what to use instead of rocks for drainage
There's an former gardening myth that it's all-time to put a layer of gravel or rocks at the lesser of a constitute pot to meliorate drainage, just how truthful is information technology? Can the practice actually harm plants more than assistance them?
The main reason for wanting to meliorate drainage in pots is because well-nigh plants don't like having 'moisture feet', otherwise known as waterlogged roots, because this leads to root rot, which can kill a plant.
Pots, planters, tubs and containers designed to hold plants e'er have drainage holes in their bases to allow any excess water to bleed out freely, preventing water accumulating at the bottom of the pot.
If pots drain because they accept holes in them, then why the demand to increment drainage? Well, it'southward because the potting medium in which the plant grows is designed to retain moisture, to a sure degree at least…
To figure out what's best for plants, lets look at the science!
Potting Mediums, Hitting the Perfect Residuum
Besides much water and plant roots rot, non enough water and plants dry out out. A good potting medium (potting mix) has to strike the perfect balance between sufficient moisture retention and good drainage for plants to thrive.
Since any decent quality potting mix must retain some moisture, information technology needs to contain material which volition absorb and retain moisture, much similar a sponge does. This wicking or absorbent property of any potting medium is the disquisitional key to understanding the behaviour of water in pots.
The Science of Plant Pots and Perched Water Tables
Water naturally runs to the lowest point under the influence of gravity, and volition all run out from a container with drainage holes in the base unless at that place is something else present to hold it there.
Absorptive materials, such every bit a wet sponge sat upright or a wet bath towel hung from the line, behave the aforementioned way. The water will motion downwards, some of it will drip away, and some of it volition be retained. The top of a wet sponge or bath towel will dry the fastest, and the lesser portions volition remain clammy for the longest catamenia of time.
Potting mediums, existence absorbent materials, acquit much like any other when wet.
To go into some basic physics, two opposing natural forces are at play inside a moisture potting medium in a pot.
- Gravity, which exerts a downward pull on the water, causing it to be drained abroad through the drainage holes.
- Capillary action, which exerts an upward pull on the water, causing it to exist retained, saturating the potting medium.
Both these forces have limitations though:
- The capillary activity can but wick the weight of the h2o upward to a express superlative confronting gravity, and no higher.
- The gravitational forcefulness can only exert a limited downward pull on the h2o against the upwards pull of the capillary action, and no more.
At some point these two opposing forces balance each other out, and when this happens, a layer of water-saturated potting medium is formed at the bottom of the pot which cannot not bleed abroad, this is termed the perched water table because the water is literally 'perched' at that place and cannot move.
It's important to empathize that the perched water table does non drain, the water stays there unless plant roots draw the water upward, or it evaporates away when the potting mix dries out, in which instance the establish won't survive!
Also, be aware that all pots filled with whatsoever kind of potting mix, potting medium or growing medium, call it what you will, have a perched water tabular array.
The size and shape of the pot makes no difference, it doesn't matter if a pot is tall and narrow or wide and shallow, neither if information technology's big or small, if the growing medium/potting mix is the same, the perched water table will always be the same height.
Different growing media will have dissimilar perched table heights, the more absorptive materials will have higher perched h2o table, and the less absorptive ones will have lower levels.
Understanding Capillary Action
In this section we'll go a bit deeper into the scientific discipline if you lot're interested, if not, please skip to the next section. I like to teach from start principles, equally I believe this way nosotros tin can really come up to a deeper level of understanding, merely then again, I've got qualifications in the biological sciences, and then I'm biased!
Gravity is self-explanatory, it's the e'er-present force on this planet which pulls everything down!
Capillary action is created past the cohesive and adhesive forces of liquids.
Cohesive forces are forces of allure between molecules of the same type. For example, molecules of h2o are able cling to each other.
Adhesive forces are forces of attraction between molecules of different types. For instance, molecules of water are able to cling to other materials.
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 force of cohesive and adhesive forces.
To explicate how this further, we need to sympathise the nature of h2o.
Water (H2O) is considered a polar molecule considering it has a negative charge on 1 side of the molecule and the positive accuse on the other. Its aptitude V-shape which gives it a partial positive accuse on the side of the hydrogen atoms and a partial negative charge on the side of the oxygen atom.
