Relative humidity and VPD are two of the most overlooked variables in indoor hydroponic growing. Growers often chase nutrient deficiencies, adjust feeding programs, and buy supplements when the real issue is environmental. This guide explains what RH and VPD are and, most usefully, how to read the actual symptoms your plants show when either one is out of range. For the full target ranges by growth stage and how to calculate and manage VPD properly, see our dedicated VPD in Australian Grow Rooms guide.
What relative humidity means for your plants
Relative humidity measures how much moisture is in the air compared to the maximum it can hold at a given temperature. It directly affects how your plants transpire, the process of releasing water vapour through their leaves. Too much moisture in the air and transpiration slows. Too little and plants lose water faster than roots can replace it.
Why VPD tells you more than RH alone
Vapour Pressure Deficit is the difference between the moisture content inside the leaf and the moisture in the surrounding air. It tells you how hard your plant is working to move water and nutrients from the roots to the leaves, and unlike RH alone, it accounts for temperature as well. Two rooms with identical RH readings can have very different VPD depending on how warm each one runs. In hydroponics specifically, plants rely entirely on water movement to transport nutrients, so if VPD is off, nutrients cannot move efficiently and growth slows or becomes inconsistent even when the feeding program is correct. This is why many growers find that everything looks right on paper but plants still struggle. For the actual target VPD ranges by growth stage and how to measure it correctly at canopy level, our VPD guide covers this in full.
Signs your humidity is too low
Low humidity is one of the most common issues in indoor grows, particularly under LED grow lights which produce less radiant heat than HPS. When the air is too dry, plants transpire too quickly and cannot keep up with water demand. Common symptoms include leaf edges curling upward, dry or crispy leaf tips, rapid water consumption, pale or dull leaf colour, and burnt-looking tips that are often mistaken for nutrient burn. Low humidity is frequently misdiagnosed as nitrogen toxicity, overfeeding, or salt buildup. In many cases the plant is not overfed, it is pulling too much water due to dry air and concentrating nutrients in the leaves.
Signs your humidity is too high
High humidity slows transpiration, meaning plants struggle to move nutrients effectively. Common symptoms include dark green overly soft leaves, drooping despite adequate watering, weak stems, slow growth, and increased risk of mould and mildew. In flowering, high humidity creates conditions for powdery mildew and bud rot. Plants may look overwatered even when irrigation is correct.
Equipment needed to control humidity
Managing your grow room environment properly requires the right tools. Most growers invest in a humidifier to raise humidity, a dehumidifier to lower it, inline exhaust fans, carbon filters with airflow control, oscillating fans for air movement, and a temperature and humidity controller. In cooler climates a grow room heater is also necessary. These tools allow you to maintain stable conditions and hit your target VPD consistently across all growth stages, covered in detail in our VPD guide.
The cost of ignoring your environment
One of the most expensive mistakes in indoor growing is trying to fix environmental problems with more products. Adding extra nutrients, buying supplements, and flushing and refeeding are all common responses to symptoms that are actually caused by incorrect temperature and humidity. Dialling in your environment reduces input costs, eliminates unnecessary products, and makes your existing feeding program work the way it is supposed to.
Environment control products
For controllers, fans, humidifiers, dehumidifiers, and climate management equipment suited to Australian grow rooms, browse the ventilation and climate control range.
What This Means for What You Buy
If you're constantly fighting RH swings rather than dialling VPD in once and leaving it, the issue is usually airflow rather than humidity itself, an oscillating fan keeping the canopy moving prevents the still, humid microclimates that cause swings in the first place. If your space runs persistently too humid regardless of airflow, a dedicated dehumidifier addresses the actual source rather than working around it.
A Bluelab Guardian Monitor or similar continuous environmental monitor is worth buying before any equipment changes, you need to actually see your RH and temperature swings over a full day-night cycle to know what you're fixing.