Topsoil depth to prevent weeds in lawn replacement project

Asked October 2, 2016, 11:55 PM EDT

Hello, I am planning to remove a lawn and replace it with native plants within the next month or two. I am planning to manually remove the lawn with a sod cutter. The lawn is in bad shape already, and I'm not too worried about it growing back, however many, many dandelions have gone to seed there. In order to prevent dandelions from coming back, I was planning to spread a layer of clean topsoil on the site after lawn removal and before planting. However, I don't know how deep the topsoil layer needs to be to prevent the dandelion seeds from germinating. (Or if this will work at all.) Can you recommend a topsoil depth to suppress weeds, and if you think it will not work, can you suggest another method to suppress weeds? Someone suggested laying down cardboard and mulching on top, but for part of it I would like to grow a native meadow from seed, so the cardboard wouldn't work. Any advice is welcome. Thank you x a million!

Marion County Oregon sustainable gardening and landscaping native plants horticulture

3 Responses

Thanks for your question about lawn removal! (I did the same thing 3-4 years ago; have attached a photo of the result.) Here's the thing. If you use a sod remover, you're going to be removing the seeds that will be on top of the soil, and unlikely to have rooted in the fall. Your real enemy is the tap roots of the dandelions that have matured (and produced seeds.) Tap roots will create new plants in the spring because they carry the DNA to do so. The cardboard (or black plastic) method of killing plants is effective, but not while the plants are dormant. Optimal time is spring through summer, when weeds are growing, forming flowers then seeds. That said, taking up sod now and covering it to prevent seeds from rooting in the spring will help. You want to plant natives, and now's the time for that! Just know that you may leave taproots, and they'll be back! If you decide to remove sod and plant natives, be sure to mulch around the plants (to suppress weed seeds in the spring). You'll need to monitor for dandelions and remove manually. Metro has a great publication listing native plants. (I'd send you a link but computer is down and ipad doesn't copy addresses.) Hope this is helpful. Good luck!

BTW, topsoil added to your garden should be between 3 and 6 inches in depth, which is about how much space plants need to form a root system. Even with native plants, water the first dry season (summer) until they establish root system.

Thank you Kristena, you included a lot of great info! Your project turned out beautifully, thanks for sharing the photo!