new raised border on turf
What is the best way to prepare a new raised border on existing grass? How high does the border need to be to mitigate moderated to poor drainage? (I am planning to plant trees). What dirt should I use? Thank you so much for providing this service to our community.
Oakland County Michigan
First, I'd like to confirm that I have a proper understanding of what you'd like to build. Reading your inquiry, I believe you want to create a raised bed in which to plant one or more trees. Is that correct? If not, please provide a bit of clarification by responding to this message.
This configuration (raised bed) is not likely to be successful over the long term. Though species and site conditions can lead to differences in growth, most tree roots develop in the top 18 - 24" of soil. That would be a good estimation of the minimal height you would want to provide. The width needed for the bed, however, presents a greater challenge. Most trees root well past their drip line (the outer edge of the leaf mass). Providing a bed large enough to accommodate this spread is difficult in most residential landscapes. Limiting the root growth can lead to a weak tree--susceptible to insect infestation and disease as well as mechanical failure. In addition, as they grow, the tree roots may well destroy the structure you have built to contain them. (The same way tree roots will push sidewalk sections out of alignment.)
Even if you are able to provide adequate space, it won't completely eliminate the drainage issue. The raised bed is basically a container, with native soil at the bottom. Water that enters the bed still needs to exit through that native soil, whatever it's drainage limitations.
The trees you plant are more likely to be successful if you plant them directly in the native soil. If drainage is poor, select trees that tolerate wet conditions, or mitigate the drainage problem by amending the soil or adding a drainage system. If grass is currently growing successfully in the spot you expect to locate the trees, drainage may be sufficient for most species.
I'm attaching links below with additional information.
If I got this wrong and what you're planning to install is simply a decorative border, the answer is different. Follow the manufacturer's instruction for the border material you select. Most turf roots only grow to a depth of 4", so if you go remove soil to that depth you'll limit the amount of grass that grows into the tree bed. The recommendation to plant the tree in the existing soil remains.