@brian6464 - can you share the thickness of the cedar boards in your beds? I'd like to buy a cedar garden bed kit and see one I like where the boards are 3/4" thick. I'd like my cedar beds to last a while (ideally 5-10 years). So Im wondering if I should invest in a bed with thicker boards: like 1"-1.25". Would love your input. Thanks in advance.

i have the pseudo named plants ... it means false ... so its a false tsuga ... what.. the nomenclature dude was too lazy to come up with an independent name ... sheesh ... dont even get me going on the maple that is a false plane/sycamore ... pseudoplanatas .. really??? a sycamore maple ... give me a break .. lol .. see below ... but i digress ... [i cant stop.. dude.. its either a maple.. or its a sycamore... it cant be both ...] anyway.. to make your life even more complicated ... conifer wise anyway ... there are two type of DFs.... and its based on which side of the mountains they come from ... the ones on the west side .. will not live in z5 .... the more cold tolerant ones.. from the east side... do take my MI winters... go figure on that ... someone else might be able to give you the full latin on those two varieties ... ken https://www.google.com/search?q=pseudoplanatas&ie=utf-8&oe=utf-8#q=esk+sunset

Let me offer up an alternative. I live in the Pacific NW, so cedar is commonly available, but the quality has declined because much of the reasonably priced stuff is now from fast-growing tree plantations and is not as rot resistant. I have a large number of 12" high raised beds, and I considered making them from cedar, but didn't want to rebuild every several years. After thinking this over, I decided to go with "composite lumber," specifically Trex. It's a wood-resin product that is highly rot resistant, and the first beds I made out of it are 7-8 years old and show no signs of deterioration. Most of my beds are about 4' X 8' and 11" high, and are held together with coated, exterior deck screws. The material comes in a variety of wood-like colors, and is an actual 1" thick X 5-1/2" high, so to get the height I want edI used it two planks high. It has a bit more flex to it than equivalently dimensioned wood, so I simply ripped some 2-1/2" strips from a plank of it and used them as cross bracing: I mounted them transversely at the midpoint of the 8' sides, and halfway up so that they span the seam where the two lengths butt against one another. This prevents bowing of the sides, and also keeps the two boards on the long sides perfectly aligned. So, for a 4' X 8' raised bed, the structure consists of two 4' X 4' sections, but the cross bracing is hidden by the dirt. The thing I like besides the fact that the material is very rot resistant is that it never warps. It also comes in lengths up to 20' and right now I am actually making some 5-1/2" high by 2' X 20' raised tomato beds, with cross bracing about every 4'. The best price I've found on these boards is about $2.50 a running foot, so they are more expensive than wood...but never having to buy materials again or needing to build them again in the future makes it economical in the long run. If you decide to go this route, be aware that there are several board designs: some have a wavy back and some have slits in the edges for concealed fasteners when you use them for decking, and some are just a plain board shape, which is what I use; they look like this: Trex composite decking The other thing is that you can special order these in 1" X 12", which I didn't know when I built mine years ago. The obvious advantage is that construction would be easier because you wouldn't have to stack two of them to get a 11-12" height.

I don't think you're going to get an answer, here, based on the scientific, laboratory study you're hoping for ... unless there is a soil scientist/chemist in the audience, and such a study has been done. Our answers will be based on the practical "gut" hunches that gardeners, horticulturalists and landscapers would have. My hunch is that your best bet of accomplishing your goal is to merely have a high height depth of soil in the raised bed. And I think 3' is adequate. If you place a permeable barrier at the bottom of the bed, it will do nothing. If you place an impermeable barrier there, it will destroy the bed by inhibiting or preventing drainage. The vast majority of garden plant roots are going to be in the top foot of soil. Some roots are going to go deeper, but probably not many roots, and only of the largest plants (that you likely would not have) are going to go to 3' depth. Lawn chemicals in the soil are going to degrade after application and move downward. They're not going to move over into your raised beds and go upward. The likelihood of your raised garden bed plants coming in contact with the "tainted" lawn soil seems very small to me, not enough to worry about.