Polar molecules human activity like magnets with due north and south poles, the (+) positive charged atoms and (-) negative charged atoms of these molecules are attracted to one another.
When the positive side on one water molecule comes about 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.
Water molecules will showroom strong agglutinative forces that allow them cling to other materials if those materials are fifty-fifty more polar (have a stronger electrical charge) than h2o itself, equally the attraction will be stronger than the attraction of water molecules to each other.
The upward movement of liquids against gravity, known as capillary activeness , is a combination of:
- The forces of attraction between h2o molecules and another textile in a higher place the water's surface which doesn't already have water clinging to information technology already (adhesion), causing the water molecules to climb up a piddling.
- The forces of allure between water molecules to each due to the hydrogen bonds they form with each other (cohesion), causing them to pull each other up.
To put it some other fashion, capillary action is a combination of the effects of agglutinative and cohesive forces displayed by water.
Now that we know why water moves upwards and creates perched water tables in growing media, nosotros tin can now re-examine our opening question from a more scientific perspective!
The Upshot of Placing Gravel at The Bottom of a Pot on the Perched Water Table
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? At present that we understand how the forces of adhesion and cohesion within liquids create capillary activity, leading to the formation of a perched h2o tabular array at the bottom of an absorptive medium, we can see that it won't take whatever effect on these forces in whatever fashion at all.
Call up, the downwards force is due to gravity, which we tin't increment, a lower layer of some other 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.
Then what effect will adding gravel at the lesser of a pot below the growing medium have?
It will reduce the volume of potting medium, and push the perched water table higher up into the pot, as shown in the diagram below.
Calculation gravel a the the bottom of a pot will create two potentially serious problem:
- Pushing the saturated h2o tabular array layer upwards, closer to the plant roots actually increases the risk of root rot, as the roots will stay wetter, longer.
- Reducing the book 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.
There is no benefit to be gained by calculation a layer of gravel or rocks to a pot when we examine the matter from scientific first principles!
The Right Style to Increase Drainage in Pots and Containers
If the same potting medium is used, irrespective of the size or shape of the pot, the perched water tabular array ever stays the same summit considering it is determined by the wicking power of the potting medium, since gravity doesn't change.
The style 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.
Some plants crave extremely well draining potting mixes in containers. 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 found), and many grow in trees. Growers of Cymbidium orchids utilize an orchid mix which is equanimous mainly of coarse 20mm (3/4″) composted pino bawl pieces. This mixture contains huge air spaces and drains extremely well, barely retaining moisture in the bark pieces, so in that location is no perched water table.
Cactus and succulent growing mediums for pots are a coarse, open up mixes made with some organic matter to retain a fiddling moisture, and enough of gritty material such as crushed quartz or other crushed stone, which act similar a sandy soil and lets water laissez passer almost straight through.
Perlite and vermiculte are materials which are used every bit soil amendments, and both are minerals that are fabricated more porous by expanding them with oestrus, much like popcorn. Considering they take large air spaces inside them, they are used to increase the drainage and aeration in potting mixes. Perlite mainly increases drainage, while vermiculite volition as well agree some moisture and aid retain nutrients likewise. Mixing either of these amendment materials right though a potting mix volition increase aeration, improve drainage and reduce the summit of the perched h2o table.
Hydroponic systems also use perlite as a potting medium, or 'dirt balls' which are in fact dirt coated pumice assurance which are very porous and weigh almost nothing. These growing media have large air spaces both inside and between the particles, and then they drain extremely well, but concur enough h2o to keep the roots moist.
Nosotros can meet that the mutual practise in horticulture to increase drainage in pots and containers is to modify the composition of the potting medium to increment the air spaces within it, and not by making changes to the the space beneath the pot.
How Did The Tradition of Putting Gravel At the Bottom of Pots Originate?
The just style that gravel at the bottom of the pot will increment drainage is if the pot has insufficient drainage, either due to not having enough drainage holes, or by having blocked drainage holes.
