After seeing many houseplant enthusiasts have great success using passive hydroponics to both grow and propagate their plants, I decided to give it a try!
I have been able to successfully root 11 cuttings to date (100% of the cuttings I tried) via passive hydroponics. Only one cutting did not make it, which was due to a fungal disease.
I want to share the method I used and what I’ve learned to date about passive hydroponics so that others can try it as well. If you decide to give it a try, please share your results with us below!
Table of Contents
- What is passive hydro?
- What I used to set up my passive hydro propagations
- A step-by-step guide to my process of rooting plant cuttings via passive hydro
- 2 months after starting the cuttings – Here are my results
- Troubleshooting – Issues I had and what I did about them
- What do I plan to do now
- Amazon links to all products mentioned in this post
- Related Posts
What is passive hydro?
Hydroponics systems allow plants to grow without the use of soil. Because soil is usually what provides plants with nutrients, the nutrients must be provided by using nutrient-rich water regularly.
These hydroponics systems can be either active or passive.
Active hydroponics systems rely on water movement in some form to bring water and nutrients to the plants (like using a pump to circulate water).
Passive hydroponics systems, on the other hand, do not use pumps or other electronic equipment to move the water and nutrients. Instead, the plant receives nutrient-rich water through wicking or capillary action using a porous growing medium.
Passive hydroponics is also referred to as Semi-hydro by many.
An example of capillary action that we are all familiar with is how sponges absorb liquid. The process through which a sponge soaks up the liquid is the same process through which a porous growing medium soaks up the liquid.
In [passive hydroponics] most basic form, it provides water only beneath the plant roots. It doesn’t recirculate. The nutrient-rich water is there until used by the plants.Tammy Clayton, Garden Culture Magazine, https://gardenculturemagazine.com/what-is-passive-hydroponics/
In my setup, I use a porous growing medium called LECA (Lightweight Expanded Clay Aggregates) to draw water up to the roots of a plant.
What I used to set up my passive hydro propagations
This is a general list of the items I am using to propagate in passive hydro.
If you are looking for links to buy any of the products, those will be included at the end. Click here to skip to that section.
Items I Needed
- Net pots
- Containers to hold net pots and water
- PH testing kit
- PH adjustment solutions
Items that are optional, but I use consistently
- Super Thrive
- Bamboo Stakes
- Plant Clips for climbing vines
- Grow lights
- Root inoculant to control algae
A step-by-step guide to my process of rooting plant cuttings via passive hydro
#1 Rinse and Soak the LECA before use
LECA is the expanded clay balls in the photo above. The name is an abbreviation describing them: Lightweight Expanded Clay Aggregates. It comes with dust in the bags from the LECA rubbing against one another.
To remove the dust, thorough rinsing is necessary.
Because the dust is clay, it is advised to discard the dirty water outdoors so the clay dust doesn’t build up in your sink’s pipes.
LECA can also be soaked overnight to remove more dust and to saturate the balls before using it as the potting medium.
I’ve noticed that if I don’t soak them for a while, I have to refill the water reservoir more often in the beginning until the LECA are saturated. I’ve also noticed that there might be a little extra clay dust fallout when not soaked.
#2 Place cuttings 2/3 into net pots and surround with LECA
Once the LECA is prepared, I fill the bottom third of the net pot with LECA.
After covering the bottom I will place my cutting in the center of the pot and fill in around it with LECA until it reaches the top of the pot.
I make sure that at least one node is covered with the clay pebbles because the node won’t be receiving moisture to hydrate the plant and grow roots otherwise.
This is also the time that I will add bamboo stakes and support clips if the cutting needs it. These items can be added later but it is easier when the net pot isn’t already full of LECA.
#3 Add water 1/3 the way up (below where the plant sits)
Once the cutting is potted, the net pot can be placed into the cachepot, bowl, or another holding container.
Then water can be added one-third of the way up the side of the net pot – which is below where the cutting or roots are placed.
As the water level decreases, more water will need to be added to maintain the 1/3 level.
