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Systems · Beginner · 2 min read

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.

Supported
Draft: awaiting expert review Updated
Any stageSoilCocoLiving soil

Being reviewed. Our editors are double-checking a figure in this article against primary sources. Marked passages may change.

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 being reviewed
Top-water flush 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 the root zone [3].
  5. Keep trays clean; standing nutrient solution grows algae and fungus gnats.

Failure modes

Problem Sign Response
Salt crust 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.

Questions this article answers

Is bottom watering good for cannabis?

Bottom watering is a good, gentle way to water cannabis in soil or airy coco 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 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 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 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 of the rest of the root zone. 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?

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.

Sources

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

Related

SupportedIntermediate · Draft

Ebb and flow (flood and drain)

A pump floods a table or tray of pots to a standpipe height, then the solution drains back to the reservoir. Each flood refreshes water and pulls fresh air into the medium as it drains. Typical schedules: clay pebbles 3–6 floods per light period of 10–15 minutes, coco or rockwool 1–3. It is robust because the media buffer pump failures for days, but salts can build up at the top of the medium.

SupportedBeginner · Draft

Growing in soil (potting mix with bottled feed)

Soil (a peat- or compost-based potting mix fed with dilute bottled nutrients) is the most forgiving system: it buffers pH and nutrients and holds water for days. Water to light runoff when the pot is light, feed at EC 0.8–1.6 mS/cm with water at pH 6.2–6.8, and remember that over-watering, not under-feeding, kills most soil plants.

Grower practiceIntermediate · Draft

Living soil (no-till, water-only organic growing)

Living soil is a large, biologically active, amended organic soil that is reused without tilling and watered with plain dechlorinated water; soil organisms release nutrients. It is forgiving and low-input once established but slow to correct, so build it well, keep it moist and mulched, never use salt-based nutrients or sanitisers on it, and don't chase pH. Most of its claims are grower practice rather than tested in cannabis.

Grower practiceBeginner · Draft

Choosing your growing system: soil, coco, rockwool or hydro

Choose by how much attention and measurement you can give, not by yield claims. Soil and living soil forgive mistakes for days; coco and drip need daily feeding; DWC, NFT and aeroponics grow fast but can lose roots within hours if a pump or air supply fails. No controlled study shows one system out-yielding the others when light and climate are equal.

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