What if there was a halfway step between full inorganic semi-hydro and soil growing that was less stressful on the plant, but still gives you the benefits of semi-hydro growing?

Semi-hydro with pumice: Swedish Plantguys’ pumice approach (SPG)

Authored by Alison J.

This approach derives from Swedish Plantguys’ pumice planting approach that I like to refer to as SPG for short. To be clear, what I detail here is similar but slightly modified with corrections/clarifications in comparison to what they outline in their YouTube channel.

For those not familiar with Swedish Plantguys, they're a wealth of information on caring for and troubleshooting problems associated with particular plant species. They also provide information about growing in pumice which they state they’ve been doing for more than 12 years. The basic premise of their pumice approach is planting a soil rootball in a pot that's completely surrounded by pumice, leaving room for a reservoir at the bottom of the pot and utilizing wicking material to carry water from the reservoir up to the base of the soil root ball. 

Sound familiar? 

If you've been planting in semi-hydro or even just reading this website, it should! Pumice has very similar properties to LECA: It absorbs moisture and wicks it throughout the pot while also having the benefit of leaving plenty of air pockets and never compacting. Over time, roots grow into the reservoir and develop water roots in exactly the same way that LECA plants do. And just as with LECA, they can and will survive that way. 

In an SPG set up, the wicking properties of pumice are used in similar ways as LECA is utilized in most semi-hydro set ups; however, these wicking properties are used in conjunction with a soil rootball. The pumice surrounding the rootball provides aeration and consistent moisture without being too much moisture since pumice only ever absorbs about 30% of its weight. Over time, roots grow into the pumice, adapting to the more airy environment it provides. With periodic flushings (same as you would do with LECA plants), remaining soil washes away. This can take several years but never fear - your plant will be perfectly fine with soil co-existing alongside pumice. The plant will have room to grow and will have plenty of air.

But won’t the roots rot from all that constant moisture? No. Lack of air is most often the culprit of root rot. In this set up, pumice provides adequate aeration in similar ways as LECA making the constant access to moisture not a problem.

Can any plant grow in this set up? Potentially, yes. As they explain in a Q&A video #5 @ 1:10, this set up can and will work even for the most drought-loving plants. The difference is that you omit the reservoir for those plants needing to dry out between waterings and just flush nutrient water through periodically, allowing longer periods between waterings.

For me personally, I reserve this set up for my sensitive water-loving plants like African violets and calatheas. My preference is for full semi-hydro with LECA but I’ll use PON and SPG depending on what plant I’m transitioning. By sensitive, I mean plants that might not transfer well or easily to LECA or PON straight away. These types of plants certainly can be transferred straight to LECA or PON but it can be tricky and dependent on approach, set-up, environmental factors, etc. That being said, I have successfully transferred plants using the SPG method with more moderate water needs such as tradescantia or philodendron brazil, and I’ve done so with a reservoir from the start. 

As a side note, the benefit of transferring vining plants to SPG is that you get to maintain the existing vining structure of the plant since it remains anchored in place in the root ball and you're just surrounding that rootball with airy, breathable substrate for it to then expand into. As you can see from the photo, this teddy bear tradescantia has retained its original shape and placement and this anchoring makes it much more manageable.

How to pot SPG-Style

Rinse the Pumice & rocks.

Step 4

Apply first layer of pumice to the pot.

If you're using a single pot, make sure you leave enough room at the bottom for a 1/3 reservoir. The soil rootball should never be in the reservoir space. If you're using an NSW pot with a wick, make sure the wick is threaded before you add pumice. Once the first layer is in the pot, pull the wick up to lay across the top of the bottom layer of pumice.

Step 5

 Add rootball and fill with pumice.

Hopefully you've eyeballed how much space you have left in the pot and the height of the rootball and adjusted your bottom layer of pumice accordingly. Place the rootball on top of the bottom layer of pumice and fill in pumice evenly around the rootball and on top. You might have to flex the sides of the pot a bit to allow the pumice to fall into empty space and crevices.

Step 6 (optional)

Add aquarium rocks to the top of the pot.

Add rinsed aquarium rocks to the top, covering visible pumice. The purpose of this is because, in my experience, the pumice on the top layer often turns brown and looks a bit nappy over time as you can see from the photo Aquarium rocks at least hide this and make the pot look clean depending on how much of the top of the pot is visible. Plants with foliage that completely covers the top of the pot don’t need this step.

The Nerdy Analysis

But isn't this exactly like what Swedish Plantguys describe?

Short answer: Yes.

Long answer: Not exactly. Their description of what's necessary to pot in this set up and how it functions is unnecessary and confusing. Here's a link to their pumice video. 

