Holy Hydroponic Houseplants, Batman!: Can you grow houseplants without soil? Yes!

Just when you thought you got the hang of growing houseplants in potting soil (or if you’re a doting plant parent, a special homemade mix someone on the internet told you to use) comes a new trend – hydroponic houseplants!  Or, “semi-hydroponic” to use the more technical term that is used when describing the trend.  How do you grow houseplants semi-hydroponically?  Do they grow this way?  But first, maybe we should ask the question – why? 

Why grow semi-hydroponically? 

I think for most casual houseplant growers, this method is attractive because it is a challenge.  Something new to try after you’ve mastered growing houseplants the old-fashioned way. And quite possibly a pandemic project to provide a distraction after being cooped up in the house for months on end.   But are there benefits to growing houseplants this way?  Turns out, there are some.    

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Many articles you find on the subject state that semi-hydroponic houseplant growth can be beneficial for those who struggle with chronic over- or under- watering.  The media used for semi-hydroponics is a big, porous puffed clay stone called hydroton or LECA (Lightweight Expanded Clay Aggregate, of course we have to have an acronym!).  It is used in some hydroponic vegetable (and other plant) production systems.  The large pore spaces it creates and the wicking action it does in the container helps keep a balance of air and water for the roots. The #1 leading cause of death among houseplants is overwatering – it creates a lack of air in the potting media, the roots lack oxygen (called hypoxia), and are either damaged or die.  This can also make it easier for fungal infections to cause root rots.  But how can you stop from overwatering plants if you’re growing them in water?  We’ll talk about that in a bit when we talk about the “how to”. 

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There are some houseplants, like epiphytes, that might also benefit from having a media that isn’t like soil.  Plants that are used to growing on tree bark, or in rocky environments in their native habitat that might actually perform better in a media that is a large, rough pebble that does kind of resemble the texture or tree bark or stones.  There are lots of tropical houseplants that also grow in areas with high levels of large particulate organic matter like chunks of wood and bark.  Plants from boggy environments that have high water requirements or grow in a more “mossy” type soil might also benefit. 

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One other application of this method is for propagation of cuttings.  Many houseplant growers like to propagate cuttings in water, but this often isn’t the best practice because the water can be depleted of oxygen (causing hypoxia and rot) or become spoiled or soured (and cause infections).  Most horticulturalists will recommend propagation in a light media like seed starting mix, perlite, or sand.  But keeping water consistent without overwatering is difficult in this situation, and media can also be a vector for disease.  The air space and wicking action of the LECA media used for semi-hydroponics can help keep cuttings hydrated without the issues of water propagation.  This method is commonly done in clear class containers, so there’s the added benefit of being able to see root growth to monitor progress. 

How do you grow semi-hydroponically?

Of course, in this short article we won’t be able to cover every detail, so if this is something you’re interested in trying, I’d suggest some self-study.  I’ll be covering some of the basics, but there’s a lot more to learn. 

First, this method somewhat resembles one of the simplest forms of hydroponic production that lots of home hydroponic gardeners use, called the Kratky method.  In this passive hydroponic method, a plant is suspended above a water-based nutrient solution.  At first planting, the nutrient solution is right below the plant, close enough for a few inches of the roots to touch the solution.  As the plant grows, the roots elongate and the nutrient solution level is reduced to keep just a few inches of the roots submerged.  This allows the roots to take up solution, but the space between the plant and the solution allows a majority of the roots to be surrounded by air to avoid the issues of hypoxia. 

In semi-hydroponic houseplant growing, a container (usually clear glass, at least for beginners) is filled with the LECA media and the plant’s roots are distributed through the media.  (The media should be washed and soaked in water first, to remove dust and allow it to hydrate.)  It is easier if it is a young plant or recent propagation so you don’t have too many roots to deal with (you may need to root prune larger plants).  Smaller plants will also withstand the shock of going to this system, especially if they’re moving from a potting soil media.  (Note: Clear glass container + nutrient solution + light = algae. Be prepared to clean up the algae from time to time.)

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A (dilute) nutrient solution is added to the container.  The roots should not be submerged in the solution, but rather it should be added to a level where it will wick up through the media to surround the roots.  The basic rule of thumb is to fill the container about 1/3 of solution, but if the container is exceptionally large or the roots are very small, you may need to fill it higher to make sure the media around the roots stays hydrated. 

