Question
Is the plastic a heavy grade or a lighter grade like plastic trash bags? Also, wouldn't the plastic keep the water from penetrating and therefore my bed will be soggy?

I was thinking of just 2 stones high, would that enable me to have roses or do I need to go 3 stones high? How do I figure out how much soil I'll need?

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Followup To
Question -
I want to create circular raised beds using those stackable (no concrete needed) blocks. Do I have to put some landscape fabric against them on the inside so that dirt when watered doesn't seep through?
Answer -
Chris,
Yes you do need some black plastic(not clear). I use plastic because many times I have seen rooted seedlings of weeds attach themselves to fabric and cause more trouble than if you had used nothing. Depending on how tall your bed is going to be,use black plastic at the bottom of the bed.The bed really doesn't have to be too tall,because you will have to use more soil. I would say stacking them in a double teir is good.
Get some black plastic in rolls.It is usually in the paint section used for drop cloths for protection. It is very inexpensive.

Then use some metal garden staples in the garden section.They are long  metal u shaped pins that hold down landscape fabric or plastic.

Make sure your plastic is as close to the stones as possible when inserting the staples. The best way to do this is when you have finished constructing your bed,unfold some of the plastic and cut a big section.Then place it on top of the newly constructed stone bed and draw around it the circular width of the bed.Use chalk or spray paint.
Then slide it down into the stones until it hits the ground and let it overlap the stones some.Then use the metal garden staples to secure it into the ground.
When you put your fresh potting soil in it, make sure the sides of the plastic don't get moved around or out of position around the stones at the bottom.
That's it your done. Now you won't have grass and weeds grow up through your bed.
You still may have stray weed seeds blown by the wind root on the top of your bed but you can see these pretty well and remove them easily when you go to water your beautifully planted bed! If it is full sun just make sure you buy plants suited for that area.This is a pretty simple project with big rewards. Good luck and have fun!

Answer
I was thinking perhaps annual bedding plants.I have not seen roses planted in a raised bed before.Usually roses are supported by a structure like a fence or trellis or an arbor.But if that's what you want to plant then go for it.

The plastic may hold water for a short period of time unless you have a really bad rainy season. I don't know your climate. I am in zone 6 in Western Ky. where we often experience severe drought in our summer growing season so this is actually helpful.We need all the moisture we can get. But if you have lots of rain you might want to punch some holes in your plastic at the bottom of the bed.

You will know how much soil you will need after you construct the bed.Then you can see how deep it is and while you are shopping for plants you can then buy the potting soil. Lowes has theirs in huge bags. Depending on the size of the bed,I would say certainly 3 of the largest bags they have and then if you need more you can get more.

The plastic should be about as thick as a heavy duty garbage bag.