You see it all on the image on the left: the plant is placed in a double container. The external pot is an earthenware container. The internal plastic envelope has slots to let water through, and is filled with clay bubbles, in which the plant grows. There is no soil.
The roots must be immersed in water, which fills a part of the system. The water level is controlled with a floating gauge. The pots have a water reserve that allow to space watering every four or five weeks.
As a fertilizer, I am using an ion exchange resin loaded with inorganic nutrients: nitrate, phosphate, potassium, magnesium, ammonium, and oligo-elements, principally zinc and iron. Liquid fertilizers can also be used, but the advantage of ion exchange resins is that they allow the nutrients to diffuse in the water according to the needs of the plants, so any overdose is avoided. One cartridge or pouch of ion exchange fertilizer lasts several months (4 to 8 months depending on plant size).
Growing according to this system is particularly useful for indoor plants. Take a “normal” plant, carefully detach the root ball and rinse it until the roots are bare, then transplant it into the clay beads. The plant’s acclimatisation time to its new soilless environment varies between three and six weeks, during which it is better not to touch it too much.
I started hydroculture in 1979 as the company for which I was working developed the ion exchange resin mixture suitable for this application. Since then, I planted:
- ficus of various sorts,
- a caphorbia*,
- an aloe vera,
- a beaucarnea (called elephant foot),
- and a few other plants of which I forgot the names.
The ficus are particularly suitable for hydroculture. From a cutting, I managed to produce a tree that grew to 1.6 m high, but eventually died, about twenty years later, due to a watering failure.
Hydroculture : principle and illustrated examples.
Welcome to Houseplant Hydroculture Site
What is Hydroculture?
Hydroculture is a method of growing plants without the use of soil. With the method I use, and detail on this page, plants are grown in an absorbent aggregate and nutrients. Some of the advantages of houseplants grown in hydroculture are the water reservoir makes it easy to see when the plant requires water, the aggregate is open therefore allowing air to circulate around the roots, and the reservoir coupled with using absorbent aggregate helps create humidity around the plant. This method of hydroculture is sometimes referred to as ‘passive hydroponics’.
There is another method, which uses nutrient solution, called ‘active hydroponics’ or often referred to just as hydroponics. This system often uses a form of nutrient delivery system to deliver the nutrients to the roots. I will only be detailing the method of houseplant hydroculture I use on this page, however there are other web sites which detail active hydroponics and also various other methods of hydroculture.
The word “hydro” derives its name from the Greek word “hudor” meaning water, hence hydroculture = water culture.
I use expanded clay pebbles which are specifically sold for hydroculture. I get my expanded clay pebbles from a hydroponics stockist who stocks them in 10 litre and 50 litre bags. These are made of clay which has been fired to a high temperature to create a hard outer shell and a honeycomb-like centre that allows for water absorption. I clean these as described in the cleaning section of this page.
The image below shows the typical inside structure of expanded clay pebbles
Aggregate performs a similar role to that of soil in that it supports the plants. Expanded clay pebbles have the ability to absorb nutrient solution and transferring it through the aggregate via a capillary action from a reservoir to the plant roots.
Expanded Clay Pebbles
A search on the internet for hydroculture, passive hydroponics or hydroponics usually leads to various suppliers sites who stock suitable aggregates, some offer it in various sizes and I usually use the smaller pebbles for fine rooted plants and the larger sizes for all others. You can use the search box below if you like.
This is the ‘food and water’ for the plants. Special nutrients are available for hydroculture, these usually come in either powder or liquid form which are added to water to make a solution, or in a resin or tablet form which is added to the aggregate or reservoir. With the resin form the food is slowly released over long periods of time, often months.
Hydroculture nutrients differ from some houseplant foods in that they contain extra trace elements. Some nutrient solutions may not be suitable for indoor houseplant hydroculture, I always check the suitability with the manufacturer first. Instructions on the use of nutrients should be supplied with them.
Nutrients are available in many different NPK (N=Nitrogen, P=Phosphorus, K=Potassium) formulations, these are expressed as percentages. By selecting different NPK formulations the growth, flowering or crop of the plant can be controlled to a certain degree. I use a general purpose nutrient for all my plants but some plants may appreciate a different formulation.
The easiest method I have found for converting a houseplant to hydroculture is to use one that has been water rooted because the root system seems to be slightly different to one that has been grown in soil. The method I use is to take a soft stemmed cutting and suspending it in a container full of water, this can be done by placing a piece of cardboard on the top of the container and putting the cutting through a hole in the centre. The cutting is put somewhere where it will get light but not direct sunlight and also somewhere that is not too hot. The water in the container is changed every few days to stop it becoming stagnent. When a good root system has developed the cardboard is carefully removed from the plant avoiding damaging the plant, the plant is then transferred to hydroculture as described in the containers section. Then I place the plant into a propagator or place a transparent plastic bag, with air holes, over it to keep the humidity high for up to 1 month and I also use only water, not nutrient solution, for this time.
