Q. Last year I planted a container-grown oak tree from a local nursery and after two months it was dead. I went back to the nursery and got another oak tree and now that one is dead too.

Both trees were bought in 15gallon containers. The nurseryman said that I may not have given the trees enough water, but I heard that you are not supposed to water oak trees because it can harm them. Please help. I want to grow an oak tree.

A. I’ve been asked this question more times than I can remember. There’s much confusion about the care of oak trees, especially in Thousand Oaks. Some of the confusion comes from misinformation, but mostly it’s a lack of education about what’s required for an oak tree to survive and thrive.

It is first necessary to understand what kind of trees we’re talking about. For our purpose I’m going to focus on the two large indigenous oak trees that inhabit the Conejo Valley. They are the Valley oak (Quercus lobata) and the Coast live oak (Quercus agrifolia).

You can identify the Valley oak by its thick, gnarled bark and deeplobed leaves. It’s a deciduous tree that loses its leaves in the fall and then grows them back every March. The new leaves appear as a blanket of millions of beautiful light green leaves that cover the entire crown of healthy trees. The Valley oak is the largest, oldest and most majestic of the trees in our area.

The Coast live oak is an evergreen tree and has grey bark that’s mostly smooth. When healthy, it’s packed with dark green oval leaves that resemble a holly leaf. It generally doesn’t grow as large or live quite as long as the Valley oak.

From here on, everything I say about oak trees will pertain to oaks of both species. The most important thing to appreciate is that both species fall into two categories: naturally germinated oaks and nursery-grown oaks. Each category has its own specific requirements, and the trees have to be treated according to those requirements if they’re going to survive and stay healthy.

Naturally germinated oak trees are those that have been growing in the Conejo Valley for 500 years or more. The trees started as acorns dropped by a parent tree and, under ideal conditions, germinated on their own.

Naturally germinated trees survive everything that nature throws at them, and they establish and mature despite harsh environmental changes. They have thousands of minute roots that search deep and wide for available groundwater.

Once established, naturally germinated oak trees survive on the sparse volume of rain that blows into our area a few months of the year. With just a few exceptions, they should never be artificially watered or fertilized at all. The fallen leaf litter should be left on the ground under the canopy, and no plants or sod should be planted anywhere under the tree. This category of oak trees has been doing just fine without our involvement for centuries and should be left alone.

Nursery-grown oaks, which make up the second category, are trees that are started as a planted acorn in a pot and watered two times a week for two years or more. They are totally dependent upon the artificial irrigation and fertilization provided them. If you stop watering them, they will die. When they’re planted in the landscape, they are still totally dependent upon the artificial irrigation they’re accustomed to. They eventually adapt to their landscape environment as long as they continue to receive ample water.

Once established, more than 90 percent of the root structure of these trees resides in the top 12 inches of soil because that’s where the water and nutrients are. Because of this, it’s easy to understand the tree will do better if it gets watered just like the rest of the plants in your yard.

If you want to plant a nurserygrown oak tree in your garden and stick flowers and potted plants all around it—or plant an oak in your front yard surrounded by a lawn— that’s okay. It is, after all, just a container-grown tree that you bought at a nursery and planted in your yard. Water it like you do the rest of your trees and plants and it will be happy and healthy.

David D. Mortimer is a certi fied arborist with more than 30 years’ experience in the tree care industry. E-mail questions to dmortimer@theacorn.com.