Astute listeners will notice that they mention you have to mix a type of clay in with the pumice in order for it to work properly. The reasoning behind this addition is specified in other videos [@ 41:14] they've posted and it has to do with cation exchange capacity (CEC). CEC is basically the degree to which a potting medium holds onto different nutrients (think magnets here). The higher the CEC, the more a medium holds onto nutrients. "The total number of cations a soil can hold -- or its total negative charge -- is the soil's cation exchange capacity. The higher the CEC, the higher the negative charge and the more cations that can be held." (source). If a medium holds onto nutrients, those nutrients become unavailable for uptake by the plant. Soils with high CEC will remain relatively stable in terms of pH and will remain fertile for longer, requiring less frequent fertilization. LECA and pumice, in contrast, have a low CEC. This is why it's so important to continuously fertilize semi-hydro plants in LECA or pumice and to do so at appropriate concentrations to make sure the plant gets the nutrients it needs. Both mediums bring nothing to the table nutrient-wise and this won't change with time. 

Swedish Plantguys are well versed in soil growing. And, I would note, they explicitly advise against bare rooting plants or disturbing plant roots as semi-hydro growers so often do. In fact, they do not recommend planting in semi-hydro set ups unless you have purchased your plant that way because, they argue [@ 5:49], it's too stressful to the plant and difficult if not impossible to be successful with such transfers. As this website demonstrates pretty clearly, it IS possible and oftentimes not terribly stressful to the plant (this can vary by plant type). Because of their stance toward semi-hydro, they don't see their pumice approach as semi-hydro (I do!). They also don’t believe that the wicking action of pumice actually carries nutrients along with water enough to distribute it throughout the pot. Instead, they believe it’s necessary to add something with a high CEC that will hold onto nutrients inside the pot so that those nutrients are sitting waiting for the roots to take them as needed. This is the logic behind their claims that adding a powdered clay product [@41:14] to the mix is absolutely essential. While I respect their knowledge and experience, they are absolutely wrong on this point.

What doesn't seem to occur to them is that having a low CEC can actually be a good thing. The problem with high CEC substrates is that not only do they bind or hold onto nutrients but they do so in a way that makes those nutrients inaccessible to the plant. This is why pH is so important in soil growing. At different pH levels, soil mixes hold onto different nutrients. Different plants have different nutrient needs thus we must be careful with the soil pH so that it doesn’t bind nutrients that a particular plant needs. In other words, having a low CEC is actually great! It means that LECA or pumice, as substrates, are not withholding necessary nutrients. Yay!

But there is another misunderstanding fueling their belief that high CEC environments are necessary in this set up. They mistakenly believe that pumice (and LECA) do not wick nutrients throughout the pot. Instead, they believe that pumice only has the capacity to wick water from one place to another in the pot. In other words, they believe that without the addition of the powdered clay, nutrients will never reach the plant roots. In this way, they believe the addition of powdered clay assists in the process by grabbing nutrient molecules and suspending them in the mix for roots to access. But this logic clearly fails given that high CEC of substrates actually works against nutrient availability by withholding nutrients from the roots. Why would you want your substrate to hoard nutrients? And rest assured, LECA and pumice both wick not only water but also the nutrients contained in your liquid fertilizer mix. And those nutrient molecules remain available to the plant to take as needed. 

In other words, if they experimented a little with just plain pumice, they would find out it actually works! Not only have I potted plants in their SPG pumice set up with success but I’ve also planted in just plain pumice. Pumice is very similar to LECA in terms of wicking ability and low CEC. And as should be clear at this point, LECA doesn’t require any modifications to alter its CEC in order to make it work. Long-time LECA growers can certainly attest to not having issues in this regard so long as you're paying attention to water type, nutrient concentration, and nutrient frequency. Pumice is no different. 

So, why take the additional, unnecessary step of adding to pumice when you don't have to? This understanding is supported by Ray Barkalow (45+ years growing orchids in LECA) and his description of pH and nutrient availability. According to him, "in orchid media, which tend to have essentially no CEC, the nutrient cations tend to stay in solution and remain available" (source).

I spoke to Ray at length about this and here’s the summary.

CEC (Cation Exchange Capacity) in soils originates primarily from microscopic organic matter and clays.

Clays are flat, plate-shaped crystalline minerals. The face surfaces and the edges have different electrical charges due to the exposed ions in the crystals. The strength of the charges of those ions, their polarity (+ or -) and the density of them determine what external ions are attracted and bound to them or not. The old "nutrient availability versus pH" charts demonstrate that. Another good example is pH control over "clayey" soils - in one pH range, the clay stacks face-to-face, making it very dense and hard to work. In another range, it is edge-to-edge, which is better, but not ideal, and in the preferred pH range, they are arranged edge-to-face like a card house, with lots of open space for air and water.

Substrates with high CEC - that is, a lot of binding sites - hold onto stuff strongly, preventing it from remaining in- or returning to solution in order to be taken up by the plants. (All uptake is via solutions.)

LECA contains no organic matter and the exposed clay crystal lattices have mostly fused to others, so the CEC is so low as to be almost nonexistent. That suggests to me that nutrients in solution will be wicked, unabated, right along with the water, rather than being "trapped" along the way.