Do juniper roots, needles affect surrounding soil?

Asked September 26, 2013, 11:56 AM EDT

5 years ago I removed a considerable amount of junipers. I left the stumps cut below grade and the roots as soil stabalization (small hill) and had very little success growing anything in that region. I have recently been told two things, please verify and much thanks in advance:
- first, is that the 30 years of needles produced a higher acidity within the soil?
- second, the roots which I left give off a chemical which stunts the growth of anything else in the region and has a 4-5 year period of doing so?
I have seen grass and weeds start to grow in this region with one exception, a 6-8' diameter "no grow zone", not even weeds. I just found that in the center of this circle I had cut a root from a neighboring blue spruce (tree is still thriving). Could this cut root be preventing any growth?

Denver County Colorado

1 Response

#2 answer is closer

If soils were made "too acidic" by needle drop, then you should be able to apply lime to the soil to raise pH, making the soil less acidic (more alkaline) and therefore productive again. If soils were made "too acidic" then you should plant blueberries and rhodendrons there! If you were to do a pH test on soils where junipers were growing and compare that with a pH test on soil from 50 feet away, it's unlikely that the pH would differ much.

There is evidence that many junipers produce allelopathic chemicals that can inhibit, stunt or prevent growth of certain other plants nearby. Another way to say this is that junipers make their own "herbicide", an amazing adaptation that allows them to minimize competition from other nearby plants. The best known example of plant allelopathy is Black Walnut, which secretes a chemical called juglone into the soil, where it stunts or prevents growth of many different plants.

There are some university research efforts aimed at identifying and extracting allelopathic chemicals from various plants for potential use as "natural" herbicides.

Most allelopathic chemicals (terpenes, phenols, alkaloids, etc) do not have extended activity in the soil as soil microbes do break them down into simpler compounds. So, one thing you could do is to add a few shovelfuls of good garden soil from your garden and some compost to the soil of this area...maybe an inch-thick layer total. Rototill or spade into affected soil, mixing well. CAUTION: rototilling may injure existing tree roots in the area.
This will add organic matter to affected soil, stimulate microbial growth and encourage microbial breakdown of allelopathic compounds. Could take a while.