Looking for advice on controlling nutgrass and put in a raised garden bed?

[AllenV]

Hello everyone,

I'm a new homeowner in QLD (moved in about 4 months ago) and have never had to maintain a lawn before so please excuse my ignorance on what may be silly questions.

I have a couch lawn (8x10m) which for the most part has been well behaved. There is however a patch of soil that is in almost perpetual shade due to the fence and roof so the grass doesn't really grow there. To make it worse, we have had nutgrass (if im identifying it correctly) growing in that area (as pictured below).

Bare patch Soil. Nutgrass?

The mrs and I have decided to have a raised flower bed in the bare soil area (the box on the ground type as opposed to the elevated types) and I'm looking for advice on how to best approach setting up the flower bed without having to worry about nutgrass growing in it. I've read about putting a barrier at the base of the flowerbed (cardboard or a weed mat) to smother the weed then top that with soil/compost and mulch. Given how nutgrass propagates from the 'nuts' underground, can they even be smothered in that manner?

For now, I've been trying to control the nutgrass by digging it out and using herbicides (Sedgehammer). I've applied Sedgehammer twice now (once 6 weeks ago and again 2 weeks ago when new sprouts started popping up). The nutgrass leaves have yellowed and no new sprouts seemed to have popped out (for now) but am wary how quick they can propagate and grow. Ideally, i'd want to eliminate the weed in the area before I put the flower bed in but it feels like that would be a long wait. I have also read that using Amgrow wintergrass killer would be a potentially better solution(?) to getting rid of the nutgrass though i dont want to stress the soil too much with herbicides too quickly.

I'd really appreciate any advice or suggestions on how to tackle this problem!!

Cheers!

[BradN]

Hi @AllenV , and welcome to the Bunnings Workshop community.

I know that nutgrass can be a real pest. In terms of herbicides, Sedgehammer is definitely the way to go. It's the herbicide that Amgrow makes specifically for nutgrass, so there would be no advantage using a different Amgrow product.

How deep is your raised flowerbed going to be? If it's sufficiently deep the nutgrass and other weeds might simply not be able to push up through it. If it's just going to be a shallow bed behind some garden edging you might want to look at some kind of barrier.

Our resident D.I.Y. expert @EricL will be back on the site this afternoon, and he might be able to offer more advice. In the meantime I'll tag horticultural experts @Noelle and @Adam_W to see if they have any thoughts.

How are you planning to construct your flower bed? If you need some inspiration you can check out Adam's step-by-step guide How to build a raised garden bed and our Top 10 most popular raised garden beds.

Please do keep us updated. We'd be happy to help with any other questions you have and we'd love to see how your project turns out.  

Brad

[AllenV]

Hi @BradN,

Thanks for the fast reply!

Our current thinking is to just put a weed mat as a barrier on the lawn surface then have a planter box 40-50cm high sitting on the weedmat. What I wasn't sure about is whether nutgrass would still survive under the mat and either penetrate the mat or even propagate and grow around the base of the planter box. Given the planter box depths im considering, would nutgrass not be a big concern? Im also unsure if it would be better in terms of weed control to dig out a bit of the lawn and soil (~10cm deep) in the shape of the planter box and then have the weedmat and planter box sit in that space? 

We havent given too much thought at this stage on what design of garden bed we wanted (admittedly i've been stubbornly focused on gettin grid of the weeds) but will certianly have a look at the link you've provided for more inspiration!

[EricL]

Hello @AllenV

Thanks for sharing your question about controlling nutgrass. Your use of Sedgehammer is spot on, but nutgrass is a very hardy grass and has a maximum dormant state of 10 years in ideal conditions. I suggest manually digging out as much of the nutgrass and its root system before you place your raised garden bed over the area.

Another option would be to cover the area with builder's plastic and mulch. I propose building your raised garden bed with a bottom independent of the ground soil. In this way, the area beneath will not receive any sun or water. This should starve out the nutgrass and prevent it from growing.

If you need further assistance, please let us know.

Eric

[AllenV]

Thanks for the advice @EricL! Given the flowers we want to have in the planter box wont have very deep root systems, I intend to take your suggestion of having a bed with an independent bottom and soil system. With the soil and mulch, I'm assuming the idea would be to have a mulch base and a top layer of soil for the plants to grow in the box. Is there any recommendation on how thick the mulch layer would need to be or just enough to covers the base of the box and plastic will do?  

Also, having never bought builders plastic before, is there any specific type or thickness that I should look out for or any type (of appropriate size) would suffice? 

[BradN]

Hi @AllenV,

The mulch would go on top of the soil. The purpose of the mulch is to help retain moisture in the soil, and also to help keep weeds down.

You might also want to check out Noelle's excellent guide How to fill a raised garden bed, which takes you right through the process of filling a garden bed from the bottom to the top, including the mulch.

I'm sure Eric will be able to advise you on the builder's plastic when he's back later today.

Brad  

[AllenV]

Ah, right. I misunderstood. Thanks for clarifying that and the handy link!

[EricL]

Hi @AllenV

Just to be clear on the sequence, the Builder's plastic goes on the ground only in the area where the nut grass is growing. It will need to be fixed in place with weed mat pins. The builder's plastic will need to be cut more or less to the shape of the raised garden bed. If the raised garden bed is built with legs and off the ground then you place the mulch over the plastic to disguise it so that you won't see the plastic. The raised garden bed assembly then goes over that area. 

In essence, the raised garden bed should have a gap underneath so that the bottom does not touch the ground. However, if you build your raised garden bed with its own bottom and place it over the plastic, there will be no need for the mulch. I hope that clears up the sequence of assembly.

As for the type of builder's plastic, I suggest getting a black one with a thickness of 200um. Make sure to measure the area you are covering so that you'll know how much you'll need. 

If you need further assistance, please let me know.

Eric

[AllenV]

Hi again @EricL, that certainly clarifies the assembly. We're intending to get a bed that has a base of it's own without legs (essentially a box on the ground) so we'll have that on the cut out plastic. 

Thanks you very much for the advice! At least now i know what to get on the next trip to Bunnings.