This is speculation on my behalf, merely I suspect that the reason gardeners traditionally used gravel in the bottom of pots is probably considering pots were traditionally made of terracotta dirt rather than plastic, and these pots only have a large single drainage hole in the base of the pot. With these pots, information technology was a traditional exercise (and still is) to sit down a very loosely fitting stone over the hole to stop the potting mix falling out. If this single hole became blocked, water would pool at the bottom of the pot and drain out very slowly, leading to waterlogging. The problem would be even worse in glazed terracotta pots, which don't seep moisture from their sides and stay moisture longer.
Adding gravel into the bottom of terracotta pots creates a small water holding area for the excess water that would unremarkably bleed out on its ain if the gravel wasn't there to collect into if the single drainage hole become blocked. That's my educated approximate, and similar most traditions, people eventually forget the reason why something was done in the first place, and just keep doing the same affair simply out of addiction.
With plastic pots, in that location are always plenty of drainage holes, and many are designed to increase drainage through the utilise of domed bases with drainage holes at the edges to take excess water away faster.
Do You However Believe Gravel in Pots Improves Drainage?
If you lot've read this far, and all the same aren't convinced that putting gravel at the bottom of a put just pushes the perched h2o tabular array upwardly, but doesn't improve drainage, so I have two diagrams and 2 direct quotations from authoritative sources to illustrate the point:
From the Northward Willamette Research and Extension Centre, Oregon Country University, Physical Properties of Container Media:
" For a given media, the perched h2o tabular array remains the aforementioned regardless of plant height. It is therefore unwise to use the aforementioned media in large containers every bit small. "
From the University of California, Agriculture and Natural Resources, Master Gardeners of Monterey Bay:
From the Academy of Illinois Extension, Urban Programs Resource Network – Successful Container Gardens, Choosing a Container for Planting – Drainage Is Critical to Found Health:
"Skip the gravel inside the lesser of individual or pot liners – It is a myth that a layer of gravel (within the lesser of an private pot) beneath the soil improves container drainage.
Instead of extra h2o draining immediately into the gravel, the water "perches" or gathers in the soil simply above the gravel. The water gathers until no air infinite is left.
In one case all the available soil air infinite fills up, then excess water drains into the gravel below. So gravel in the bottom does petty to keep soil above it from being saturated by overwatering."
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 reduces drainage, so putting gravel at in the bottom of a pot actually has the opposite issue:
"Media Cavalcade Elevation/Containers – Another gene relating media to air/water relations in the root zone is the size of the growing container. With media in containers, the amount of air and h2o held in a given media is a function of the height of the column of media. The taller the column,the smaller the ratio of water to air spaces.
This is near important in plug production where the minor cells drain very poorly or not at all, resulting in poor root zone aeration…
In all containers, in that location will exist a certain amount of saturated media at the bottom of the container afterwards drainage. This is due to what is called a perched water table. The saturation zone is a larger part of the total volume of the growing media in a very short container, such as a plug prison cell.
A good way to illustrate the issue of container pinnacle is to use a sponge. A sponge of the dimensions 2@ x 4.25@ ten eight.five@ (72.25 cubic inches or ane,184 milliliters) represents the media in a container.
When fully saturated, the sponge holds 950 ml; that is, the total porosity is eighty pct.
Property the sponge and then it is 2 inches high results in most 50 ml water draining out, resulting in a book air space of 4.2percent.
If it then is held so it is iv.25 inches high, another 125 ml drains out, resulting in a volume air space of 14.8 percent.
If the sponge is then held so information technology is eight.v inches loftier, another 375 ml drains out, resulting in a volume air infinite of 46.5 pct.
Starting with the aforementioned book of media (sponge), the effect of container acme (sponge top) on media air infinite is dramatic. We can conclude that the choice of containers is important in managing water/air relations in the root zone, specially of plugs."
Hopefully that's convincing! If non, then see for yourself. I'll show you lot how to examination this, all you lot demand is some empty soft potable bottles. This is a proper controlled experiment, so please don't go changing the experiment design parameters on whim!
A Simple Experiment to Test Whether Adding Gravel at the Bottom of a Pot Improves Drainage
Here is a elementary experiment that can be set up to determine whether adding gravel at the bottom of a pot improves drainage or not. This would brand a nifty classroom experiment.