Although water rooting is possibly the most reliable method of getting a houseplant into hydroculture, it is not my preferred method. I’m a little impatient and I like to see quick results, therefore I prefer to convert a soil (compost) rooted houseplant. I always use young houseplants as large or established ones may be more difficult to convert to hydroculture. The method I use is detailed below:
I soak the plant, in its pot, in room temperature tapwater up to the height of the top of the soil for approximately one hour. This helps soften the soil from around the roots.
Remove the plant from the pot, place the plant roots back into a bucket of clean room temperature water and agitate to remove most of the soil.
Remove the plant from the bucket and run room temperaure clean water over the roots until all the soil is removed. It is important to remove ALL the soil.
Cut off any dead roots. Trimming the roots a little seems to help the plant establish better.
The plant is place into a hydroculture pot as described in the containers section.
Water only is added, no nutrients.
I don’t add nutrient solution for approximately 4 weeks, only water. The plant will be very sensitive until it has grown a new root system so it will need nurturing for a few weeks, to do this I place the plant into a propagator or place a transparent plastic bag, with air holes, over it to keep the humidity high.
I currently use the pot and saucer method. This uses pots made of an inert material such as plastic. Plants need converting before they are used in hydroculture.
This method uses a standard plant pot, with bottom drainage holes, which is placed into a large plant pot saucer. A saucer which is larger than the usual size for the pot is chosen so that it can work as a reservoir, I try to select one that will hold enough nutrient solution for approximately 1 week. The nutrient solution is stored in the saucer. It is possible to get transparent saucers and these make the checking of the nutrient solution level even easier. If several plants are grown in close proximity then, instead of using a seperate saucer for each plant, a large watertight tray can be used to house several pots. A tray without drainage holes is used such as a garden tray or a gravel tray, this is filled with enough nutrient solution to last for approximately 1 week.
This is very similar to the pot and saucer method above except that the plant pot is placed into an watertight pot container instead of a saucer. A plastic pot container which is slightly larger internally than the plant pot, by approximately 1cm all round, is chosen. This allows space which will act as the reservoir. With this method it is more difficult to see the nutrient level, so the pot either needs taking out or a level indicator needs to be used in order to check the nutrient level. Alternatively a transparent plastic pot container can be used but this possibly defeats one of the reason for using a pot container i.e. it does not disguise the plant pot. With this method I try to get a nutrient level height of approximately 1/4 of the inner pot. The level is measured when the system is assembled, i.e. when the fully potted plant pot is placed inside the plant pot container.
It’s possible to make a level indicator. I’ve tried several methods over the years and one of the most successful uses plastic tube, such as 20/22mm rigid plastic water pipe or rigid plastic conduit, and a plastic drinking straw. The plastic tube goes in the inner pot and sits on the bottom. There’s a small notch in the bottom of the tube to ensure that water can enter and exit it, making sure that the notch isn’t large enough to allow aggregate to enter the tube. The straw has the bottom end sealed, if it’s not sealed then it will not float. Two marks are drawn on the straw to indicate minimum and maximum nutrient level. To set the marks, I assemble the inner and outer pot without any aggregate and put the plastic tube in place. Put the straw into the tube and draw a line around it level with the top of the tube, this is the minimum level. Now I put water in to approximately 1/4 of the height of the pots and draw another mark around the straw, this is the maximum level.
There are commercial hydroculture kits available which use an outer watertight pot and an inner pot which has drainage holes or slots. These kits usually come with a nutrient level indicator which makes it very easy to see exactly when they need topping up. These level indicator usually have three markings, those being MIN, MAX and OPT. Typically one would top up to OPT, when MIN has been reached, but if extended watering periods are necessary then one could top up to MAX. The manufacturers/suppliers of these kits can usually supply nutrients too.
Commercial Level Indicator
Houseplants need converting prior to being used in hydroculture, refer to the converting plants section on this page. Potting is done in a similar way to potting in soil except that aggregate is used. Aggregate is place in the bottom of the pot to approximately 1/3 the height of the pot, the plant roots are held suspended in the pot then aggregate is poured around the roots to the same height as the plant was originally potted. I waggle the plant and tap the pot to ensure that the aggregate fully surrounds the roots then run room temperature tap water through it.
When a plant is first transferred to hydroculture the roots sit in the clay pebbles and not the water/nutrient solution, the plants roots will receive moisture from the reservoir through the capillary action of the aggregate. This is why the base of the pot is filled with clay pebbles and the roots are placed on top of them. It is also necessary to ensure that the reservoir is not too deep as this will also cause the roots to sit in the water/nutrient solution. When the plant is established it may grow roots downwards into the reservoir, this seems to be OK with the plants I have grown.
A nutrient solution is made following the manufacturers instructions and this is then added to the pot and allowed to drain through into the reservoir, being careful to not overfill the reservoir. I allow the level to fall until no nutrient solution is in contact with the bottom of the pot before adding more. Some plant pots have feet on the bottom of them which stand the base of the pot off of the reservoir by a few millimetres, with these I don’t let the reservoir empty, I top it up when the nutrient level falls to the top of the feet. When the reservoir has emptied the aggregate can dry out quickly, especially on hot days. Some plants are not tolerant of dry aggregate so I add nutrient solution as soon as the reservoir empties.
