killing the roots of trees after cutting them down - as organically as possible

Asked May 3, 2016, 10:29 PM EDT

Hi - recently I had my septic tank cleaned and was told that the roots of two young walnut trees that I had let grow (because they were providing shade to a part of my yard where it was nice) were impinging on my drain field, and potentially threatening my septic tank. At the company's suggestion I've had the trees cut down, leaving about 10# of trunk. I was told to drill holes into the trunks and put in "poison" and then seal the trunks by varathaning them - so the chemical couldn't escape through the air and would definitely be drawn into the roots. The tree trunks are about 5" in circumference. I've attached a photo. I want to do whatever I need to do to protect my septic system, but I'd like to use the least toxic chemical possible. I've gardened organically here for more than 35 years. What can you suggest? Thanks very much, Virginia

Multnomah County Oregon

3 Responses

You may have a couple of issues. I would recommend that you simply have the stumps ground up, but I don't know if that would affect your drain field. When I've had a stump ground, the tree usually does not send up suckers from the roots (but I have seen this from other grafted fruit trees and some other woody shrubs like laurels and roses. Since these trees are close to your drain field that may be why it was recommended to use something to pour into the stump. If the stumps are only 5 inches in diameter, you may be able to hand dig out the stumps and leave the roots in the ground. They will eventually rot. It's hard work but it's organic!

Hi - thanks for your answer. The roots are almost over the septic tank and definitely into the septic field, so I'm very hesitant about digging them out - and they are significant in size and depth. The septic person said that he has seen instances where the roots continue to grow and spread after the trunk is removed. That's why I'm a little less confident than you that the roots will definitely "eventually rot". He said the only way to be sure the poison would be absorbed by the roots - and therefore kill them eventually - was to seal them in. He spoke about roots "shattering" and "crushing" septic tanks because of their force. Does that seem melodramatic to you, or could the roots of a fast-growing tree - which these volunteers have been - have the potential to have that much force? I'm not sure if your background is as a forester. Do you have a recommendation for a non-organic treatment that would definitely kill all the root growth? At this point I am most concerned about the possible damage to my septic system. Thanks again for your time, Virginia

Whether or not the roots will continue to grow is more likely related to the species of plant. A 'fast-growing' tree could have that effect. I am not a forester (just a master gardener.) I think if you are really concerned, you should contact a certified arborist. (For tips on choosing an arborist http://usna.usda.gov/Gardens/faqs/SelectArborist.html) A consultation on your options may cost you some $ but it will be a fraction of the cost of fixing your septic system.

I found this presentation from U of California regarding woody weeds http://ucanr.org/sites/csnce/files/57600.pdf This document provides the information on active ingredients that are effective, the availability to a home owner and application options.

For 'sealing in' the product, you can simply use any latex paint you have around, you don't need a varathane sealant. A couple of coats is all you need.