Nine years ago I bought a house where the owners had dumped tons of river rock in every possible part of the yard where you might want to plant something. Tons. Over the 30 years that they lived here, the rocks settled in, debris accumulated over them, weeds grew between them, and they dumped more rocks. I've hired men to help me clear it out; I spend countless hours "harvesting" and hauling rocks. Just to plant a small something from the nursery in a new spot requires digging out rocks embedded in the dirt nearly a foot deep.

Put down the mat.

Debris will still blow in or wash in with rains - regardless of whether you lay down the mat or not - and it does accumulate, and provides a growing substrate for any type of seeds that also blow, wash, or fall into the area. Maintenance/clean out will still be on the agenda.

You can save yourself some money by initially using a weed killer (or outright plant killer) of some sort such as Round-Up (not sure if this is available in NZ but surely there's something available) first - and then covering the area(s) where you intend to set stones down with wettened layers of old newspaper. Layers of wettened newspaper will be just as effective and have the added benefit that, unlike "landscaping fabric" (and other names it goes by), when stuff does try to grow up through it and on top of it (it still will with landscaping fabric, debris and seeds aren't biased one way or the other), you can actually tend to the newspaper much easier than landscaping fabric. You can also use old cardboard (like boxes broken down flat and soaked with water).

There's a blueberry "patch" on my property (was put in by previous owners) and they religiously used "landscaping fabric" around all of the bushes. Now what I have to contend with is yards and yards of landscaping fabric with debris that has collected on top of it - become a prime growing environment not only for grass/weeds, but also has "stuff" growing up through it. Unlike newspaper or cardboard - the landscaping fabric is VERY time-consuming and labour-demanding to try and pull up/free and I have to CUT it in little (tiny/very small) sections as I get it to turn loose. If they had used newspaper or cardboard (even if they allowed all of the debris to remain on top of it), then I could just kill the "stuff" off with a spray and re-layer it with newspaper/cardboard or I could at least till it under and start from scratch. The landscaping fabric can't be tilled - it gets tangled in the tines, screws up the tiller, and is just a hair-pulling personified Frustration that makes one want to scream.

Newspaper and/or cardboard wettened and laid down before your stones go in, will be much less work-intensive in the longrun and definitely cheaper.

I spend no small amount of my volunteer gardening/landscaping time for other people's yards/landscapes/gardens, trying to rip out landscaping fabric and tarps and heavy, non-degrading plastic that 5 or more years beforehand they were told was exactly what they needed to lay down before they put in stones, pebbles, etc. I have yet to encounter anyone who has used such who "down the road" (whether a few years or a decade or more) didn't end up wholly regretting the choice/decision and the expense, which in retrospect they found to be a complete waste of their money.

Me, I've ingrained it in my memory so as to learn by their mistakes.

I've seen (and have had to contend with the aftermath) of the same mistake being made by people using old full pieces or scraps of linoleum and other similar "floor coverings" in their efforts to prevent weeds and grass from coming up in landscaped areas. The only advantage I have found to the linoleum is that it tends to tear less/get fewer holes for things to grow up through and when it is laden on top with accumulated debris, if I can get a good hold on it with the right equipment and attach to a vehicle or tractor or have several extra pairs of hands/arms, it can be pulled/tugged loose and out a fair bit easier than landscaping fabric that has its days numbered.

So, aside from going with biodegradable paper products that you can recycle/reuse in landscaping, the next thing to remember is perpetual maintenance to clean out debris regularly to slow down eventual accumulation. You can still use plant/weed-killing sprays over rocks and in between them as well - and unlike resistant landscaping fabrics and plastics, that spray can soak down through paper-based material to continue to inhibit growth underneath the layers beneath the stones.

Oh, and paper-based reused material also "breathes" better - and you have less issue with moisture-retention or build-up underneath - something definitely to be taken into consideration if you are working around the foundations of a home, outbuildings, etc. You don't want more moisture trapped and "driven" towards the edges to escape right around the foundation of such buildings. The paper-based reused materials also make it conducive if you decide that you want to clear a little circle and dig down to put in a specimen plant or plants amongst the stones - such as cacti, succulents, and other "rock garden" conducive plants. Trying to cut through already laid landscaping fabric is a PITA! and rain would also run towards such openings and could end up flooding out future plants and causing them to rot.

Hope this saves you some effort and money... best wishes for your gardening - I am semi-ignorantly envious of your locale (in other words, I've never been to NZ and don't know the climate[s] of NZ firsthand, but from what I've seen... *drool* ya know?) and gardening possibilities!

Hermitonthehill, you sure saved me a lot of typing!
I did yard makeovers for about 25 years and had nothing but trouble with all of the above. One place had three layers of black plastic interspaced with ground up bark. You guessed it, weeds were growing on top like crazy. What a time consuming mess to undo!
Fabrics, breathable or non breathable, might work in desert climates, but not any place with 'normal' rainfall.

Dude Hermitonthehill, hope you don't have Carpal Tunnel Syndrome after your long-winded post.
* If you don't put the mat down, you will be sorry when you see those weeds peeking through your beautiful rocks.
* Think about it, the stone vendor is the expert. They will not retire off of selling some sucker some extra weed mats. I'm pretty sure they know what they are talking about.....