Overwatering Your Plants
Interview: Can You Really Overwater a Plant? Understanding Root Hypoxia and Transpiration
Caller: Hey Aquaponic Dumme, I keep hearing people say you can “overwater” your plants. Is that actually true?
Aquaponic Dumme: Technically, you can’t overwater a plant. If overwatering were truly possible, systems like Deep Water Culture (DWC) wouldn’t exist. The term “overwatering” is commonly used to describe various negative effects on plants, but the real issue isn’t too much water — it’s anaerobic conditions. This has a name: root hypoxia. It’s the lack of oxygen in the substrate and rhizosphere that prevents roots from performing respiration. Roots need to breathe.
Caller: So what actually causes that lack of oxygen?
Aquaponic Dumme: There are a few reasons. Sometimes growers water too frequently, saturating the soil and creating anaerobic conditions. Other times, the soil is too compacted, preventing oxygen from reaching the roots. And sometimes, it’s simply that the root mass itself becomes so dense that oxygen can’t circulate properly. In all these cases, the roots suffocate due to a lack of O₂.
Caller: But in DWC, the roots are underwater all the time. How does that work?
Aquaponic Dumme: That’s the key point. In DWC systems, the roots are submerged 100% of the time, but the water is aerobic — it’s full of dissolved oxygen. That proves it’s not the quantity of water that causes problems, but the amount of oxygen available in that water. As long as oxygen levels are high, roots can thrive even when fully submerged.
Caller: So if a plant looks overwatered, what’s really going on?
Aquaponic Dumme: If your watering schedule is healthy but the plant isn’t taking up water, it could be that the water is just sitting in the soil or substrate with nowhere to go. When transpiration slows down, water uptake slows too. That stagnant water sits there until it evaporates naturally, which can take a while. During that time, oxygen levels drop, and anaerobic conditions develop.
Caller: What exactly is transpiration?
Aquaponic Dumme: Transpiration is what drives water movement through the plant. It relies on cohesion and adhesion — water molecules stick together and to the plant’s internal surfaces, forming a continuous column from root to leaf. As water evaporates from the leaves, more is pulled upward from the roots. It’s a constant cycle that keeps the plant hydrated and oxygenated.
Caller: And that happens through the stomata, right?
Aquaponic Dumme: Exactly. Water exits through tiny pores called stomata, which control how much water the plant takes up. When the stomata are open, water moves freely through the plant. When they’re closed, water uptake slows or stops entirely.
Caller: What makes the stomata open or close?
Aquaponic Dumme: Each stoma has two guard cells that act like a mouth. They open and close based on internal signals and environmental cues. One key factor is light. When light photons hit the guard cells, potassium ions move into them, converting sugars into malate. This lowers the water potential inside the cells, drawing in more water through osmosis. The cells become turgid, the stomata open, and water uptake increases. When the lights go off, the process reverses — the cells lose turgidity, and the stomata close.
Caller: So that’s why plants look like they “wake up” when the lights come on?
Aquaponic Dumme: Exactly. When the lights turn on, the stomata open, water uptake increases, and the plant perks up. If the plant isn’t getting enough light, the stomata stay closed longer, which slows water uptake and can lead to oxygen issues in the root zone.
Caller: What about temperature and humidity? Do they affect this too?
Aquaponic Dumme: Definitely. As room temperature increases, the air’s ability to hold water also increases. This changes the Vapor Pressure Deficit (VPD) — the difference between the moisture in the air and the moisture inside the leaf. VPD directly affects transpiration rates. If humidity is too high and temperatures are too low, transpiration slows. On the other hand, if it’s too hot — above about 82°F (27.7°C) — cannabis plants start producing more abscisic acid, which signals the stomata to close. That reduces transpiration, slows water uptake, and limits gas exchange.
Caller: So “overwatering” is really just poor oxygen and transpiration management?
Aquaponic Dumme: Exactly. It’s not about how much water you give — it’s about how much oxygen is available and how efficiently the plant can move that water. If transpiration rates slow, water will sit in the soil or substrate until natural evaporation takes place, which can take a while. Warmer temperatures also cause oxygen to gas off faster, leading to anaerobic conditions.
It might sound like a lot of science, but understanding this helps prevent one of the most common issues new growers face. “Overwatering” isn’t about too much water — it’s about not enough oxygen.
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