Best houseplant soil mix and filler for use in self-watering planter?

November 18, 2013 3:11 PM

Should the capillary action which will allow water to soak upward from the bottom of my self-watering planters be a factor in selecting a potting mix for Arrowhead Vine houseplants, or is the ideal potting mix the same regardless of whether they receive most of their water from the surface? Also, what would be the best lightweight "filler" for the bottom portion of the planters?

There is an arch/nook above my living room which I would like to decorate with houseplants. Since the area is somewhat out of reach, I purchased three large self-watering planters to limit the frequency at which I'll have to break out the step stool to tend to the plants. The planters are those Mayne Fairfield window boxes that are mostly intended for outdoor use, and since houseplants don't seem to be a common application for this type of planter I'm wondering if the reservoir/trough watering style might effect which type of potting mix would be best. The instruction packet for the planters also recommends using a layer of lightweight filler in the bottom third of the planter, beneath the actual potting mix, "to encourage faster root growth down into the troughs." It suggests lava rock, mulch, or woodchips for this purpose. Anyone out there with a green thumb have an opinion about which material would work best for an Arrowhead Vine houseplant?

posted by lmpatte2 to Home & Garden (2 answers total)

Best answer: I would choose something organic (meaning non-mineral) and fibrous. I like cocnut coir because it is pH neiutral, does not become hydrophobic when it dries out, and maintains good aeration even when wet. You want shredded, not chunky. Large pore spaces stop capillary wicking. You can use a blend with coir, but all coir is really nice and light. You may need to keep an eye out for any nutrient deficiencies, and use a slow release houseplant fertilizer at 1/3 normal dosage if plants start to look peaked.

I don't really understand their instructions about lava rock &c. For one, that change to a coarse material stops water movement downward as well as upward- only gravitational (oversaturated) water will move from a media with small pore sizes to a media with much larger*. What happens in containers is that the bottom creates a condition where the bottom third or so of the soil does not really drain: there is a resistance to water moving downward. At some point if you never water, this soil will dry out and your plant will die, but under normal conditions that area of saturation remains. If you use a filler in the bottom of your pots, you basically shorten the soil column within the pot. This may be desirable in some cases, but normally it won't promote root growth into that layer of coarser media- unless that area stays full of water and your plant likes that. I might put a layer of coarse media in the bottom to keep plant root balls somewhat above that layer of gravitational water that has drained out of the soil, however. That would impede "wicking", but I think it's better to water more frequently and have a sort of reservoir of lava rock to collect gravitational water, rather than having plants with their lowest roots in very wet, mucky soil.


* many people think adding a layer of gravel increases "drainage", but this is a myth. Coarse material allows for collection of oversaturated or gravitational water: when the soil exceeds it's carrying capacity excess water will finally move into those large pore spaces, but not before.
posted by oneirodynia at 4:40 PM on November 18, 2013 [1 favorite]

Response by poster: That makes a lot of sense and is exactly the kind of information I was looking for. Thanks so much!!
posted by lmpatte2 at 8:13 PM on November 18, 2013