- Make some clear plastic pots past cutting the tops of clear plastic soft drink bottles and then the perched water table can be viewed through the sides.
- Make the same number of decent sized holes (around 6mm or 1/iv″) in the lesser of each clear pot.
- In the first pot, place only potting mix in it, fill to merely inside 2.5cm (1″) of the pot, leaving a gap from the surface of the potting mix to the top of the pot to make watering easier. When filling the pots, just tap the sides gently to settle the potting mix slightly, don't shrink it down.
- In all the other pots, add together increasing amounts of gravel at the bottom, then make full with potting mix to within two.5cm (1″) of the pot. Tap the sides gently to settle the potting mix slightly, don't compress it down.
- Water each pot with the same volume of water.
- Wait till all pots drain well, this volition depend on the type of potting medium used and the volume of the containers.
- Notice the perched water table.
If the physics is truthful, and so the perched water table, the wet bottom layer of the potting medium volition be the same thickness in every pot, and the gravel will simply push button information technology upward higher in the pot because it'southward pushing all the potting mix upward higher in the pot.
That said, now lets play some mind games!
The Permaculture Design Approach, Turning Drainage Problems Into Solutions!
If nosotros expect at the Permaculture Attitudinal design principle – "Everything Works Both Means", nosotros see information technology states that whether we see something equally positive or negative, as a 'problem' or as a useful resource, depends on our attitude.
So how can we turn the problems created by adding gravel at the bottom of pots into solutions? This is a real exercise in lateral thinking, or more accurately, Permaculture holistic solutions thinking.
If we do a Permaculture functional analysis of the outcomes our outputs, nosotros meet that the technique reduces soil volume and raises the saturated perched water table.
One of the problems gardeners run across frequently is unknowingly planting a tiny plant into an overly large pot. Small plants don't have plenty roots to take upward huge quantities of water, and in large pots the potting mix stays also wet for also long, causing root rot once again. The growing medium wont be every bit saturated as the perched water table, but it will nevertheless exist wet enough for way also long to exist detrimental to the found. At that place is wisdom in the gardening advice to plant up to the next size pot when repotting, and increase pot size gradually rather than plant into the biggest pot available at the showtime.
A shallow rooted plant in a tall narrow pot will have similar issues, there will exist likewise much overly wet potting mix which the roots will never be able to reach, and if the potting mix stays too moisture for too long information technology will break down much faster, and sink downwards, dropping the level of the constitute in the pot. Filling the bottom of the pot with fibroid 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.
The only kind of plants which beloved a saturated growing medium are marginal aquatic plants, and in that location are plenty of useful edible ones such as watercress, taro, kangkong and h2o chestnuts for example. With these plants it's much better to remove the drainage altogether and saturate all of the growing medium though, or sit down the pots in a saucer of h2o.
There are always exceptions to the rules, equally we've discussed in this department, but in general, it's all-time not to identify gravel, stones, pebbles, scoria, terracotta pot shards or whatsoever other materials at the bottoms of pots beneath the growing medium.
Give plants as much space to spread their roots out, relative to what they can apply or need. The more moisture retentive growing medium/potting mix available, the less often a plant will need to exist watered, as long as the pot is not too big. Nigh all plants adopt a natural wet-dry cycle, every bit that's what they experience in nature.
Most people will place a stone or pebble over drainage holes in pots, especially the large primal ones at the base of operations of terracotta pots, to prevent the potting mix falling out and making a mess. The point is not to block the pigsty, but to simply create a loose-fitting barrier to prevent the loss of growing medium while nonetheless allowing water to freely drain out.
As a concluding idea worth pondering, it's curious how gardening has every bit its foundations the applied sciences of horticulture and agriculture, yet it's filled with so much dogma and myths, very strange indeed…
To learn how to meliorate drainage in pots, please meet my article – How to Improve Drainage in Plant Pots, The Proper Way to Practise It!
Source: https://deepgreenpermaculture.com/2019/09/06/should-you-put-gravel-or-rocks-at-the-bottom-of-plant-pots-for-drainage/
0 Response to "what to use instead of rocks for drainage"
Post a Comment