Ask Dr. Universe – Evergreen Trees

Dr. Universe: Why are evergreen trees green all year? – Emily, 10, Silverdale, Wash.

Dear Emily,

Whenever I go for a hike in the woods, I can’t help but admire the tall evergreen trees. No matter what time of year it is, the pines, hemlocks, cedars, and spruces are usually all green.

My friend Bert Cregg is also very curious about the lives of trees. He graduated from Washington State University and is a professor at Michigan State University.

Cregg told me that evergreens have lots of needles, which are their leaves. We have even seen some trees, such as bristlecone pines, that have had the same needles for more than 16 years.

Each tiny needle on a great big evergreen is working hard to make food for the tree. It all happens through a process called photosynthesis. Here’s how it works:

The tree’s needles contain something called chlorophyll that gives them their green color. But the chlorophyll also has another important job. The chlorophyll absorbs sunlight which the tree can use to turn carbon dioxide from the air and water into sugars. It is these sugars that help the tree grow and stay green.

But while some trees, such as maples, stop doing photosynthesis in the colder months, evergreens keep on photosynthesizing (pho-toe-synth-uh-size-ing). In addition to sugars, evergreen trees also need something called mineral nutrients to help them grow.

In fact, humans also need mineral nutrients, such as calcium, potassium, and iron to help them grow. But while humans get their nutrients from food, trees get a lot of their own kinds of nutrients from the soil.

Cregg said that evergreens are really good at living in cold places where there aren’t a lot of nutrients in the soil.

“Once you have worked hard to take up those nutrients,” Cregg said, “you want to hang on them.”

Evergreens store up all those nutrients and can use them through the winter months. These types of trees are also good at storing up water in their needles which can help them stay green, too.

The nutrients help trees to do all kinds of things, including go through photosynthesis. But I also found out that some even evergreens do lose at least some of their green color. We might see some of their needles at the bottom of the tree start to turn orange. That means those needles are at the end of their lifespan.

“They drop their needles but they don’t do it all at one time,” Cregg said.

We have quite a lot of evergreen trees in Washington state, as you can tell from the state’s nickname “The Evergreen State.” The next time you look up to an evergreen, think about how each little needle is doing the job of keeping the tree green and growing. What kinds of evergreen tree species are growing in your state or neighborhood? Can you find some of their needles or pinecones Or do you have other kinds of trees in your part of the world? Tell us about what you see some time at

Dr. Universe