I'd skip it.

The Myth of Paper-Based Sheet Mulch

“Newspaper and cardboard sheet mulches are excellent ways to reduce weeds and maintain soil health
in permanent landscapes”

https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/newspaper-mulch.pdf

My initial thought is "What's the point?", since a think layer of much will prevent weed seeds from sprouting. But then I reread about your grass roots, and that may be a justification for the cardboard. It will add an extra temporary barrier for invasive grasses. But "temporary" is the operative word here because come fall it will be halfway decomposed and no more a barrier than the mulch.

But if you want to do it you will need to lay it around (or cut out to allow) existing plants. If you intend to fertilize before the fall, do that first. Be sure and water the cardboard thoroughly after you put it down and before you mulch. You don't want a water barrier. Then add mulch and water thoroughly again.

A better (but more expensive) solution would be a landscape fabric. It will last for a couple of years and easily pass water and nutrients to the soil. At the same time it will be a much more effective barrier for invasive roots trying to send up shoots.

"A better (but more expensive) solution would be a landscape fabric."

That is highly debatable :-) There are literally hundreds of discussion on these forums regarding the use of landscape fabric in planting beds to help keep weeds down and the overwhelming consensus is that a) it really doesn't work all that efficiently to control weeds; and b) it is a huge PITA to work with. And it can cause improper water penetration down into the soil profile and also can hamper nutrient availability. Most professional horticulturists and landscape professionals will urge you strongly to avoid. Under hardscaping, fine.....in planting beds, no way!!

A decently thick layer of mulch - 4-6" - will do as much to control weeds as will anything but weeds will still proliferate despite whatever methods you follow. It is just the nature of the beast.

Something with persistent perennial root-stocks like Quack Grass is likely to creep out to the edge of the cardboard and then emerge, unless the remaining root pieces are all near the center of a broad, continuous sheet of it. And might even be able to just hold on until the paper melts away, then come up.

The Myth of Landscape Fabric

"Landscape fabric provides permanent weed control for ornamental landscapes"

https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/landscape-fabric.pdf

So I've dug up the grass along with the roots. Some 4 inches long. It was tedious. I'm sure there are bits still left in the soil. I doubt anyone could completely get rid of them.

I already have flowers and plants in the flower bed.

I have watched videos on the fabric and don't really want to use it because of the reasons already soe of you have mentioned. If mulch alone is enough, I will go with it. However, if I can put that cardboard down, and mulch on top of it, I feel I may have more weed free time.. and the soil might gain some nutrients?

For the fabric, they suggest pinning it down, how to hold the cardboard down? Just soak it and then let mulch weigh it down?

I expect there may be growth around the edges of the flower beds... there

One of things you need to do to keep a flowerbed neat around the edges is to maintain a good sharp border. This prevents grass from creeping into the bed. You can do it with a sharp spade by cutting vertically down 4 - inches and creating a small v-shaped trench. Then you need to regularly cut the grass along the lawn edge with edging shear.

How long ago did you plant the plants? Ideally you would not have planted straight into a newly prepared bed but waited to see what weeds returned and dealt with them. I know this is probably more trouble than you want, but if the plants have only been in a couple of weeks I, personally, would dig the whole lot up and thoroughly cultivate the bed again. Using a garden fork I would dig the entire bed over to at least one fork depth removing all roots as I went. It always pays in the long run to cultivate thoroughly before planting or you can have a never ending battle on your hands against perennial weeds. After planting I would just mulch and forget the cardboard. I do use it but only between fruit bushes where I will never need to cultivate again, only add more mulch. In a flower bed where you will be doing much more regular work and probably occasionally walking on the soil cardboard can tend to compaction and also, as the mulch decomposes, you'll get bits of card exposed.

As for landscape fabric, dismiss it from your mind entirely.

As has been said on these forums before 'garden' is a verb as well as a noun. There is no way to avoid some regular maintenance. And for many of us the gardening is as important as the garden.

Two researchers at Michigan State University spent 9 years studying mulches and found that depth helps, mulches 4 to 6 inches thick helped control weeds best. However, mulches that deep also created conditions that invasive grass species, such as quack grass, like those growing conditions and do really good in them, a nice moist soil.

Janet Macunovich describes her method of keeping invasive grasses, such as quack grass, was to dig an 18 inch deep trench put in plastic carpet runner, vertically, as a barrier. A neat edge will not stop many of the invasive grasses because it is not, usually, deep enough.

Edging as I described works for me and thousands of other UK gardeners. The maintenance with shears is important as is recutting the soil edge periodically. At the moment we don't even know if the bed is surrounded by lawn nor, if it is, what grass species the lawn comprises.

floral_uk z8/9 SW UK - What an ID to type out! :) Thank you. My husband had already planted a few things - raspberry bush, blueberry busy, Gladiolas and another flowering plant (don't recall the name), and there were three small Hostas that reappeared as well.

I could of course wait till when the Glaiolas stop blooming to dig them up and put away the bulbs for winter, then do as you suggested. I do agree with your method but not sure how easy it would be to do it with all these plants in there already. The flower bed kind of starts from the lawn and is surrounded by a stone wall/border as it curves and then meets the grass again at the other end. I will try to post a photo shortly.

kimmq - Thank you for your input! I noticed the plastic runners on the edges of a few vertical (ish) flower beds on hillsides, or curbs and wondered why they had been used. Now I know I guess :)

Also. stopped thinking about the cardboard for now. Just going to do mulch.

What is the grass and where is this garden?

Transnationalq - just copy and paste! Or call me Floral;-)

Getting rid of the persistent perennial weeds is especially important if you are growing permanent plantings like your fruit bushes.

lazy_gardens - NW Massachusetts. Not sure, but I think its blade grass that I've been pulling out.

Finally got the time to get photos on here. This is the one I've been talking about mostly.

The ladder is not for decorative purposes :P It'll be gone in a few days.

Below is the other flower bed with a lot more planted and growing in it. Its towards the front corner of the plot, along the drive way and takes all the snow shovelled in the winter. All the perennials seem to grow back. This too had to be cleaned out. You can still see the 'blade grass' along the corner of the plot (bottom right corner towards postbox).

Nope! Landscape fabric WILL NOT WORK! If you have nut grass or bermuda grass it will grow through it. Several shrubs will root and grow through it. Then you have weeds and landscape fabric to cut out. (Ask me how I know).

In my beds that have plants - I use newspaper (no shiny pages) under the mulch. It's easier to fold around the plants. I lay it thicker where I think it needs to be. I use cardboard in the big areas. I've found that quite a few of the weeds that then grow there are easy to pull up since they haven't rooted through the paper/cardboard.