3R Lighting/ tutorial Humidity and Plants

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3R Lighting Tutorial: Humidity and Plants

Q: Why is humidity so important to plants?
A: Plants do not have a heart or a bloodstream like we do.

What a plant does is absorb water and nutrients through the root hairs and evaporates water only out the leaves, leaving the nutrients behind. This method of moving the nutrient inside the plant's own system is called "transpiration".

In high humidity, it becomes more difficult for the plant to evaporate the moisture it needs to for transpiration. This can result in nutrient deficiencies and color changes on the leaves.

Q: What is the best relative humidity?
A: Most books say 50% .

We do not know where the research for this number comes from, but virtually everyone sticks to this number. It's an easy fix--not too dry, not too wet. We suspect, however, that this research comes exclusively from plants grown in soil. From experience, we know that the best relative humidity depends on a number of factors and that for experienced hydroponic growers, with their plants growing very quickly, lower humidities than 50% have proven to help with speed.

In soil, though, it is best to stick to 50%.

Q: Can Humidity get too low?
A: Well, maybe, sorta'. There is research that seems to point toward low humidity being a factor in Blossom End Rot.

Q: What is Blossom End Rot?
A: Blossom End Rot is often called a disease, but it is really a syndrome. It is not caused by a bacteria or virus that we can identify. It affects fruit, but not the leaves.

At the cell level, Blossom End Rot is simply a lack of Calcium in the fruit.

Q: So, just add more Calcium fertilizer. What's the problem?
A: Making sure that there is enough Calcium fertilizer has to be the first step to fixing Blossom End Rot. It's kinda' like fixing a TV and checking to see if the TV is plugged in as the first step. You have to make sure Calcium is available to the plant.

The next thing is to make sure that the pH in the root zone is between 6.0 to 6.5 or else there can be Calcium in the root zone, but the plant has trouble absorbing it.

Here's where the humidity comes in: the plants have little holes or stomates in the leaves where the plant evaporates water and the plant nutirents are left behind. When the humidity is low, it is easy for the plant to move water out of the leaves through the stomate holes.

In fact, if the plant moves more water out of the leaves than it is sucking up through the roots, the plant starts to look limp and wilt. This is because the plant is drying itself out faster than it can replace the water. When new water--or more specifically--new water nutrient mix is absorbed by the plant, it goes straight to the leaves. The plant is desperately trying to fill the leaves back up with water pressure so the plant will stiffen up.

So because Blossom End Rot happens in the fruit. When the plant gets to the point it starts to wilt, it forgets about the fruit. During severe drought conditions, the plant will even drop the fruit, to try to stay alive. So even though there is sufficient Calcium in the root zone, and even though the plant is absorbing Calcium, the Calcium may not be getting into the fruit.

This is only one of many factors that can contribute to problems in some cases. If you are growing herbs, they often appreciate lower humidities and growing conditions. These plants are not grown for their fruit, but for the leaves themselves.

If you have enough time to measure the humidity range and length of time when your plants first start to show signs of wilt you can get an idea what is best for your plants in your micro climate.