Since we talked about watering our gardens on Tuesday, let’s talk about the way water moves through plants by something we call transpiration.

Transpiration is the passive process that enables water to move up from the roots, through the branches, into the leaves, and then out into the atmosphere. It not only helps deliver nutrients to the various parts of the plant, but also helps to cool the plant.

For transpiration to work, water is drawn up into the leaves and these leaves have tiny holes in them called stomata. These stomata open and close to help water evaporate which in turn helps keep the plant cooler. During this process, the leaves also absorb carbon dioxide which is required for photosynthesis.

As you might imagine, plants with larger leaves have larger stomata enabling larger amounts of water to be released on hot days. But, when it’s really hot, more water can get released through the stomata faster than it can be replenished, causing the leaves to get droopy on your plant. A good example of this would be hydrangea or hibiscus leaves. For plants with these large leaves, it’s not unusual for them to look pretty droopy at the end of a hot day even though there is plenty of water available for their roots. In this case, the water replenishment will likely catch back up overnight, and the plant will look fine in the morning.

But here’s the interesting part, only about 5% of the water being drawn up by the plant remains part of the plant. The rest exits the plant via the stomata. But this remaining water helps maintain plant rigidity. For an example of this, if you have a hydrangea or hibiscus, go out and feel those leaves when they are droopy and when they are more rigid. The difference is the amount of water within those leaves.

Here’s where guessing the water needs of your plant based on droopy leaves gets a little complicated; one of the symptoms of both over watering and under watering is droopy leaves. Healthy garden soil is about 25% water and 25% air, with the remaining 50% being the solids. Over watering pushes out the air which ironically inhibits the plant’s ability to uptake water. As a result, you get droopy leaves from both under watering and over watering. The downside to long term overwatering is that it will cause root rot which is typically fatal for the plant.

For plants in the ground, to be sure, you’ll need to stick your finger in the ground to check moisture levels. Or there are inexpensive water moisture meters which accomplish the same purpose.

Potted plants are the ones we tend to over water though because over time the drainage holes in the pot can get plugged. So, we water the plant like we always have, it keeps looking droopy, so obviously, it needs more water, so we add more water, and the next thing you know the plant has root rot.

To check moisture in your pots, you can use your finger, or a moisture meter, or you can just lift to pot off the ground or off the hanging plant holder. If it feels too heavy, it likely has a plugged drainage hole. At this point, you need to open up that drainage hole before continuing to water.

After the 6 to 10 inches of rain we got within the last week to 10 days, be sure to check for too much water on your plants before resuming watering. This will help minimize any potential damage from overwatering. Good luck.

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org. Photo: Tom Ingram