As mentioned there are slow release resin nutrients available, these are usually added to the aggregate or reservoir, depending on the manufacturers instructions, and will release nutrients slowly for several months. If these are used then it is only necessary to add water to the plants. The resin will need re-adding when they are exhausted.
Tapwater is used for making the nutrient solution. I allow this to stand for several hours to reach room temperature.
Click here for a list of plants reported to be suitable for hydroculture which was kindly supplied by PurLec Hydroculture. This is not a definitive list, but a starting point. If you know of any suitable plants for hydroculture that you would like to tell me about then I can be contacted via the feedback form on my FAQs & Contact page.
I tend to experiment with commonly available plants and see if they will convert to hydroculture. The majority of the plants I have tried convert successfully but I have had a few failures. It’s impossible for me to say if the failures are due to the plant not liking hydroculture or whether it’s because I tend to experiment with techniques and may get it wrong sometimes.
Like all plants, house plants also need good light. Whilst a bright window, or even better a conservatory, can provide adequate and sometime too much light in the summer months, this is certainly not the case for my plants in the winter months, where the day-length significantly decreases. Many of my plants come from more tropical climates, where the day-light would be longer throughout the year, so I like to provide some supplementary artificial lighting to extend the day-length. In fact, plants that are in the room and not directly next to a window receive much less light, so in these cases I sometimes use artificial lighting as the main light source. Plants require light in the Photosythetically Active Radiation (PAR) band.
I typically use fluorescent lighting, in either traditional tube form or compact fluorescent lamps (CFL). For a single plant, I have in the past used a desk lamp equipped with a standard CFL and plant growth has been good. For a better spread though, to cover more than one plant, I use a fluorescent light fixture, containing several tubes. When using fluorescent fixtures, a good inbuilt reflector is important, in order to direct the light down onto the plants, where is it required. Another option are high power CFL, which are available for horticultural use, together with suitable fixtures.
Based on some NASA research into efficient ways of growing plants in space, comes LED grow lights. LEDs have a very narrow bandwidth, so can be chosen for the specific wavelengths required for photosyntheses. Most light sources output some portion of the green spectrum, which is of little use to some plants – the reason we see most plants as green is because they are reflecting green light. Originally, LED grow lights were primarily a combination of red and blue, with there being a higher proportion of reds to blues. Red and blue, at the correct wavelengths, are the main chlorophyll absorption bands. Nowadays however, some LED grow lights are available with more colours, to cover some of the auxiliary spectral bands. Some also include white LEDs, mixed in with the other colours, for which the plants can make use of some of the spectrum, but more importantly make the plants appears more attractive to the human eye.
Cleanliness of the equipment helps to reduce the risk of plant disease. The hydroculture equipment I use is cleaned, before it is used, using the following methods:
Thoroughly rinse with clean water until the water is clear.
Place aggregate into a bucket.
Fill the bucket with tapwater
Leave to soak for approximately 1hour.
Place pots into a bucket containing clean water and a small amount of washing-up liquid (hand dishwashing liquid).
Using a small soft clean brush scrub the inside and outside of the pots.
Thoroughly rinse the pots with clean water.
Apart from ensuring that the plant receives nutrient solution, when required, there is very little aftercare involved.
The plants appreciate being washed from time to time, this helps to remove any dust which has settled on the leafs and salt or nutrient build-up in the aggregate, I also wash out the saucer/pot container at the same time. I do this once a month. The pot is removed from the reservoir and placed under a room temperaure running shower or slow running tap. After cleaning is finished excess water is allowed to drain then the pot is placed back into the reservoir and nutrient solution is added.
Plants need repotting into larger pots as they mature. I do this when the roots start to grow through the drainage holes in the pot. Care is taken when removing the plant from the pot to avoid ripping or breaking too many roots.
If it’s necessary to extend the watering period then I use a larger reservoir such as a garden or gravel tray without drainage holes. The pot is removed from the reservoir and placed in the tray, the tray is then filled with water up to the height that nutrient solution is usually added. It’s best to not do this too often as the larger reservoir means that the aggregate will stay wet for longer and some plants like to dry out slightly between watering.
Articles & Videos
Hydroculture: Growing Plants Without Soil
A nicely written blog on growing plants in hydroculture.
Inside Urban Green – Hydroculture
Inside Urban Green hydroculture section has plenty of examples or hydroculture including some alternative methods to those on this site. There are also useful links to other alternative growing methods, such as aeroponics, aquaculture, hydroponics etc. A real wealth of information on modern methods of growing.
How to grow house plants in water
A lens site written by a fellow hydroculture enthusiast. This lens explains hydroculture, helps you get started, and provides valuable resources.
Hydroponics information on Wikipedia [opens a new browser window].
Growing houseplants in hydroculture – passive hydroponics.