Should I line my planter box with plastic? That is not right. . If the physics is true, then the perched water table will be the same height from the bottom of the potting in every pot, and the gravel will simply locate it higher up in the pot because it’s pushing up the potting mix. Lauren Dunec Hoang – November 1, 2017 | Updated June 4, 2020 The way to increase drainage of the perched water table is to add materials throughout all of the potting medium which reduce capillary action by increasing the air spaces in the mix, which is why we sometimes add potting medium amendment materials such as perlite so potting mix drain better. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. From your experience stated I am guessing you’ve probably grown lots of plants in containers with very different requirement. However, if you already have a plastic sheet at home, the DIY option is also fine. 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. It is not recommended to use Styrofoam peanuts for potted plants now, because they may break down in water and soil, leaving you with sunken in containers. Why putting rocks in potted plants can cause problems. Now that we know why water moves upwards and creates perched water tables in growing media, we can now re-examine our opening question from a more scientific perspective! Overwatering a potted plant can cause root rot, a disease that results in the slow death of the plant. Or perhaps it was simply something passed down from one plant owner to the next. When I mixed them with the compost, it was 50% clay balls and 50% soil on the bottom of the pot, then slightly decreased the clay balls, somewhere around 40% clay balls and 60% soil in the middle of the pot and on the surface it was 30% clay balls and 70% soil. Each Important Function is Supported by Many Elements, 4. Capillary action is created by the cohesive and adhesive forces of liquids. If your plant and pot are light enough to … ( Log Out /  That is, when people ask why put rocks in potted plants, they usually have some idea that this will help with drainage. If we do a Permaculture functional analysis of the outcomes our outputs, we see that the technique reduces soil volume and raises the saturated perched water table. If you find yourself with a large amount of Styrofoam from product packing and question: “Should I line potted plants with Styrofoam,” there is a way to test the Styrofoam. Why Is My Aloe Vera Plant Turning Yellow and Brown? 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. It might need a better explanation, although you do well as always. If this single hole became blocked, water would pool at the bottom of the pot and drain out very slowly, leading to waterlogging. To figure out what’s best for plants, lets look at the science! Perlite and vermiculte are materials which are used as soil amendments, and both are minerals that are made more porous by expanding them with heat, much like popcorn. Adhesive forces are forces of attraction between molecules of different types. Coincidentally, these forces are also how plants get water from their roots to their leaves! You can put rocks on top of soil as they do provide some benefits. The gravitational force can only exert a limited downward pull on the water against the upward pull of the capillary action, and no more. While you can mist your indoor plants as one way to help with this, using a pebble tray is also another easy solution. A better solution is perlite, a heat-expanded mineral used in hydroponic systems, from experience, it works really well. 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. They will protect your plants from weeds, keep pests out, prevent soil erosion, avoid splashing soil on the foliage, and prevent fungal diseases. My nest article “How to Improve Drainage in Plant Pots, The Proper Way to Do It” is coming soon! What's worse is that these annoying insects can be incredibly prolific during their rather short lifespans, generally only living a single week as an adult gnat. 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. You won’t believe how simple this is! It also doesn’t mean that air is circulating more easily inside the sponge. One option is to simply make sure you always soil that’s excellent for drainage. So what effect will adding gravel at the bottom of a pot below the growing medium have? Putting rocks on top of potted plants is a perfectly acceptable method to cover the soil decoratively. I’ll show you how to test this, all you need is some empty soft drink bottles. Here is a simple experiment that can be set up to determine whether adding gravel at the bottom of a pot improves drainage or not. ( Log Out /  Do you need to put rocks in the bottom of plant pots? You do not need to put rocks in the bottom of plant pots as they do not help with drainage or air circulation. If you are a cat owner, you may have encountered an issue with your pet using potted house plants as their own personal restrooms. That is, to continue the analogy used above, you need to make sure the sponge is big enough for your plant. This post may contain references to products from one or more of our partners. Double potting makes it possible to sink individual potted plants into the landscape (or remove them) without disturbing the roots. This means that putting rocks on top of potted plants is great both for the plants themselves and the room where you keep your houseplant, especially if it’s near a door or a window that’s occasionally open and lets the wind in. What do you put in the bottom of a planter? Cohesive forces are forces of attraction between molecules of the same type. Gravity, which exerts a downward pull on the water, causing it to be drained away through the drainage holes. Remember, the downwards force is due to gravity, which we can’t increase, a lower layer of another material won’t change the adhesive forces between the growing medium and the water molecules, nor will it alter the cohesive hydrogen bonds between water. This means you should make sure the soil you put in these pots helps with drainage to avoid root rot. Food for the thought! With plastic pots, there are always plenty of drainage holes, and many are designed to increase drainage through the use of domed bases with drainage holes at the edges to take excess water away faster. Rocks contain micro nutrients, which erode over time, Roots latch onto these rocks and can slowly suck these rocks, if the plant lives for many years in one container this could be of use for extra minerals 3. Find out more here. Simply take your plant saucer and add a single layer of pebbles, which can include either small decorative stones or simply some small rocks you find outside. My apologies for not being clearer in my explanation, so I appreciate you bringing the point up. Can the practice actually harm plants more than help them? Polar molecules act like magnets with north and south poles, the (+) positive charged atoms and (-) negative charged atoms of these molecules are attracted to one another. Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. The only way that gravel at the bottom of the pot will increase drainage is if the pot has insufficient drainage, not enough drainage holes for example. Hi Javier, thanks for your comment. Perhaps it was a misunderstanding of the purpose of the rock you can put over each drainage hole. Filling the bottom of the pot with coarse scoria, which is light in weight, will eliminate the unusable space in a tall, narrow pot and effectively reduce pot size to a more suitable volume. If you are working with a liquid nutrient medium (hydroponics, essentially), then no, stones or marbles are not an impediment to growth. You can use more natural-colored decorative gravel for plant pots, although I’m personally a fan of the clean elegance of these white stones, like in the picture below. Change ), You are commenting using your Twitter account. The drainage and aeration should also be okay as there are no particles smaller than 1mm in the mix. 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. If you’d rather improve the soil you already have though and are looking for how to add drainage to soil, some options include: Lining your planter box with plastic is a good idea for houseplants as it helps make sure that the soil does not escape through the bottom. Post was not sent - check your email addresses! Would be great to add an explanation of why the water doesn’t cross into the gravel layer. 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 water will move downward, some of it will drip away, and some of it will be retained. Potting Mix acts like a sponge and will not release water until it is saturated. 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! Well, golly! 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. 2. You may also be interested in: 8 Simple Ways to Make Rooting Hormone at Home. Now, your article makes me curious to see how perched water table differs from let’s say compost versus sand. Make some clear plastic pots by cutting the tops of clear plastic soft drink bottles so the perched water table can be viewed through the sides. We can see that it’s a common practice in horticulture to increase drainage in a pot by altering the potting medium, and not the space beneath the pot. Sorry, your blog cannot share posts by email. Well, it’s because the potting medium in which the plant grows is designed to retain moisture, to a certain degree at least…. With outdoor plants, there’s a risk they could be rained on, whereas you can control how much water your houseplants get. 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. Tricks to Make Your Potted Plants Look Extra Stylish A potted plant may be attractive on its own, but nestled in a bed of polished river rocks or surrounded by tiny succulents makes it extra stylish. Make the same number of decent sized holes (around 6mm or 1/4″) in the bottom of each clear pot. The more absorbent the potting medium, the taller the perched water table will be. 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. If you’ve read this far, and still aren’t convinced that putting gravel at the bottom of a put just pushes the perched water table up, but doesn’t improve drainage, then I have two diagrams and two direct quotations from authoritative sources to illustrate the point: Hopefully that’s convincing! You should also make sure your plant isn’t too small for a pot like this as without root mass filling your pot, it will instead be filled with soil that will hold more water for longer. This mixture contains huge air spaces and drains extremely well, barely retaining moisture in the bark pieces, so there is no perched water table. And it’s not even clear where this idea came from. There are only two forces at play on water in a pot of growing medium. 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. Will the wick also allow water to travel upward as needed without having the roots be overly moist. 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. The point is not to block the hole, but to simply create a loose-fitting barrier to prevent the loss of growing medium while still allowing water to freely drain out. Some plants require extremely well draining potting mixes in containers. Hopefully, what you have learned here will make your indoor herbs and plants … . Crocking was supposed to encourage water to pass down from the potting mix into the gaps in the coarse layer below and out through the drainage hole. This is a proper controlled experiment, so please don’t go changing the experiment design parameters on whim! Now, we’re not talking about squeezing your plants here (although feel free!). Since any decent quality potting mix must retain some moisture, it needs to contain material which will absorb and retain moisture, much like a sponge does. It keeps the potting mixture from getting on the plant foliage, and it gives the pot a tidy, finished look. No one’s really a fan of coming home and finding soil scattered all over the floor, right? Too much water and plant roots rot, not enough water and plants dry out. Think of it like this: if you soak a sponge, it will hold on to its water. 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. Absorbent materials, such as a wet sponge sat upright or a wet bath towel hung from the line, behave the same way. Excerpts and links may be used, provided that full and clear credit is given to Angelo Eliades and Deep Green Permaculture with appropriate and specific direction to the original content. Having a layer of gravel makes no difference to the system when using a wick.