The right water depth for bottom watering is halfway to three quarters up the side of your pot. This range comes from NParks Singapore. The depth lets water touch the drain holes for wick action. It also keeps water from flooding the top of the soil.
I tested this rule on my own plants to dial in the right depth for each pot size. My 4 inch (10 cm) African violets did best with 1.5 inch (4 cm) of water in the tray. My 8 inch (20 cm) peace lilies needed 3 inch (8 cm) of water for a full soak.
Get the depth wrong and your plant pays the price one way or another. Too low and the water never reaches the drain holes for the wick action to start. Too high and water enters the pot from the top and floods the soil all at once.
Your bottom watering water level needs to touch the drain holes at the base of the pot. The fine soil pores pull water up from these holes through capillary action. Water cannot climb up if the holes sit above the water line in the tray.
The NParks Singapore protocol gives you a clear range to work with for any pot. Fill the tray to halfway up the pot side for a quick soak, or up to three quarters for a slower deeper drink. Adjust the depth based on your pot size and how thirsty your plants are that week.
Soil type changes the math a bit when you set the water level. Fine peat mix pulls water up fast and needs less depth in the tray. Chunky bark mix needs more depth to push water up against the air gaps in the soil column.
Watch for two clear warning signs that your how much water in tray choice is off. Water on top of the soil after 15 minutes means the depth is too high and water came in over the top. Dry soil after 40 minutes means the depth is too low and the wick action never started.
I had a peace lily that kept getting top flooded in my early days of bottom watering. I had set the water at 5 inches (13 cm) for a 7 inch (18 cm) pot, well over the safe line. Drop the level to 3 inch (8 cm) and the problem went away in one watering.
Use a clear tray when you can so you can see the water line at a glance. I marked my plastic trays with a black sharpie at each fill line for fast setup. You can do this in 5 minutes for your whole plant collection and save time on every future soak.
Check the tray water level after 10 minutes to see how much your plant has pulled up. A drop of half an inch (1.3 cm) or more means the capillary action is working well. Less than that means the depth is wrong or the soil mix is blocking the flow.
Get the water depth right and your plants will drink the right amount every time. Match the pot size to the chart above and you skip most of the trial and error. Your soak sessions will run smooth and your plants will thank you with steady new growth.
Read the full article: Bottom Watering Plants: 8 Pro Tips
