# Bottom-watering, soak trays and wick (self-watering) systems

> Bottom-watering lets pots draw solution up from a tray, either by a timed soak (flood 2–5 cm for 10–30 minutes, then drain) or by a constant shallow level held by a float valve or wick. It is gentle and labour-saving and suits soil, but salts accumulate at the top of the pot, so feed at lower EC than top-feeding and top-water to runoff every week or two.

- Canonical: https://genetixbay.com/knowledge/systems/bottom-watering-wick.html
- Category: Systems · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · Systems: Soil, Coco, Living soil
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**Is bottom watering good for cannabis?** Bottom watering is a good, gentle way to water cannabis in soil or airy [coco](https://genetixbay.com/knowledge/glossary/coco-coir.md) blends, as long as you manage salts. Pots drink from a tray, either by a 15–30 minute soak at 2–5 cm depth or from a constant shallow level. Evaporation pulls salts to the top of the pot, so feed weaker and top-water to [runoff](https://genetixbay.com/knowledge/glossary/runoff.md) every 7–14 days.

**Do self-watering pots work for cannabis?** Self-watering wick and float-valve pots do work for cannabis grown in soil or a free-draining coco and [perlite](https://genetixbay.com/knowledge/glossary/perlite.md) mix, and they suit busy growers. Feed about 20–30% below top-feed strength (grower practice), give the tray a dry day every 7–10 days to re-oxygenate the pot base, and avoid dense media, which wick [too much water](https://genetixbay.com/knowledge/glossary/overwatering.md) and stay soggy.

**Why is there white crust on top of my bottom-watered pots?** White crust on bottom-watered pots is accumulated fertiliser salt. Extension research shows evaporation draws salts upward in subirrigated pots, so the top layer can reach several times the [EC](https://genetixbay.com/knowledge/glossary/ec.md) of the rest of the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md). Fix it by top-watering to 10–20% runoff and lowering feed EC, and switch to heavy top-watering gradually so the salt layer doesn't hit the roots at once.

**Should I bottom water [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md)?** Cannabis seedlings in large pots should be top-watered first, not bottom-watered. Water from above until the roots reach the bottom of the pot, then switch to tray soaks or a wick. Bottom-watering a big pot before roots reach the base is a common mistake, because the wet lower mix has no roots to use the water and stays soggy.

**Short answer.** Pots stand in a tray and drink from below. Two flavours: **soak** (fill the tray 2–5 cm / 1–2 in for 10–30 min, drain, repeat 1–2× a day as needed) and **constant level** (a float valve or wick keeps a shallow reservoir topped up) [4]. It's the gentlest method on hardware and roots and suits soil well [4]. The catch: evaporation pulls salts to the **top** of the pot, where EC can reach several times the rest of the root zone [1][2][3]. **Feed weaker than you would top-feed** and **top-water to runoff every 7–14 days**.

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| Soak depth | 2–5 cm (1–2 in) | [4] |
| Soak dwell | 15–30 min, then drain | [4] |
| Feed EC | ~20–30 % below top-feed equivalent | |
| Top-water [flush](https://genetixbay.com/knowledge/glossary/flushing.md) | Every 7–14 days, to 10–20 % runoff | Pushes the salt layer down and out |
| Constant-level systems | "Dry the tray" day every 7–10 days | Re-oxygenates the pot base [4] |
| Media | Soil, airy coco/perlite blends | Dense media wick too much, stay soggy |

## How to run it

1. Start seedlings top-watered until roots reach the pot bottom; then switch.
2. Use a free-draining mix with a coarse bottom layer for wick/float systems.
3. Weigh pots or use a VWC probe to decide soak frequency.
4. Sample EC from the top layer separately if tips burn; discard the top 1–2 cm when [sampling](https://genetixbay.com/knowledge/glossary/homogenisation.md) the root zone [3].
5. Keep trays clean; standing nutrient solution grows algae and [fungus gnats](https://genetixbay.com/knowledge/glossary/fungus-gnat.md).

## Failure modes

| Problem | Sign | Response |
|---|---|---|
| [Salt crust](https://genetixbay.com/knowledge/glossary/salt-build-up.md) on top | White crust, burnt leaf tips | Top-water flush; lower EC |
| Waterlogged base | Slow growth, brown bottom roots | Dry-tray days; airier mix |
| Stagnant tray | Smell, algae | Refresh; Jardin can add an air-stone option [4] |
| Tray overflow | Water on floor | Level sensor + leak pad [4] |

A warning when switching methods: if you stop bottom-watering and start heavy top-watering, the salt layer moves down into the roots at once [3]. Flush gradually.

## Jardin notes

Jardin method "Sub-irrigation / soak / wick": tray level (float or pressure) and reservoir mass required; soak = fill to level, dwell, drain (max-on timer 1.5 × learned fill); wick = continuous hold with periodic dry days. Time-to-harm: days. Automation level 3 [4].

## Common mistakes

- Hydro-strength feed in a wick system.
- Never top-watering.
- Bottom-watering seedlings in big pots before roots reach the base.

## Evidence

Top-layer salt accumulation and the switching risk are *Established* extension knowledge [1][2][3]. Cannabis-specific EC reductions are *Grower practice*.

## Sources

1. UMass Extension. Subirrigation for greenhouse crops. https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/fact-sheets/subirrigation-for-greenhouse-crops
2. NC State University. Subirrigation (top-layer EC). https://hortscans.ces.ncsu.edu/uploads/s/u/subirrig_51e435ff2e468.pdf
3. University of Arkansas. Greenhouse management, Unit 9 §6 (salts in the upper inch; risk when switching to overhead watering). https://greenhouse.hosted.uark.edu/Unit09/Section06.html
4. GenetixBay Jardin engineering notes, 2026 (system profiles and failure margins).