Sometimes I have accidentally let passive hydro propagations dry out and it hasn’t been an issue.
PH adjustment to a plant’s ideal range: 5.5 to 6.5
Some people adjust the PH of their water to ensure it is in the ideal range for plants – 5.5 to 6.5.
This can be done using a hydroponics PH kit, like the one below.
PH can be adjusted for both the water used to refill the reservoir and the nutrient-rich water.
This particular kit makes testing very easy. I simply add a few drops from the small bottle to a sample of the water I plan to use and then see what color the water turns. That color can be compared against the label on the bottle itself, which has the PH range color-coded on it.
I can then adjust accordingly using the other two solutions, PH Up and PH Down.
My test results are displayed below, showing that my distilled water had a PH of 6, so no adjustments are needed.
#4 Rinse LECA as needed to reduce the mineral buildup
(the white crystallized buildup on LECA)
You may notice that white dust or crystal-looking buildup forms on the LECA balls over time. This is mineral buildup from dissolved minerals in our water as the water evaporates.
While a little bit of mineral buildup is not a huge problem, allowing the buildup to continue to accrue may damage or burn the roots.
To prevent this from happening, the pots should be flushed regularly – which just means that a strong stream of water should flow through the pot to dislodge and wash out the mineral buildup.
This buildup can also be reduced by using filtered or distilled water.
Common filters like those installed in refrigerators or purchased separately, like Brita, can help to reduce a little of the impurities present in water. Distilled water is more effective at this by evaporating the water in a controlled space and then condensing it once more, leaving most of the impurities behind.
#5 Add monthly nutrients to provide what the plants would normally get from soil
This is the most complicated part of passive hydro for me.
To provide the plant with the nutrients it needs, I mix a gallon of water with hydroponics nutrients. I use General Hydroponics nutrient kit which contains FloraBloom, FloraGrow, and FloraMicro (linked to the Amazon product page).
The kit advises that FloraMicro should be mixed first. The order I do each time is Micro, Grow, and lastly Bloom.
I started off using a smaller dosage than the bottle states I can use in a gallon of water. I wanted to make sure I didn’t damage the plants with too strong of a solution. I’m not sure whether this was necessary, but it proved successful for me.
At the end of the first month of having the cuttings in passive hydro, I added 1 ml of each nutrient to my gallon of water.
At the end of the second month, I added 1.5 ml of each nutrient.
This month (month 3) I added 2 ml of each nutrient.
I also add SuperThrive (linked to Amazon) to my nutrient solution each month. This is not something the plants need to survive and thrive. It is basically a vitamin boost for plants to give them an even better chance of rooting and thriving.
I do think that it has helped with growing and rooting my plants because I have gotten really wonderful results in a short period of time so I plan to continue using it. I’ve been adding 1 ml of Super Thrive each month.
#6 Continue maintenance regularly
The steps above are repeated as often as needed to maintain the health of the plants.
The water reservoir is refilled when low.
The PH of the water is continually tested for refills and to create a nutrient solution. (though after a few times testing a water type, it may not be necessary to test each time if the results are consistent)
All plants will continue to need monthly nutrients unless transferred to soil.
2 months after starting the cuttings – Here are my results
Here are the results of my passive hydro propagation experiment. To summarize, it was a huge success.
Most of my cuttings need to be repotted due to the overwhelming amount of roots they have grown. I did lose 1 cutting to fungal disease, but even that cutting had rooted before it fell ill. Below I will show a few of the cuttings I tried with photos documenting their progress.
This cutting came from a grower in Hawaii. It arrived in beautiful condition. As you can tell from the picture on the top right, the leaves started to yellow during rooting and I wasn’t sure the plant would make it. Fortunately, I was wrong to worry because the plant has rooted beautifully!
Hoya heuschkeliana variegata
This cutting also came from the same grower in Hawaii as the one above. It took quite a while for the cutting to begin to root. The bottom leaves were still soft a few weeks into rooting, but all a sudden the leaves firmed and roots started popping out everywhere! This cutting also grew two new leaves during the process, making it one of the biggest achievers in growth and development.
