Clearing under raised bed?

[Ablor]

I notice instructional videos clearing out grass/weeds before laying out a raised bed. Why is this necessary? If I’m going to dump more than a foot of soil (or sand + pebbles+ soil) on top of the weeds/grass, won’t that kill them? I’m asking because the ground here has burdock and other difficult weed roots underneath, and with many things planned out, I’d rather not do what I don’t have to. Thanks.

 

[Oliver Buckle]

If it were me I would mow it very short first, then if it had those sorts of deep rooted weeds I would put something down to suppress them, but which would rot down over time. In the old days it would have been paper or cardboard, but they put a lot of plastic in those products nowadays. I would probably go for the cheapest tongue and groove cladding I could find.
On the other hand most things won't make it past what you put on top, as you say, so maybe just dig out or poison what does.

 

[Black_Thumb]

I would think if you put down a thick layer of newspaper, it will block the sunlight and kill the weeds and their roots in relatively short order. The newspaper is also biodegradable, and no worries about chemicals that way either. I'd cut it very short, like Oliver said, and cover it. That's exactly what we're going to do around our three big Dogwood trees in the front yard. We want to make one large circle around all three of them, and then fill it in with some red mulch. We'll be putting down a thick layer of newspaper to kill the grass and weeds, and then put down the mulch.

 

[Mr_Yan]

At home I've always had good luck with cardboard or newspaper as the bottom layer then pile soil over that and it would smother weeds just fine.

BUT

My old church back in Rockford IL ran a community garden and built raised beds in what was an old mowed field. These were at least 18" of soil over the sod layer and there was bindweed / devil's bindweed / wild morning glory in that filed. That crap had no problem growing up through the 18" of soil and persisted for years. I can't talk to burdock specifically but something like that wild morning glory will have you wanting to salt the earth and give up.

 

[Ablor]

At home I've always had good luck with cardboard or newspaper as the bottom layer then pile soil over that and it would smother weeds just fine.

BUT

My old church back in Rockford IL ran a community garden and built raised beds in what was an old mowed field. These were at least 18" of soil over the sod layer and there was bindweed / devil's bindweed / wild morning glory in that filed. That crap had no problem growing up through the 18" of soil and persisted for years. I can't talk to burdock specifically but something like that wild morning glory will have you wanting to salt the earth and give up.

Hmm.. Thanks for this example. This is what I was wondering about. Burdock is very persistent and deep rooted, although I don't know if it can do what this devil plant did. There were also all manner of other tall weeds in this area - Joe Pye, etc. I will grant that some don't consider these plants weeds - that's fine, I have nothing against any of them in general. Perhaps I will lay down newspaper or cardboard first. Not sure if I should cover and wait a month before filling with soil or cover and fill straightaway. I don't think it makes a difference either way.

Now, Black_Thumb: Isn't there a concern covering your tree roots with cardboard and mulch. I have heard that you shouldn't cover/raise soil level, etc to existing tree roots as this diminishes oxygen access or something. I thought this affects tree life. On the other hand, I have a lot of ground-cover (Pachysandra) around our Maples and I am looking for a way to kill them and replace with something else. The reason is that this ground-cover spreads and is slowly creeping across grass. I dislike anything that spreads and would rather eliminate all such from my yard.

 

[Mike313]

Personally, even though it's a hassle, I would dig out any strong-rooted weeds. I don't like using cardboard or newspaper as weed suppressors underneath the soil - even if they are 'biodegradable'. Various chemicals are used in the manufacture of paper & newsprint inks and I don't know if there is any possibility of them being water-soluble and taken up through the roots of edible plants. So I prefer to err on the side of caution.
Edit: Just noticed this is an old thread but it came up as 'new' in the e-mail I received . . . .

 

[Meadowlark]

... I don't like using cardboard or newspaper as weed suppressors underneath the soil - even if they are 'biodegradable'.

I'm with you 100% on that.