A sub-irrigated pot changes the shape of the wet-dry cycle, not the amount of water a plant gets. In a living-soil system that matters more than it does in an inert medium. One well-known living-soil grower switched from conventional pots to self-watering pots for all his flowering plants. His reason is that a distinct type of root develops in the capillary zone, and that claim deserves a careful look. The source is The Rev, True Living Organics: The Ultimate Guide to Growing All-Natural Marijuana Indoors, 2nd edition (Green Candy Press, 2016), referred to below as “the book” and “the author”. This article covers how the pots work, rough pot sizes by canopy, when bottom watering makes sense, and the reservoir risks that are specific to a tropical climate. It also separates what is evidence from what is observation.
1. The problem: irrigation is a shock event
In a conventional pot, every watering is a sudden change. The medium goes from partly dry to saturated in a few minutes, then dries out over several days. With an inert medium and a bottled feed programme this is fine, and arguably useful, because the wet-dry cycle drives root growth.
Living soil is different, because you are managing a microbial population as well as a plant. Two problems stand out:
Dry pockets. If the medium has dried unevenly, water runs along the easiest paths and straight out of the drainage holes, leaving dry regions inside the pot. Roots in those regions are damaged, and the microbes there go dormant or die. The book links calcium problems to dry pockets.
Anaerobic zones. This is the opposite problem. If a pot sits in standing runoff, the lower root zone runs out of oxygen. The main damage comes from roots being unable to respire. Oxygen-starved root tissue then switches to fermentation and builds up ethanol and acetaldehyde. In waterlogged soil, anaerobic microbes also produce organic acids that inhibit growth, and in strongly reduced conditions they can produce sulphide (H₂S). Warm, wet, stagnant root zones also favour Pythium root rot (see topic 1263).
The usual conventional-pot fixes are double watering and correctly sized saucers, covered in bai135. Self-watering pots tackle both problems by removing the sudden change altogether.
2. How a sub-irrigated pot works
The design is simple: a reservoir chamber below, a growing chamber above, and a wicking connection between them. Water rises through the medium by capillary action, at whatever rate the medium and the plant’s transpiration demand.
The author’s build, in outline: before adding any soil, he puts a layer of perlite at the bottom of the growing chamber. Perlite/vermiculite or perlite/coco also work. The layer should be deep enough to cover the holes between the chambers. He moistens that layer and adds a thin layer of soil mix, which becomes the base for his usual layering. The root ball goes in next, and the pot is filled and core-fed as normal (see bai127 for core feeding and layering).
Perlite is used because it wicks readily, does not compact and, most importantly, keeps the interface between reservoir and soil open to air. In a well-designed pot, roots at the bottom of the growing chamber are not submerged.
The consequence the author stresses: air keeps moving through the medium, so aerobic organisms thrive. In his view, oxygen rather than water saving is the main reason these pots suit living soil. For the wider background on root-zone air and moisture, see topics 1342 and 1343.
3. “Water roots”: the claim, examined
The author reports that roots reaching the perlite layer grow differently there. He describes them as thick roots that take up water quickly and tolerate saturation. He calls them “water roots” and compares them to what plants do in nature when they reach a shallow water table.
Assessment. Plants do produce different kinds of roots under different moisture and oxygen conditions. Under waterlogging, many species form adventitious roots and aerenchyma (air channels inside the root that carry oxygen down from the shoot), and they change their root architecture. Plant physiology reviews document this well. See for example M. Sauter, “Root responses to flooding”, Current Opinion in Plant Biology 16 (2013): 282–286, and W. Jia et al., “Plant Morphological, Physiological and Anatomical Adaption to Flooding Stress and the Underlying Molecular Mechanisms”, International Journal of Molecular Sciences 22(3) (2021): 1088, doi:10.3390/ijms22031088.
What we could not find is any published description of this in cannabis grown in a sub-irrigated container. Treat the phenomenon as plausible and consistent with general plant physiology. Treat the specific “water roots” description as the author’s field observation, not an established finding.
The practical claim built on it, that plants grown this way tolerate overwatering better, has the same status: plausible, but not proven in cannabis. However often an observation is repeated, it is still anecdote.
4. Pot size: a rough guide by canopy, not lamp watts
The book sizes pots by HID lamp wattage. Today most growers in Thailand and Southeast Asia use LED, and LED wattage is not comparable: a “1000 W” LED fixture does not equal a 1000 W HPS. The figures below summarise the author’s own experience with self-watering pots, expressed as approximate canopy area. They are one grower’s practice, not a standard.
| Canopy per light (approx.) | Original HID reference | Pot size the author found workable |
|---|---|---|
| ~0.6–0.8 m² | 400 W HPS/MH | ~28 cm, ~9.5 L |
| ~1.2–1.5 m² | 1000 W HPS | ~33–38 cm, ~13–17 L |
| Smaller canopy, very long flowering (~16 weeks) | 400 W | One size up, ~13 L |
The canopy areas are rough conversions added for this article, not figures from the book. With LED, size the pot to your canopy and the fixture’s rated PPF/PPFD, not to its wattage. Plant size and flowering time also matter. The last row reflects the author’s reasoning: a long-flowering plant needs a pot that carries more fertility and water buffer over the whole cycle, even under the same light.
Two related rules of thumb from the book:
- A beginner’s starting point: about 3.8 L of pot volume per 30 cm of plant height.