Hoya ‘Rebecca’ and Hoya memoria came from a grower in Florida. Hoya ‘Rebecca’ not only established roots pretty quickly, but it also grew a peduncle and flowered within the 2 months of rooting this cutting.
Hoya memoria, also from a Florida grower, was another that was slow to root, but has rooted none-the-less!
Hoya caudata ‘Sumatra’
Hoya caudata ‘Sumatra’ was one of the first to begin rooting, but coincidentally has the smallest ball of roots. It just goes to show how different each plant develops.
Hoya finlaysonii ‘Olfe’
Hoya finlaysonii ‘Olfe’ wasn’t rooted for a long time and the stem began to rot. The original cutting, which I forgot to take a photo of had 2 leaves, both of which were above the LECA. I had a feeling that what I thought was a node beneath the first leaf was not, so I decided to cut off the first leaf and put that node beneath the LECA. Sure enough, the remaining leaf firmed up quickly, telling me that it was rooting. Look at it now!
Troubleshooting – Issues I had and what I did about them
Because the water in my home town is very hard, crystallized minerals built up on my LECA super often. The only 2 ways I’ve found to control the buildup is to do very frequent flushes of my net pots or to use distilled water instead of tap water.
Many of my propagations grew algae, green or red, in the glass container where the nutrient solution was sitting. To control this buildup I bought a root inoculant (pictured below) which I add to my nutrient solution to reduce algae growth.
You can see the green algae below growing in the glass jar holding my Hoya heushkeliana variegata.
Another way to control algae growth is to use an opaque container as your water reservoir instead of a transparent one. I prefer to be able to see the water level and root growth at a glance, which is why I used clear containers.
One cutting not rooting
As mentioned above, this cutting (Hoya finlaysonii ‘Olfe’) wasn’t rooting because there was no node in the LECA (I misidentified a stem section as a node).
The cutting came to me with two leaves (each of which signifies a node).
To make a node available that I could plant in the LECA, I cut off the lower leaf. If I had a larger net pot, I might have been able to just bury a bit of the leaf in LECA to root it instead of sacrificing the leaf.
I lost one cutting to fungal disease – but all the rest survived
Here is my rooting Hoya caudata (not the same as Hoya caudata ‘Sumatra’ above). It developed fungus and I got rid of this plant to avoid potentially infecting others. I was lucky and all the others remained healthy.
Mold on bamboo stakes
The bamboo stakes grew black mold, as you can see below, because they were staying wet consistently along with the plant they were supporting.
I asked what I could do about the mold growth and another hobbyist said that he uses a silicone waterproof sealant on the portions of the bamboo that will be submerged. I haven’t tried this yet, but I plan to.
What do I plan to do now
Will I grow in LECA or transfer to soil?
I am thinking I will do both. I am not someone who currently wants to convert all of my plants to passive hydro, but I am going to keep some of the harder to grow specimens in LECA for a while since they are there currently, happy and healthy.
Would I propagate again using LECA?
Yes, absolutely!! I went into this experiment with the expectation that not all of the cuttings would root. It is normal for some cuttings not to make it, for various reasons. I also had no experience prior to these past few months with passive hydro and figured that I may not be able to keep some of the cuttings happy because I was learning as I went.
I am so pleasantly surprised that all of them survived and that passive hydro seems to be fairly easy as long as the basics are done right. I’m so glad I decided to explore passive hydroponics!
Amazon links to all products mentioned in this post
Containers to hold net pots and water
I used small mason jars and Pyrex containers to hold the net pots. These were inexpensive options for me because I already had them. Really, many containers could support your net pots, but I will include the links to the ones I used.
I like that when I am done I can sanitize them and use them for other purposes in my kitchen.
Plant Clips for climbing vines
Root inoculant to control algae
Gallon Jug for Nutrient Solution
PH testing and control kit
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