Question
QUESTION: Good Day Jim,

I have 4 trees that I believe to be some type of Sequoias and Redwoods along the fence line in my backyard. They are at least 30-50 feet tall and have been in their current locations for at least 10-15 years. I am in Fresno, CA.

My dilemma is that their shade cover and root systems prevent anything else from growing within that area of the garden. I would also like to visually and physically separate this area from the lawn area.

The potential solution I have in mind is to build a stone retaining wall block raised garden bed. It would be about 60 feet by 5 feet around the perimeter and about 12 to 16 inches high.

I am, however, concerned about covering the lower 12 to 16 inches of those trees with soil. Will this rot the bark area and/or kill the tree? I love these trees and wish them many more years right where they sit so I don't want to cause them any harm. Needless to say, I would also be less than pleased were one to fall given that there are houses within 20 feet in every direction.

Any guidance you can offer would be greatly appreciated. Thank you for time, help, and dedication to this service.

Be Well,

Ben

ANSWER: Not a good idea! 90 percent of a tree's roots occupy the top 12 to 18 inches of soil. These fine feeder roots spread out, pancake-like, away from the trunk. Roots can extend from the trunk to a distance of 1 to 2 times the width of the tree. Roots take up water, nutrients and oxygen, and release carbon dioxide. Because roots need oxygen, any activity that compacts the soil or raises/lowers the existing soil level can damage tree roots and diminish tree health.

Changing the grade drastically is not something that trees can tolerate very well. At the most, a 2 inch layer of sand at a time is the most one should apply.
The very shallow roots are responsible for absorbing all of the water for the plant, for the uptake of dissolved minerals, and for holding the plant in place. Those top few inches of top soil provide all that the roots will seek, including plenty of oxygen without which they (roots) will die. By adding too much more soil on top, you will be smothering those roots. 2 or so inches during a season and the roots can slowly grow UP, before adding another 2 inches. Use sand. You can then add grass sod over the area.

Also keep the soil away from the tree trunk about a foot. the tree needs the swell of the trunk as it enters the ground-covering it will effect the health of the tree.  


---------- FOLLOW-UP ----------

QUESTION: Thank you for the prompt and extremely helpful response Jim.

I think I will opt to go with one level of border stones to frame the bed but will definitely stay away from adding a full 12" to 16" raised retaining wall around these trees. For new plants, I will probably just use above ground planters such as whiskey barrel halves.

May I ask if adding a 1/2" to 1" of top soil over the hard pan to give the area a fresher look would be safe?

Thanks again

ANSWER: That small amount of soil will be ok. The problem with more than that would be the weight of the soil will compress the soil and deplete the soil of oxygen that the roots need. A 1/2 or 1 inch will be OK.

---------- FOLLOW-UP ----------

QUESTION: That's great to hear. Thanks again Jim.

If it's okay I'd like to take advantage of your expertise a little while longer.

The trees, as previously mentioned, are about 30-50 feet tall. In this case, are they still largely dependent on being watered from the surface down or, given their height, is it likely they've found an underground water source? The city is moving to a metered (as opposed to flat rate) water system so I am trying to budget what I use.

Answer
If you're Redwood is 45 ft. tall then it should be getting ground water from someplace. A 45 foot tree shouldn't have to be watered due to the expanse of the root system. If this tree were newly planted then I would say give it a deep soaking once a week, but this tree is 45 ft. tall, unless you are having a severe drought, which you wouldn't be wasting water on a tree then stop worrying it will be just fine.