Transpiration – Why you need to water your trees

Watering Can
Watering Can (Editor_B, Creative Commons)

By Brighton West

Trees need water. That’s why Friends of Trees staff and volunteers tell every new tree owner to give new trees 10 to 20 gallons per week during the dry season. We’re glad to see lots of ooze tubes and five-gallon tree-watering buckets around Portland.

But did you ever wonder why trees need so much water?

First, let’s look at photosynthesis. Water + Carbon Dioxide = Plant Material + Oxygen. You probably knew that already. Plants need water to make oxygen.

But there’s more—a lot more. It turns out that only ten percent of the water that’s sucked up by a plant’s roots is used for photosynthesis. The rest is used for transpiration.

Through transpiration, water evaporates from the undersides of leaves and is drawn up from the roots. This process cools the leaves, exchanges oxygen and carbon dioxide, and moves nutrients up the tree.

No water in the soil = no cooling of the leaves = no nutrient transfer and no photosynthesis.

So water those trees when it’s dry outside. And if you want a more detailed explanation of transpiration, complete with words like stomata, xylem, and lysimeter, check out Wikipedia.

–West is Program Director for Friends of Trees.