Hi all, I just pulled everything from my raised beds for the fall and would like to use the opportunity of empty beds to take action on my weed problem. When building the beds, I placed gardening fabric under the soil, but over many years, the few grass seeds that found their way in have grown, gone to seed themselves, and managed to spread a very healthy system of roots. As much as I pull, and as carefully, there remain enough rhizomes and errant seeds mixed into the soil that I can't get a handle on it.

Is there a chemical or other method that I can use to kill the rhizomes as well as any seeds that remain in the bed? I have all fall and winter to let any chemicals run their course. I won't be replanting until the spring.

Thanks so much!

Covering the soil with black plastic could kill everything - especially if you have some warm, sunny weather.

@porkpal, thanks! This will kill seeds as well? How hot for how long? It's getting cooler here, but we'll probably have some hot days yet.

Use a vegetation killer.
Then roto-till it and apply it again.
Plastic will not kill the rhizomes, or main crab-grass root which can be far, far deeper than you would ever imagine..

IF, if the grass is still there, use a systemic crab-grass killer but it works best if you dig down to the rhizomes, or main root and there IS a main root.

Thanks @RpR. I don't know if a rototiller will work in my little raised beds, and I'm afraid it would mutilate the fabric at the bottom if it went too deep. Is the intention to make sure the plant killer touches all of the rhizomes? I could probably turn it all by hand.

If you have rhizomes, first identify. Plants which grow under ground with rhizomes are difficult to control. The two majors are Bermuda grass and nutsedge. Each requires a different approach.

@farmerdill great question. Looking online at the seed heads from both of those, I think I have seen both. Right now the garden is bare and I don't have the ability to identify them. Is there a solution that would cover both?

Just one that I know. Hand digging. In the case of Bermuda carefully shovel out the soil, being care to avoid breaking off pieces of the rhisomes and removing all the pieces that you break. In the case of the nutsedge you will need to screen out the tubers in addition. Bermuda can be killed with repeated use of glyphosate. It is neither quick nor easy. Nutsedge requires Sledgehammer or Ortho Nutsedge Killer. Again not a quick fix. In a small raised bed with a covered bottom, it may be easier to remove and replace the growing medium.

@farmerdill Thanks. They're just big enough to be a pain in the ass and expensive to replace. 2 at 8'x4' each, about a foot deep. The medium is many years of mel's mix, amended with peat moss, vermiculite and compost each year.

What makes these chemical solutions not quick or easy? I have 6 months (including winter months) until I need to grow anything again.

dscheuerman said:Thanks @RpR. I don't know if a rototiller will work in my little raised beds, and I'm afraid it would mutilate the fabric at the bottom if it went too deep. Is the intention to make sure the plant killer touches all of the rhizomes? I could probably turn it all by hand.

The first application kills what is up high, after tilling it kills what was down lower by bringng it up higher.
Yes you can do it by hand.

One thing I have found in my garden is quack-grass, if you have it , does not like to be roto-tilled as the small bits can be fairly easily pulled, IF, IF you pull them early on, and will not be part of a long system.
BUT then you have to chop them up pretty well.

Most vegetation killers need to be applied to the foliage to be absorbed and will not work on roots, rhizomes, tubers, or seeds. I think your best bet is to carefully sift through the soil removing all the plant material you can, allow it to dry out and stay dry all winter then apply a pre-emergent herbicide in the spring and plant your bed with started seedlings not seeds. Re- apply the pre-emergent as directed on the label, and promptly pull or paint with glyphosate (or sedge hammer if a sedge) any unwanted sprouts that appear. Good luck!

@rpr thanks. i'll give it a shot!

@porkpal dang. i thought i was doing a good thing by pulling everything. I'll do the preemergent thing.
Good idea.

I should tell you, depending on type of fabric you used, some rhizome weeds, can/will over the years find their way through the fabric, or in the case of crab-grass around the fabric.
I have several retaining walls, heavy 70 plus pound blocks , two to five courses high, and have found quack grass growing out of the blocks two courses down, this is over a foot below soil level, PLUS when I pull the blocks to remove the root/s, I find it often runs for feet along side the retaining wall sending rhizome out towards where the day lilies are planted.

PLUS, PLUS there is another wall above this one, small blocks five layers high behind which there are Boxwood bushes and crab-grass , coming from the high side has grown down the height of that wall and is now coming out at the bottom.

Now I do not know what you have, for Nut Sedge only killer works, but it is worth finding the root that sends out rhizome (In areas where the problem is years old there will be a root) and paint it with systemic herbicide.
IF you finally do redo the planters do not waste time putting fabric down as it is a feel good thing that in soil 12 inches deep does nothing unless you want it to keep the moisture from getting out as easily.

In the years I did landscaping I/we pulled up a lot of fabric, or worse plastic, and you would be amazed at what can work its way actually through some types of fabric.

If I were dealing with that problem, I would dig out all the soil and probably the bottom barrier too and start over with new barrier and new soil. It might cost a little more, but it would be sure solution. If not careful, you will still have the same problem next year that you had this year.

So what are any and all thoughts on using old carpeting to snuff out invasive weeds with rhizomes, or actually, just weeds in general? Our front area that we wish to add landscape too is constantly overrun with weeds, some of which is crabgrass. Unfortunately I don't know the names of any of the other weeds. I've heard before that carpet can kill vegetation by blocking out sunlight.

In your climate it should work pretty well but crab grass seeds can sit in the ground for a long, long, long time before showing their face.

I would expect many weeds to be able to grow up through the carpet. Plastic tarps would work better.