This nutrient solution is one of the trickier parts.  You can use a general all-purpose hydroponic nutrient mix, available in lots of garden centers now or online.  You can also try some of the general houseplant fertilizers or ones specific to whatever houseplant you’re trying to grow.  You’ll want one with micronutrients as well as the macronutrients like N-P-K – since we’re growing without soil or an organic matter based media you’re going to have to supply all of the plant’s nutritional needs.  You’ll want to mix the solution between ¼ – ½ the recommended strength – you’ll need to see what works for you and your plant.  And then you’ll want to pH balance the water to create the right environment for the plant and make sure that nutrients are available for uptake.  The pH range for most plants is between 6.0 and 7.0 (aim for 6.5), unless you have one with specific needs.  For this you’ll either need pH test strips or a meter (which you can now get for less than $20 online) and some acidic and basic solutions to adjust pH (you can use some household items like vinegar to do this, but your best bet would be solutions specifically prepared for adjusting hydroponic or aquarium pH levels commonly referred to as “pH up” and “pH down”).  This pH adjustment is a lot easier (and maybe unnecessary) if you start with distilled or reverse osmosis water (or if you have a really good water filter that removes dissolved solids).  The pH levels and dissolved solids in some tap water makes it hard to adjust (my water here in Nebraska is very basic because it is very heavy due to high calcium levels, which also throws off the nutrient balance). Rainwater or melted snow can also work (though may not be pH balanced). 

You want to keep the solution topped off so that the media stays sufficiently moist. As with hydroponic production, plants pull nutrients out of the solution at different rates, so you can get build-up of some nutrient salts over time that could result in poor growth and even toxicity.  To avoid this, every few weeks (or more often if your plant is a heavy drinker) you might need to perform a flush, where you drain off the nutrient solution, give a quick rinse with tap water, and start over with fresh nutrient solution. 

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More experienced growers might graduate to using this method in containers other than clear glass.  This adds a level of challenge, since you can’t automatically assess the level of nutrient solution by visual inspection.  The use of self-watering pots that have net pot or hole-y insert pots are commonly used for this.  Or you can buy net pot or orchid pot plastic inserts to use in any non-porous container you desire.  Growing in net pots can make the flushing process easier, since you can just pop it out of the container and run tap water through it.  Otherwise, you’ll have to find a way to pour the tap water out of the container or

completely remove the plant and wash the media. 

What can I grow semi-hydroponically?

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Well, you can try with a lot of different plants.  I don’t know that there’s a list of plants out there for do’s and don’ts, but there are a few good candidates to try. Most tropical houseplants are good candidates. I’ve seen lots of articles on orchids, and I just recently put a rescue phalaenopsis in semi-hydroponics.  Other epiphytes like holiday cacti and bromeliads are also good candidates – think of things that like to grow on trees/treebark. Hoya, which are all the rage in houseplant circles, are also candidates due to their mostly epiphytic habits.  Lots of tropicals like Monstera, Philodendron, and Pothos also do well in this system.

Things that probably won’t do the best in this system are ones that don’t like to have “wet feet” – I’m thinking mostly desert cacti and succulents. But some of the LECA lovers that I talked to said that some succulents, like “string of pearls” and other strings of things (hearts, dolphins, turtles, etc) do grow well. But if we take a look at their natural habitat, where they grow over rocky outcroppings, it makes sense.

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There isn’t really an exhaustive list, so you might want to experiment if you’re wanting to try it out. As long as it isn’t an expensive plant (and there are lots of expensive houseplants out there), a little experimentation can help you find the plants that would work best for you and your situation. 

In conclusion…..

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Growing houseplants semi-hydroponically isn’t for everyone.  Getting everything just right can have a learning curve, especially if you weren’t great in chemistry class.  But, it can be a fun way to challenge yourself and may also benefit your plants in the right situations.  It is becoming so common that the materials are getting easier to find – many garden centers now carry the LECA and hydroponic supplies, you can always order them online, and you can even find small bags of the LECA/hydroton in the ever expanding houseplant section at IKEA (of all places, if you’re lucky to have one).  So if you’re up for a challenge, give it a try!  You might find a fun new way to grow houseplants….or a new way to kill houseplants!  But the fun will be in the trying. 

Sources:

• Kratky Hydroponic Method: Upstart Farmers
• Houseplant Hydroculture
• Semi-hydroponics: A new way to pot orchids