- For a truly water-only grow (layering and core feeding, no teas), he treats roughly 13 L or more as the minimum. In smaller pots, expect to top up with core feeding or teas.
5. When to bottom-water, and when not to
The book uses reservoir filling selectively rather than exclusively:
| Situation | Approach |
|---|---|
| Unsure whether the pot needs a full watering, and you will be out all day | Fill the reservoir only. This protects against both overwatering and drought |
| Hot days | Fill the reservoir |
| Large plant in a small pot on a hot day | Top-water first, wait about 15 minutes, then fill the reservoir |
| Final 10 days before harvest | The author’s practice: reservoir only, plain RO water, no additives. This is effectively a pre-harvest flush, and the evidence on flushing is disputed (see topic 1136, Pre-Harvest Flushing: Science or Myth?) |
| Normal conditions | Top-water. The author top-waters by default |
He top-waters by default for a specific reason. Watering from above activates biological activity in the top layer and wets the whole microbial population and root system. The top layer holds the nitrogen-rich layering amendment and the mulch, and it is the most aerobic zone in the container. A pot that is only ever bottom-watered never activates that zone.
Hard water: a guideline, not a ban. Bottom watering, like saucer reabsorption, keeps dissolved minerals in the pot instead of letting them drain away, so salts build up faster around the roots. Measure your source water first. As a rough guideline, if its EC is above about 0.5 mS/cm or its hardness is above about 150–200 ppm CaCO₃, use RO water or rainwater in the reservoir. Alternatively, top-water regularly until you get runoff, to leach accumulated salts. See bai135 for runoff and saucer technique.
6. Tropical reservoir risks
Most self-watering advice comes from temperate indoor rooms. In Southeast Asia, an open water reservoir brings risks of its own:
- Mosquitoes (dengue). Any standing clean water can breed Aedes mosquitoes. Cover the reservoir, the fill tube and the overflow hole with fine mesh or a tight cap. Mosquitoes get in through any gap the mesh leaves.
- Algae. Light reaching the water grows algae, which competes for oxygen, can clog wicks and fill tubes, and feeds other microbes. Use fully opaque containers and keep the fill tube capped.
- Heat and low oxygen. Warm water holds less dissolved oxygen, and warm roots use more of it. A reservoir at 30 °C or above in a hot room or in direct sun is a poor oxygen supply and a good environment for Pythium (topic 1263). Keep pots out of direct sun, use light-coloured or shaded outer containers, and don’t top up with sun-warmed water.
- Hygiene. Empty, scrub and refill the reservoir regularly, at least weekly in hot weather, rather than just topping it up. Stale water in the bottom chamber is where most of these problems start.
7. If you cannot buy a self-watering pot
Buy a locally available sub-irrigated planter. Self-watering planters are widely sold in Thailand and across Southeast Asia for ornamental and balcony gardening. They work the same way as grow-specific versions. What matters is that the reservoir is separate from the root zone, not just a deep saucer.
Build one. The standard DIY design is bucket-in-bucket: a perforated inner container sitting in a sealed outer container, with a wicking column, a fill tube and an overflow hole. Use opaque, food-grade containers. Drill the overflow hole in the outer container just below the base of the growing chamber, so the reservoir can never rise into the soil. The overflow hole is the step people skip, and it is the one that stops the plant from drowning. Mesh it as described in section 6.
Staying with conventional or fabric pots? For double watering and correctly sized saucers, see bai135. For core feeding, see bai127. Fabric pots take the opposite approach, with maximum air exchange and maximum evaporation. In a hot room without air conditioning, they need to be larger or watered very often. See topic 1345 (Grow Bags). Choose based on your main risk: self-watering pots guard against drying out, fabric pots against overwatering.
8. Summary
- Self-watering pots suit living soil mainly because they keep air exchange while removing the soak-then-dry cycle of conventional watering.
- “Water roots” fit what is known from general plant physiology about waterlogging responses, but in cannabis they remain a field observation, not a documented finding.
- Pot sizes are one grower’s experience: roughly 9.5 L for a ~0.6–0.8 m² canopy, 13–17 L for ~1.2–1.5 m², and one size up for long-flowering plants. With LED, size by canopy, not watts.
- Top-water by default and use the reservoir selectively. The RO-only final 10 days is the author’s practice, not established science.
- With hard water, use RO water or rainwater in the reservoir, or leach regularly.
- In the tropics, mesh against mosquitoes, use opaque containers, keep reservoirs cool and clean them weekly.
Has anyone here run the same genetics in a self-watering pot and a fabric pot side by side in a room without air conditioning? In this climate the two designs pull in opposite directions, and the choice probably matters more here than in the temperate rooms most of this advice comes from. Post your pot volumes, room temperature, reservoir water temperature and watering frequency.
Related reading: bai127 (core feeding and layering) · bai135 (double watering, saucers and runoff) · bai139 (perpetual harvest) · topic 1136 (Pre-Harvest Flushing: Science or Myth?) · topic 1263 (Pythium root rot) · topics 1342 and 1343 (the root zone) · topic 1345 (Grow Bags)
This article is general horticultural education, not legal advice. In Thailand, cannabis is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis) B.E. 2568. Cultivation and any dealing in cannabis flower must follow that Notification and the related licensing rules. Recreational use is not permitted, and online sale or advertising of flower is prohibited. The Cannabis Control Act is still a draft: the Cabinet approved it on 22/09/2026 and it is now with the House. Readers in countries where cannabis is banned must not grow it. Check the current law where you live before doing anything.