Ebb and Flow Flood Cycle Timer Calculator

Ebb and flow (flood and drain) systems work by periodically flooding the grow tray with nutrient solution, then draining it back to the reservoir. Timing the flood cycles correctly ensures roots get nutrients and oxygen in the right balance.

Key Variables:

• Grow media type: different media retain water differently
• Plant growth stage: seedlings need less frequent flooding than fruiting plants
• Temperature and humidity: hot, dry conditions require more frequent cycles
• Light period: most flooding happens during lights-on; reduce or stop during lights-off

Media Water Retention:

Growth Stage Multiplier:

• Seedling/Clone: ×0.5 (half frequency, roots are delicate)
• Vegetative: ×1.0 (standard frequency)
• Flowering/Fruiting: ×1.2 (slightly more frequent, higher demand)

Temperature Adjustment:

• Below 70°F (21°C): reduce frequency by 25%
• 70–80°F (21–27°C): standard frequency
• Above 80°F (27°C): increase frequency by 25%

Flood Duration: The tray should fill to 1–2 inches below the top of the media. Standard flood time is 10-15 minutes, enough for the solution to wick to the top before it drains completely.

Worked Example: LECA media, vegetative stage, 78°F

• Base interval (LECA): 2.5 hours
• Stage multiplier (veg): ×1.0
• Temp adjustment: standard
• Flood every 2.5 hours during lights-on
• Flood duration: 15 minutes, plus about 3 minutes to drain
• For 18-hour light period: 18 / 2.5 = 7 floods per day
• No dark-period flood in vegetative growth

Lights-Off Rule: During the dark period, reduce to 0-1 flood cycles. Roots take up most of their oxygen when the lights are off and the plant has stopped transpiring, so a tray sitting full overnight is the fastest way to get root rot. The calculator adds a single dark cycle only for flowering and fruiting plants, which keep drawing water at night, and only when there is a real dark period to put it in.

A note on failure modes

Ebb and flow has one genuinely dangerous failure: the pump runs and does not stop. Set the timer to run the pump for a fixed number of minutes rather than filling to a level, and fit the overflow standpipe lower than the tray rim. Done that way, a stuck timer empties the reservoir into the tray and stops, which is a mess. Done the other way, it empties the reservoir onto the floor and keeps going.

The second failure is quieter. If the drain is partly blocked, each cycle leaves a little water behind, the tray never fully drains, and roots sit wet between floods. Watch the tray empty completely at least once a week rather than assuming it does.