Hi there. I have searched your forums and am not sure I’m seeing the exact answer to this. I’m wondering if you might be able to advise on an attempted fix for some junipers that are not doing well in our clay-like soil. Before planting, we did research and understood that these trees needed good drainage to thrive – we thus took advice from many gardening websites and added good amounts of peat moss and topsoil before planting, but a few years later most of them seem to be dying. We think there may be insufficient drainage. Some websites say that adding builders sand or organic compost may help to bring them back to life – ar either of these things likely to help, or is there anything else you can suggest? Thank you in advance for your guidance/advice!
Thank you for contacting the Toronto Master Gardeners. You are right that drainage is really important for your junipers (Juniperus scopulorum ‘Skyrocket’ aka Juniperus virginiana ‘Skyrocket’). This plant adapts to a wide range of soils with good drainage, however it will not tolerate wet soil. It is considered to be drought tolerant when established, and requires little watering at that point.
Many gardeners amend their soil with organic material and use this amended soil to backfill the planting holes, but I think this could be what has caused the problem you have now with your junipers. Recommendations about transplanting trees and shrubs and amending soil are evolving and incorporating new research findings. Here is why I think that your junipers are in trouble :
- The roots are running into a ‘wall’ as they try to grow beyond the planting hole. Plants typically thrive in the short-term in their new rich environment, and then have problems trying to grow roots beyond the planting hole into the unamended native soil. This slows down the rate of growth, and the plant may eventually become a hazard because it does not have the roots needed to support itself.
- The movement of water through the ‘wall’ is also a problem. The amended soil is more porous and water will wick away to the finer textured native soil. In the summer moisture in the planting hole will be depleted by the plant and not replaced by water held more tightly in the native soil, resulting in water stress to the plant unless it is watered often. And when the weather is wet, water will move quickly through the amended soil and then it will be blocked by the more slowly draining native soil, resulting in a ‘bathtub’ in the planting hole which will eventually kill the plant.
- Organic material eventually decomposes. The result is a sunken compacted area which makes the flooding in wet weather worse.
- The soil structure is damaged. Amending soil involves digging and tilling which damages the soil structure and the critical pore spaces that hold fluctuating amounts of oxygen and water in the soil.
Textural continuity between the planting hole and the surrounding soil is important for the critical movement of water and oxygen and subsequent root development. It is also important to maintain the integrity of soil structure for the same reasons. Therefore amending the backfill soil is not recommended. Only native soil should be used in the planting hole, and the planting hole should be as small as possible so that there is minimal disruption to the soil.
As to what to do now :
- Mulch : If you are regularly amending the soil where your junipers are planted, you should consider stopping this practice and instead mulch with coarse, chunky organic materials (like wood chips). This is more effective and less damaging to soil health. Coarse, woody mulches protect the soil, reduce compaction and provide a slow feed of nutrients.
- Soil test : You should also consider having a soil test to determine the levels of organic material and nutrients in your soil. It could be that this mulching is all that is needed to provide required nutrients.
- Wait a while : Although your junipers are not thriving now, they might adapt over time.
- Replace your junipers : You might need to consider this if it looks like your junipers just aren’t going to make it. You could either replace them with the same junipers and plant them without any soil amendment, or replace them with different plants that will thrive in your soil as it is without any amendment.
Best of luck with your junipers !
June 7, 2021