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how thick does mulch have to be to stop weeds?

Posted by rutica z7, Maryland (My Page) on
Mon, Aug 8, 05 at 12:36

i made a border around my flower garden where i used a weed wacker to dig up grass, then i put down some mulch.

2 weeks later, i now see grass poking thru the mulch. if i buy more mulch to put on top of the existing mulch, will that choke out the grass?

or do i have to pick up the mulch and put down a weed barrier and then put the mulch back? i usually cut up black plastic trash bags for my weed barrier.


Follow-Up Postings:

RE: how thick does mulch have to be to stop weeds?

By 'border' do you mean several inches that you have no intention of planting? Or are you going to plant within it?

Black plastic as a weed barrier truly creates problems for the soil/root systems. The soil system is a life system in and of itself. Covering with plastic puts a halt on many natural events such as gas exchange between soil atmosphere and the air, water infiltration and percolation, breakdown of organic matter to feed the microorganisms, and more! Plant roots need oxygen just as much as they need water. If you are not going to plant in this location, the plastic would not be so much of a problem.

By using a weed eater, you simply chopped the blades from the grass but did not kill the root systems. Grass can be very stubborn and is likely to continue to grow up through your mulch until it can see daylight!

Personally, I think that this is one of the instances where an application of RoundUp is called for. Established grass is much harder to kill than many weeds. BUT, even annual weed seeds can germinate in the mulch, so you'll have to keep after it, at least a little bit. Many people use several thicknesses of newspaper as a weed barrier. Apply the mulch on top.

RE: how thick does mulch have to be to stop weeds?

  • Posted by rutica z7, Maryland (My Page) on
    Mon, Aug 8, 05 at 16:31

Thank you so much for writing and for your information.

i have included 2 pictures.
This is the garden with the border. i am not going to plant in the border.

Image Hosted by

and this shows the grass peeking thru the mulch:

Image Hosted by

so you're saying, no matter how high i pile the mulch, it probably won't stop the grass?

If i spray Roundup, do i have to spray continuously--for example, every time it rains? or does one application usually take care of the grass/weeds? and if i spray thru the mulch, do you think that will be effective in reaching the grass to kill it since i can't see the roots?

thanks again,

RE: how thick does mulch have to be to stop weeds?

Roundup is a herbicide and will kill the plant to the roots!

However be very careful not to get any on the plants you are trying to save! Read the directions for the Roundup! It works to kill the grass in serveral days and needs to be done in the right temperatures!

RE: how thick does mulch have to be to stop weeds?

  • Posted by Rosa 4-ish CO Rockies (My Page) on
    Mon, Aug 8, 05 at 21:22

You don't spray thru the mulch. You only spray the green part of the plant. It will take in the chemical and transport it the roots. Depends on various factors (temps, spray concentration, etc) as to how long it will take to work but give it a good two weeks before even thinking about spraying anything again.

And yes, roundup will need to be applied periodically as it only kills what is green and growing when you spray it. Do read the label and call the 800 number if you have any questions about using the product.

Rhizo's idea of putting down layers of newspaper could save you later. You might want to consider that instead of the roundup. Really, not that much work for the size area you are dealing with, and the grass is really young so it won't take much to smother it. Cheaper than buying the chemical.

RE: how thick does mulch have to be to stop weeds?

Weed whackers and rototillers do not kill grass or weeds, so they will grow up through the mulch. Actually, with grass whacking and tilling cuts the grass roots into many tiny parts each of which will grow into a new plant.

Weed seeds in the soil under the mulch will have trouble growing through it but established roots will push through. Weed seeds will still land on and germinate in the mulch but their growth will be slowed and they will be easy to pull.

Round up is likely the easiest application for your bed, as it will be tricky to get newspaper to fit the round, narrow bed. Remove the mulch and spray the green parts of the grass and weeds with Round up; it takes about a week for the plants to start to turn yellow. Wait 10 days and spray anything that is still green again. Then replace the mulch, you should use 3" to 4". The mulch in your picture looks to be about 1" thick which is not enough. You should also cover the bare earth beyond the mulch edge as bare earth is an invitation to weed seeds.

You will have an ongoing problem with grass invading into the mulch from the lawn. Lawn grasses have long underground roots (tillers) that will extend from the lawn, under the mulched area and up. You will need to edge the outside of the mulched area once or twice a year with an edger or shovel or install 6" deep plastic edging to hold the grass roots back.

RE: how thick does mulch have to be to stop weeds?

  • Posted by rutica z7, Maryland (My Page) on
    Wed, Aug 10, 05 at 23:10

Thank you all so much for writing and for your suggestions. I was hoping to avoid having to pick up the mulch, but i guess i'll have to do it. i'll get my shovel and a wheelbarrow, so i guess it won't be too bad.

i decided i'm going to pick up the mulch and the white rocks, put landscape fabric down and put the mulch and rocks back.

then i'll use Roundup for the area between the mulch and the grass. i wanted to leave a gap between the mulch and the grass to make it easy when i use the lawnmower, so i didn't want to lay mulch in that area.

Thanks again!

RE: how thick does mulch have to be to stop weeds?

Landscape fabric is a bad idea for a flower bed. It works under stones in pathways, but if you cut it to allow planting, weeds grow.

You cannot leave a bare patch of earth between the mulch and the grass, both the grass and weeds will soon invade. You will need to use edging -- plastic, wood, bricks or you will need to dig a straight 6" deep cut at the edge and redo the cut three or four times a year. The edge then provides a guide for your lawn mower wheels. Even with edging, you will likely need to use the weed whacker or string grass trimmer to cut the grass close to the edge.

Here is a link that might be useful: Edging

RE: how thick does mulch have to be to stop weeds?

  • Posted by rutica z7, Maryland (My Page) on
    Mon, Aug 15, 05 at 22:49

thank you for writing! well, on sunday, i picked up the mulch, put it in a wheelbarrow and laid down landscape fabric. then i put the mulch back.

it might not be the best solution, but i had used it in other parts of my yard and it stopped the weeds.

i took your advice and decided to forego the bare patch of earth between the mulch and the grass. Since i didn't want to use edging or dig a cut at the edge, i just included that area with landscape fabric and mulch as well.

The main point of my post was trying to avoid picking up the mulch since that was a pain. it took a few hours, but hopefully, with my landscape fabric and mulch down, the weeds will stay away and i won't have to worry. of course, i have lots of weeds growing inside the circle, but at least now my border is weed-free.

thanks again for writing and for your advice. i read the link you provided. That was helpful and interesting as well.

RE: how thick does mulch have to be to stop weeds?

As an alternative to the bare earth strip to make mowing around the planter easier, you could try one of the planter border products that have the built-in skirt for just that purpose. A good example can be found at the included link but I've seen other styles that are similar.

Here is a link that might be useful: Alsot's Bedrock Garden Border

RE: how thick does mulch have to be to stop weeds?

  • Posted by rutica z7, Maryland (My Page) on
    Wed, Aug 24, 05 at 23:39

thanks for writing. i've seen those for sale at Wal-Mart also. they look practical and useful.

for my garden, i ended up covering the bare earth strip with landscape fabric and mulch. it's not what i ideally wanted, but i thought it would be the easiest. and it's been about 2 weeks, and so far, no weeds.

if i'm careful with my lawnmower, i can put the 2 lawnmower wheels on the mulch in order to closely cut the grass around the garden.

but i'm always looking for ideas for my next garden so thanks for posting and including the link.

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