For your gravel area I'd get something that kills both weeds and grass. I don't know if Roundup makes any, I think Ortho does. It seems to work better than just weed killer, I think it's stronger.

We keep our mulch around 4 inches deep. It varies from person to person though. Our winters get very cold so the mulch helps insulate the plants. If we were further south we could get away with less and not worry about weeds.

Thanks for the reply Firethorn :) It's very hard to manually weed the gravel area because they are very small weed-lings, look like grass or groundcover at this stage. I also need to mulch the garden bed as these weed-lings are appearing on certain spots there too.

Will putting a thick (2-4 inches) layer of pine mulch starve them of sunlight and kill them (the tiny weeds)? As for the larger weeds, is pulling them out, leaving them on the ground, and then putting mulch over the ground (with the pulled weeds under it) ok? Will they grow through the mulch if the weeds (mostly grasses) are already established?

I've only found resources on mulching on fresh ground so far.

PS: on the gravel there's some parts will moss/lichen too, I don't think I want them there, is generic weed spray ok for them? I always wondered how roadside kerbs with the same type of gravel don't usually have grass/weeds...wonder what is used there...

Roundup is a non-selective weed killer and will kill both grass and broadeaf plants.
Without something under the mulch (newspaper, plastic, landscape fabric) the weeds will continue to come thru the mulch. But, at least the mulch make many unwanted plants a lot easier to hand pull as long as you are not dealing with really deep seated perennial types like bindweed and thistles.

Avoid landscape fabric- it really doesn't help much. Pull the big weeds and cover the little ones. The little ones may grwo, or they may get smothered. Beware of leaving the weeds on the soil and mulching over- some will re-root, especially grasses. If your let the roots dry for a few days, that would be ok.

Some folks use a pre-emergent in their beds to keep new weedsfrom sprouting. Preen is one,and Miracle gro now has one. Corn meal Gluten is an organic one. They need to be applied every 3 months. They won't stop all weeds,but you will have fewer.

I'd agree with everyone else, the mulch should smother the smaller weeds. The good thing is: Keep up with mulching and weeding, and after a few years no more weeds! I wouldn't put plastic under the mulch though. If it rains, there is nowhere for the water to go and the mulch will wash away. Newspaper is a cheap, yet effective, weed preventer/killer. Lay down a couple layers before you mulch and it'll smother the weeds and help keep them from growing.

I have no idea if weedkiller kills moss and lichens. Moss..maybe. I wouldn't think lichens though. They're tough buggers.

Here's what I'd do... Between the plants I'd cut or mash down the bigger weeds then over lap newspaper, about 4 or 5 sheets thick, over everything except the plants, water it well, put down a little soil, just enough to barely cover the newspaper, apply 3 or 4 inches of mulch and water it again...

As for the gravel.... I think I'd go over it with a heavy rake first... Do a small section at a time, rake down an inch or so and move it around to one side then level it off... If you're happy with the way it looks do the rest... That should get rid of the little weeds...

Now then for the moss and lichen, I don't think regular weed killer with have any effect.... They do make a spray for moss but the name escapes me right now, I'm sure your local nursery could advise you... You may want to think about putting down some more gravel because the moss and lichen usually form on anything that's been wet a good part of the time...

Rita

Okay, let's do it the easy way. As RitaOtay says, cut down the big plants. Now you have a choice (other than applying poison): easy and inexpensive: pour near-boiling water over the plants... apply just enough to think that the top inch of root feels the heat. Just as easy but $, use a flamer and simmer the greenery. You don't char the leaves, just use enough heat to wilt. In either case, the worst event is to re-apply the severe heat in a week (and according to those folks whom I know who use chemical, re-applying is required in almost every case which means pouring more poison-yuk!).

a flame weeder is the only effective tool I've ever found for controlling weeds in gravel.

it doesn't take much to learn to use one (you're trying to cook the plant so the steam bursts the cell walls, not crisp and burn it) and it goes much faster than any other application method except those big garden hose sprayers- and they're guaranteed to get stuff on things you don't want spray (yourself, your house, the family pet, not to mention the garden bed to the side of the gravel!)

One of the sources of seed in your gravel is actually your vehicle, particularly if you do any country running. Another is birds on the clothes line and in any nearby trees/bushes. It's an ongoing issue.

As soon as the gravel packs down it forms a base for the accumulation of leaf-litter, humus, and anything else that can form a soil-like base.

A thick layer of gravel that is still mobile helps to keep the weed population down. As it makes for hard walking you may need to add stepping slabs to make it less effort to hang out the laundry.

If you plan to use Roundup know that it is pretty useless on clovers, dandelions and other things with tap roots. They'll look sick for a few weeks and then be away again. Careful digging that loosens the roots but doesn't bring soil to the surface can get rid of the problem for quite a while until new seedlings emerge. Wait until you've had autumn rain and the first crop of seedlings comes through before spraying.

There will be a second flush in spring, unless you have a dry season.

Keeping the gravel disturbed by raking it over - particularly the ridge in the section between the wheel tracks of your vehicle/s - helps to dislodge new seedlings.

Raking will also disturb the mosses and lichens, but know that they will drift down through the pebbles to start forming a soil and seed bed in time. Even if you dowse them with something lethal this will happen.

You could try a chlorine-based bleach on any dangerous patches of moss growth or algae. They form over the winter and can make wooden steps or decks highly slippery when wet. There'll probably be instructions on the container.

i've read resources about discarding old mulch whenever you remulch...why is this necessary? I already have a thin layer of mulch...can I reuse it somehow (try to put newspaper under problem areas and move the old mulch, and new mulch on it)?

Now, with the gravel area...it's actually more of sand/dirt material than pebbly gravel by the way. I was trying out Rita's suggestion on raking it deep, but found that the grass roots just stayed there...and I wasn't strong enough to rake it deep enough barely a cm) as it's reaching the harder layer of dirt below. I'm so annoyed that the unslightly weeds are flourishing there despite it being barren and claylike orange soil there.

I've been looking at public kerb sides, and car parks with dirt roads like mine. Also some parks have grassed areas with a strip of dirt where bicycles and pedestrians form paths across the grass. These areas are usually so clean or green living creatures. I'm wondering why does happen? Is this due to natural agitation of the soil surface, or is someone spraying something that prevents plants in such areas...? What could it be?

There is no reason to discard old mulch. I suspect the advice is based upon the idea that as mulch breaks down it forms a soil like substance and can accumulate weed seeds. Wouldn't worry much about it, old mulch conditions the soil. It would be like throwing away compost.

Your dirt/sand gravel is an ideal growing medium for weeds and not much else. The reason it doesn't grow weeds in other areas you have seen it is likely because those areas get more foot/vehicle travel. Using preen or some other pre-emergent controls new weeds sprouting from seeds for a couple months, but doesn't kill existing vegetation. You want Round-Up for that.

The thing to keep in mind is that there is nothing you can do to prevent weeds in the long term, you will have to keep at it forever, using a pre-emergent and/or roundup every 1-3 months. The only long term solution is to put asphalt or concrete over the area and once it forms cracks they must be repaired or weeds grow in the cracks. It is just how it is.